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* [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
@ 2021-04-01 18:37 Axel Rasmussen
  2021-04-07  6:14 ` Hugh Dickins
  0 siblings, 1 reply; 7+ messages in thread
From: Axel Rasmussen @ 2021-04-01 18:37 UTC (permalink / raw)
  To: Alexander Viro, Andrea Arcangeli, Andrew Morton, Hugh Dickins,
	Jerome Glisse, Joe Perches, Lokesh Gidra, Mike Rapoport,
	Peter Xu, Shaohua Li, Shuah Khan, Stephen Rothwell, Wang Qing
  Cc: linux-kernel, linux-fsdevel, linux-mm, linux-kselftest,
	Axel Rasmussen, Brian Geffon, Cannon Matthews,
	Dr . David Alan Gilbert, David Rientjes, Michel Lespinasse,
	Mina Almasry, Oliver Upton

Previously, the continue implementation in shmem_mcopy_atomic_pte was
incorrect for two main reasons:

- It didn't correctly skip some sections of code which make sense for
  newly allocated pages, but absolutely don't make sense for
  pre-existing page cache pages.

- Because shmem_mcopy_continue_pte is called only if VM_SHARED is
  detected in mm/userfaultd.c, we were incorrectly not supporting the
  case where a tmpfs file had been mmap()-ed with MAP_PRIVATE.

So, this patch does the following:

In mm/userfaultfd.c, break the logic to install PTEs out of
mcopy_atomic_pte, into a separate helper function.

In mfill_atomic_pte, for the MCOPY_ATOMIC_CONTINUE case, simply look
up the existing page in the page cache, and then use the PTE
installation helper to setup the mapping. This addresses the two issues
noted above.

The previous code's bugs manifested clearly in the error handling path.
So, to detect this kind of issue in the future, modify the selftest to
exercise the error handling path as well.

Note that this patch is based on linux-next/akpm; the "fixes" line
refers to a SHA1 in that branch.

Changes since v3:
- Significantly refactored the patch. Continue handling now happens in
  mm/userfaultfd.c, via a PTE installation helper. Most of the
  mm/shmem.c changes from the patch being fixed [1] are reverted.

Changes since v2:
- Drop the ClearPageDirty() entirely, instead of trying to remember the
  old value.
- Modify both pgoff / max_off checks to use pgoff. It's equivalent to
  offset, but offset wasn't initialized until the first check (which
  we're skipping).
- Keep the second pgoff / max_off check in the continue case.

Changes since v1:
- Refactor to skip ahead with goto, instead of adding several more
  "if (!is_continue)".
- Fix unconditional ClearPageDirty().
- Don't pte_mkwrite() when is_continue && !VM_SHARED.

[1] https://lore.kernel.org/patchwork/patch/1392464/

Fixes: 00da60b9d0a0 ("userfaultfd: support minor fault handling for shmem")
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 mm/shmem.c       |  56 ++++++---------
 mm/userfaultfd.c | 183 ++++++++++++++++++++++++++++++++++-------------
 2 files changed, 156 insertions(+), 83 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 5cfd2fb6e52b..9d9a9f254f33 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2366,7 +2366,6 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 			   unsigned long dst_addr, unsigned long src_addr,
 			   enum mcopy_atomic_mode mode, struct page **pagep)
 {
-	bool is_continue = (mode == MCOPY_ATOMIC_CONTINUE);
 	struct inode *inode = file_inode(dst_vma->vm_file);
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct address_space *mapping = inode->i_mapping;
@@ -2377,18 +2376,17 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 	struct page *page;
 	pte_t _dst_pte, *dst_pte;
 	int ret;
-	pgoff_t offset, max_off;
+	pgoff_t max_off;
+
+	/* Handled by mcontinue_atomic_pte instead. */
+	if (WARN_ON_ONCE(mode == MCOPY_ATOMIC_CONTINUE))
+		return -EINVAL;

 	ret = -ENOMEM;
 	if (!shmem_inode_acct_block(inode, 1))
 		goto out;

-	if (is_continue) {
-		ret = -EFAULT;
-		page = find_lock_page(mapping, pgoff);
-		if (!page)
-			goto out_unacct_blocks;
-	} else if (!*pagep) {
+	if (!*pagep) {
 		page = shmem_alloc_page(gfp, info, pgoff);
 		if (!page)
 			goto out_unacct_blocks;
@@ -2415,27 +2413,21 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 		*pagep = NULL;
 	}

-	if (!is_continue) {
-		VM_BUG_ON(PageSwapBacked(page));
-		VM_BUG_ON(PageLocked(page));
-		__SetPageLocked(page);
-		__SetPageSwapBacked(page);
-		__SetPageUptodate(page);
-	}
+	VM_BUG_ON(PageSwapBacked(page));
+	VM_BUG_ON(PageLocked(page));
+	__SetPageLocked(page);
+	__SetPageSwapBacked(page);
+	__SetPageUptodate(page);

 	ret = -EFAULT;
-	offset = linear_page_index(dst_vma, dst_addr);
 	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-	if (unlikely(offset >= max_off))
+	if (unlikely(pgoff >= max_off))
 		goto out_release;

-	/* If page wasn't already in the page cache, add it. */
-	if (!is_continue) {
-		ret = shmem_add_to_page_cache(page, mapping, pgoff, NULL,
-					      gfp & GFP_RECLAIM_MASK, dst_mm);
-		if (ret)
-			goto out_release;
-	}
+	ret = shmem_add_to_page_cache(page, mapping, pgoff, NULL,
+				      gfp & GFP_RECLAIM_MASK, dst_mm);
+	if (ret)
+		goto out_release;

 	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
 	if (dst_vma->vm_flags & VM_WRITE)
@@ -2455,22 +2447,20 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,

 	ret = -EFAULT;
 	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-	if (unlikely(offset >= max_off))
+	if (unlikely(pgoff >= max_off))
 		goto out_release_unlock;

 	ret = -EEXIST;
 	if (!pte_none(*dst_pte))
 		goto out_release_unlock;

-	if (!is_continue) {
-		lru_cache_add(page);
+	lru_cache_add(page);

-		spin_lock_irq(&info->lock);
-		info->alloced++;
-		inode->i_blocks += BLOCKS_PER_PAGE;
-		shmem_recalc_inode(inode);
-		spin_unlock_irq(&info->lock);
-	}
+	spin_lock_irq(&info->lock);
+	info->alloced++;
+	inode->i_blocks += BLOCKS_PER_PAGE;
+	shmem_recalc_inode(inode);
+	spin_unlock_irq(&info->lock);

 	inc_mm_counter(dst_mm, mm_counter_file(page));
 	page_add_file_rmap(page, false);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index cbb7c8d79a4d..286d0657fbe2 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -48,21 +48,103 @@ struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
 	return dst_vma;
 }

+/*
+ * Install PTEs, to map dst_addr (within dst_vma) to page.
+ *
+ * This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),
+ * whether or not dst_vma is VM_SHARED. It also handles the more general
+ * MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file
+ * backed, or not).
+ *
+ * Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by
+ * shmem_mcopy_atomic_pte instead.
+ */
+static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
+				     struct vm_area_struct *dst_vma,
+				     unsigned long dst_addr, struct page *page,
+				     enum mcopy_atomic_mode mode, bool wp_copy)
+{
+	int ret;
+	pte_t _dst_pte, *dst_pte;
+	bool is_continue = mode == MCOPY_ATOMIC_CONTINUE;
+	int writable;
+	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
+	bool is_file_backed = dst_vma->vm_file;
+	spinlock_t *ptl;
+	struct inode *inode;
+	pgoff_t offset, max_off;
+
+	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
+	writable = dst_vma->vm_flags & VM_WRITE;
+	/* For CONTINUE on a non-shared VMA, don't pte_mkwrite for CoW. */
+	if (is_continue && !vm_shared)
+		writable = 0;
+
+	if (writable) {
+		_dst_pte = pte_mkdirty(_dst_pte);
+		if (wp_copy)
+			_dst_pte = pte_mkuffd_wp(_dst_pte);
+		else
+			_dst_pte = pte_mkwrite(_dst_pte);
+	} else if (vm_shared) {
+		/*
+		 * Since we didn't pte_mkdirty(), mark the page dirty or it
+		 * could be freed from under us. We could do this
+		 * unconditionally, but doing it only if !writable is faster.
+		 */
+		set_page_dirty(page);
+	}
+
+	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
+
+	if (is_file_backed) {
+		/* The shmem MAP_PRIVATE case requires checking the i_size */
+		inode = dst_vma->vm_file->f_inode;
+		offset = linear_page_index(dst_vma, dst_addr);
+		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
+		ret = -EFAULT;
+		if (unlikely(offset >= max_off))
+			goto out_unlock;
+	}
+
+	ret = -EEXIST;
+	if (!pte_none(*dst_pte))
+		goto out_unlock;
+
+	inc_mm_counter(dst_mm, mm_counter(page));
+	if (is_file_backed)
+		page_add_file_rmap(page, false);
+	else
+		page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
+
+	if (!is_continue)
+		lru_cache_add_inactive_or_unevictable(page, dst_vma);
+
+	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
+
+	/* No need to invalidate - it was non-present before */
+	update_mmu_cache(dst_vma, dst_addr, dst_pte);
+	pte_unmap_unlock(dst_pte, ptl);
+	ret = 0;
+out:
+	return ret;
+out_unlock:
+	pte_unmap_unlock(dst_pte, ptl);
+	goto out;
+}
+
 static int mcopy_atomic_pte(struct mm_struct *dst_mm,
 			    pmd_t *dst_pmd,
 			    struct vm_area_struct *dst_vma,
 			    unsigned long dst_addr,
 			    unsigned long src_addr,
 			    struct page **pagep,
+			    enum mcopy_atomic_mode mode,
 			    bool wp_copy)
 {
-	pte_t _dst_pte, *dst_pte;
-	spinlock_t *ptl;
 	void *page_kaddr;
 	int ret;
 	struct page *page;
-	pgoff_t offset, max_off;
-	struct inode *inode;

 	if (!*pagep) {
 		ret = -ENOMEM;
@@ -99,43 +181,12 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
 	if (mem_cgroup_charge(page, dst_mm, GFP_KERNEL))
 		goto out_release;

-	_dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));
-	if (dst_vma->vm_flags & VM_WRITE) {
-		if (wp_copy)
-			_dst_pte = pte_mkuffd_wp(_dst_pte);
-		else
-			_dst_pte = pte_mkwrite(_dst_pte);
-	}
-
-	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
-	if (dst_vma->vm_file) {
-		/* the shmem MAP_PRIVATE case requires checking the i_size */
-		inode = dst_vma->vm_file->f_inode;
-		offset = linear_page_index(dst_vma, dst_addr);
-		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-		ret = -EFAULT;
-		if (unlikely(offset >= max_off))
-			goto out_release_uncharge_unlock;
-	}
-	ret = -EEXIST;
-	if (!pte_none(*dst_pte))
-		goto out_release_uncharge_unlock;
-
-	inc_mm_counter(dst_mm, MM_ANONPAGES);
-	page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
-	lru_cache_add_inactive_or_unevictable(page, dst_vma);
-
-	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
-
-	/* No need to invalidate - it was non-present before */
-	update_mmu_cache(dst_vma, dst_addr, dst_pte);
-
-	pte_unmap_unlock(dst_pte, ptl);
-	ret = 0;
+	ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,
+					page, mode, wp_copy);
+	if (ret)
+		goto out_release;
 out:
 	return ret;
-out_release_uncharge_unlock:
-	pte_unmap_unlock(dst_pte, ptl);
 out_release:
 	put_page(page);
 	goto out;
@@ -176,6 +227,38 @@ static int mfill_zeropage_pte(struct mm_struct *dst_mm,
 	return ret;
 }

+static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
+				pmd_t *dst_pmd,
+				struct vm_area_struct *dst_vma,
+				unsigned long dst_addr,
+				bool wp_copy)
+{
+	struct inode *inode = file_inode(dst_vma->vm_file);
+	struct address_space *mapping = inode->i_mapping;
+	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
+	struct page *page;
+	int ret;
+
+	ret = -EFAULT;
+	page = find_lock_page(mapping, pgoff);
+	if (!page)
+		goto out;
+
+	ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,
+					page, MCOPY_ATOMIC_CONTINUE, wp_copy);
+	if (ret)
+		goto out_release;
+
+	unlock_page(page);
+	ret = 0;
+out:
+	return ret;
+out_release:
+	unlock_page(page);
+	put_page(page);
+	goto out;
+}
+
 static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
 {
 	pgd_t *pgd;
@@ -418,7 +501,13 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 						enum mcopy_atomic_mode mode,
 						bool wp_copy)
 {
-	ssize_t err;
+	ssize_t err = 0;
+
+	if (mode == MCOPY_ATOMIC_CONTINUE) {
+		err = mcontinue_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
+					   wp_copy);
+		goto out;
+	}

 	/*
 	 * The normal page fault path for a shmem will invoke the
@@ -431,26 +520,20 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 	 * and not in the radix tree.
 	 */
 	if (!(dst_vma->vm_flags & VM_SHARED)) {
-		switch (mode) {
-		case MCOPY_ATOMIC_NORMAL:
+		if (mode == MCOPY_ATOMIC_NORMAL)
 			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
 					       dst_addr, src_addr, page,
-					       wp_copy);
-			break;
-		case MCOPY_ATOMIC_ZEROPAGE:
+					       mode, wp_copy);
+		else if (mode == MCOPY_ATOMIC_ZEROPAGE)
 			err = mfill_zeropage_pte(dst_mm, dst_pmd,
 						 dst_vma, dst_addr);
-			break;
-		case MCOPY_ATOMIC_CONTINUE:
-			err = -EINVAL;
-			break;
-		}
 	} else {
 		VM_WARN_ON_ONCE(wp_copy);
 		err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
 					     src_addr, mode, page);
 	}

+out:
 	return err;
 }

--
2.31.0.208.g409f899ff0-goog


^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
  2021-04-01 18:37 [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior Axel Rasmussen
@ 2021-04-07  6:14 ` Hugh Dickins
  2021-04-07 20:33   ` Axel Rasmussen
  2021-04-12 21:54   ` Peter Xu
  0 siblings, 2 replies; 7+ messages in thread
From: Hugh Dickins @ 2021-04-07  6:14 UTC (permalink / raw)
  To: Axel Rasmussen
  Cc: Alexander Viro, Andrea Arcangeli, Andrew Morton, Hugh Dickins,
	Jerome Glisse, Joe Perches, Lokesh Gidra, Mike Rapoport,
	Peter Xu, Shaohua Li, Shuah Khan, Stephen Rothwell, Wang Qing,
	linux-kernel, linux-fsdevel, linux-mm, linux-kselftest,
	Brian Geffon, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Michel Lespinasse, Mina Almasry, Oliver Upton

[PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
was a significant rework, so here I'm reviewing a synthetic patch
merged from 5.12-rc5's 2021-03-31 mmotm patches:
  userfaultfd-support-minor-fault-handling-for-shmem.patch
  userfaultfd-support-minor-fault-handling-for-shmem-fix.patch
  userfaultfd-support-minor-fault-handling-for-shmem-fix-2.patch
Plus the PATCH v4 which akpm added the next day as fix-3:
  userfaultfd-support-minor-fault-handling-for-shmem-fix-3.patch

[PATCH v5] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
was the same as v4, except for adding a change in selftests, which
would not apply at this stage of the series: so I've ignored it.

>  fs/userfaultfd.c                 |    6 
>  include/linux/shmem_fs.h         |   26 +--
>  include/uapi/linux/userfaultfd.h |    4 
>  mm/memory.c                      |    8 -
>  mm/shmem.c                       |   65 +++------
>  mm/userfaultfd.c                 |  192 ++++++++++++++++++++---------
>  6 files changed, 186 insertions(+), 115 deletions(-)
> 
> diff -purN 5125m243/fs/userfaultfd.c 5125m247/fs/userfaultfd.c
> --- 5125m243/fs/userfaultfd.c	2021-04-04 22:32:32.018244547 -0700
> +++ 5125m247/fs/userfaultfd.c	2021-04-04 22:34:14.946860343 -0700
> @@ -1267,8 +1267,7 @@ static inline bool vma_can_userfault(str
>  	}
>  
>  	if (vm_flags & VM_UFFD_MINOR) {
> -		/* FIXME: Add minor fault interception for shmem. */
> -		if (!is_vm_hugetlb_page(vma))
> +		if (!(is_vm_hugetlb_page(vma) || vma_is_shmem(vma)))
>  			return false;
>  	}
>  
> @@ -1941,7 +1940,8 @@ static int userfaultfd_api(struct userfa
>  	/* report all available features and ioctls to userland */
>  	uffdio_api.features = UFFD_API_FEATURES;
>  #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
> -	uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS;
> +	uffdio_api.features &=
> +		~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
>  #endif
>  	uffdio_api.ioctls = UFFD_API_IOCTLS;
>  	ret = -EFAULT;
> diff -purN 5125m243/include/linux/shmem_fs.h 5125m247/include/linux/shmem_fs.h
> --- 5125m243/include/linux/shmem_fs.h	2021-02-14 14:32:24.000000000 -0800
> +++ 5125m247/include/linux/shmem_fs.h	2021-04-04 22:34:14.958860415 -0700
> @@ -9,6 +9,7 @@
>  #include <linux/percpu_counter.h>
>  #include <linux/xattr.h>
>  #include <linux/fs_parser.h>
> +#include <linux/userfaultfd_k.h>

I'd much rather not include userfaultfd_k.h in shmem_fs.h, and go back
to including it in mm/shmem.c: it's better to minimize everyone's header
file inclusion, where reasonably possible.  A small change below for that.

I advise the same for include/linux/hugetlb.h and mm/hugetlb.c,
but those are outside the scope of this userfaultfd/shmem patch.

>  
>  /* inode in-kernel data */
>  
> @@ -122,21 +123,16 @@ static inline bool shmem_file(struct fil
>  extern bool shmem_charge(struct inode *inode, long pages);
>  extern void shmem_uncharge(struct inode *inode, long pages);
>  
> +#ifdef CONFIG_USERFAULTFD
>  #ifdef CONFIG_SHMEM
> -extern int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> -				  struct vm_area_struct *dst_vma,
> -				  unsigned long dst_addr,
> -				  unsigned long src_addr,
> -				  struct page **pagep);
> -extern int shmem_mfill_zeropage_pte(struct mm_struct *dst_mm,
> -				    pmd_t *dst_pmd,
> -				    struct vm_area_struct *dst_vma,
> -				    unsigned long dst_addr);
> -#else
> -#define shmem_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma, dst_addr, \
> -			       src_addr, pagep)        ({ BUG(); 0; })
> -#define shmem_mfill_zeropage_pte(dst_mm, dst_pmd, dst_vma, \
> -				 dst_addr)      ({ BUG(); 0; })
> -#endif

Please add
enum mcopy_atomic_mode;
here, so the compiler can understand it without needing userfaultfd_k.h.

> +int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> +			   struct vm_area_struct *dst_vma,
> +			   unsigned long dst_addr, unsigned long src_addr,
> +			   enum mcopy_atomic_mode mode, struct page **pagep);
> +#else /* !CONFIG_SHMEM */
> +#define shmem_mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr, \
> +			       src_addr, mode, pagep)        ({ BUG(); 0; })
> +#endif /* CONFIG_SHMEM */
> +#endif /* CONFIG_USERFAULTFD */
>  
>  #endif
> diff -purN 5125m243/include/uapi/linux/userfaultfd.h 5125m247/include/uapi/linux/userfaultfd.h
> --- 5125m243/include/uapi/linux/userfaultfd.h	2021-04-04 22:32:32.042244690 -0700
> +++ 5125m247/include/uapi/linux/userfaultfd.h	2021-04-04 22:34:14.962860439 -0700
> @@ -31,7 +31,8 @@
>  			   UFFD_FEATURE_MISSING_SHMEM |		\
>  			   UFFD_FEATURE_SIGBUS |		\
>  			   UFFD_FEATURE_THREAD_ID |		\
> -			   UFFD_FEATURE_MINOR_HUGETLBFS)
> +			   UFFD_FEATURE_MINOR_HUGETLBFS |	\
> +			   UFFD_FEATURE_MINOR_SHMEM)
>  #define UFFD_API_IOCTLS				\
>  	((__u64)1 << _UFFDIO_REGISTER |		\
>  	 (__u64)1 << _UFFDIO_UNREGISTER |	\
> @@ -196,6 +197,7 @@ struct uffdio_api {
>  #define UFFD_FEATURE_SIGBUS			(1<<7)
>  #define UFFD_FEATURE_THREAD_ID			(1<<8)
>  #define UFFD_FEATURE_MINOR_HUGETLBFS		(1<<9)
> +#define UFFD_FEATURE_MINOR_SHMEM		(1<<10)

That's fine, but looking at the file itself, UFFD_FEATURE_MINOR_HUGETLBFS
has been given a comment above this list, so UFFD_FEATURE_MINOR_SHMEM is
feeling lonely without one.

>  	__u64 features;
>  
>  	__u64 ioctls;
> diff -purN 5125m243/mm/memory.c 5125m247/mm/memory.c
> --- 5125m243/mm/memory.c	2021-04-04 22:32:32.082244929 -0700
> +++ 5125m247/mm/memory.c	2021-04-04 22:34:15.002860678 -0700
> @@ -3972,9 +3972,11 @@ static vm_fault_t do_read_fault(struct v
>  	 * something).
>  	 */
>  	if (vma->vm_ops->map_pages && fault_around_bytes >> PAGE_SHIFT > 1) {
> -		ret = do_fault_around(vmf);
> -		if (ret)
> -			return ret;
> +		if (likely(!userfaultfd_minor(vmf->vma))) {
> +			ret = do_fault_around(vmf);
> +			if (ret)
> +				return ret;
> +		}

Ah yes, that's important, well spotted.

>  	}
>  
>  	ret = __do_fault(vmf);
> diff -purN 5125m243/mm/shmem.c 5125m247/mm/shmem.c
> --- 5125m243/mm/shmem.c	2021-02-28 18:30:29.692414467 -0800
> +++ 5125m247/mm/shmem.c	2021-04-04 22:34:15.014860751 -0700
> @@ -77,7 +77,6 @@ static struct vfsmount *shm_mnt;
>  #include <linux/syscalls.h>
>  #include <linux/fcntl.h>
>  #include <uapi/linux/memfd.h>
> -#include <linux/userfaultfd_k.h>
>  #include <linux/rmap.h>
>  #include <linux/uuid.h>
>  
> @@ -1785,8 +1784,8 @@ unlock:
>   * vm. If we swap it in we mark it dirty since we also free the swap
>   * entry since a page cannot live in both the swap and page cache.
>   *
> - * vmf and fault_type are only supplied by shmem_fault:
> - * otherwise they are NULL.
> + * vma, vmf, and fault_type are only supplied by shmem_fault: otherwise they

Yes, you're right (though I did prefer the newline after ":" as before).

> + * are NULL.
>   */
>  static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
>  	struct page **pagep, enum sgp_type sgp, gfp_t gfp,
> @@ -1830,6 +1829,13 @@ repeat:
>  		return error;
>  	}
>  
> +	if (page && vma && userfaultfd_minor(vma)) {
> +		unlock_page(page);
> +		put_page(page);
> +		*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
> +		return 0;
> +	}
> +

Yes and no. The problem here is that just above is an early return
from the xa_is_value() shmem_swapin_page() case: if the page had been
out on swap, VM_UFFD_MINOR must return there too.

(I haven't looked into the neatest way of coding that: it would
be preferable to have just one userfaultfd_minor() check and one
handle_userfault() call for both swapped-out and in-cache cases;
but maybe doing it that way would turn out to uglify the flow.)

(Should shmem_getpage_gfp() return before doing shmem_swapin_page(),
if swap is found when VM_UFFD_MINOR? Leaving it to userspace to touch
the page and swap it in? That could be more efficient, letting userspace
do it without the mmap_lock; but that would not be a robust interface,
and the exceptional swap path does not need such optmization.)

>  	if (page)
>  		hindex = page->index;
>  	if (page && sgp == SGP_WRITE)
> @@ -2354,13 +2360,11 @@ static struct inode *shmem_get_inode(str
>  	return inode;
>  }
>  
> -static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> -				  pmd_t *dst_pmd,
> -				  struct vm_area_struct *dst_vma,
> -				  unsigned long dst_addr,
> -				  unsigned long src_addr,
> -				  bool zeropage,
> -				  struct page **pagep)
> +#ifdef CONFIG_USERFAULTFD

Thank you! It's particularly helpful, given that "shmem_mcopy_atomic_pte"
gives no hint that it's for userfaultfd. (And I had to read Documentation
to understand why it likes to say "atomic" here.)  Okay, it's not your job
to change userfaultfd naming and organization; but on these rare occasions
that I have to revisit it, I do wish it were easier to follow.

> +int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> +			   struct vm_area_struct *dst_vma,
> +			   unsigned long dst_addr, unsigned long src_addr,
> +			   enum mcopy_atomic_mode mode, struct page **pagep)
>  {
>  	struct inode *inode = file_inode(dst_vma->vm_file);
>  	struct shmem_inode_info *info = SHMEM_I(inode);
> @@ -2372,7 +2376,11 @@ static int shmem_mfill_atomic_pte(struct
>  	struct page *page;
>  	pte_t _dst_pte, *dst_pte;
>  	int ret;
> -	pgoff_t offset, max_off;
> +	pgoff_t max_off;
> +
> +	/* Handled by mcontinue_atomic_pte instead. */
> +	if (WARN_ON_ONCE(mode == MCOPY_ATOMIC_CONTINUE))
> +		return -EINVAL;
>  
>  	ret = -ENOMEM;
>  	if (!shmem_inode_acct_block(inode, 1))
> @@ -2383,7 +2391,7 @@ static int shmem_mfill_atomic_pte(struct
>  		if (!page)
>  			goto out_unacct_blocks;
>  
> -		if (!zeropage) {	/* mcopy_atomic */
> +		if (mode == MCOPY_ATOMIC_NORMAL) {	/* mcopy_atomic */
>  			page_kaddr = kmap_atomic(page);
>  			ret = copy_from_user(page_kaddr,
>  					     (const void __user *)src_addr,
> @@ -2397,7 +2405,7 @@ static int shmem_mfill_atomic_pte(struct
>  				/* don't free the page */
>  				return -ENOENT;
>  			}
> -		} else {		/* mfill_zeropage_atomic */
> +		} else {		/* zeropage */
>  			clear_highpage(page);
>  		}
>  	} else {
> @@ -2405,15 +2413,15 @@ static int shmem_mfill_atomic_pte(struct
>  		*pagep = NULL;
>  	}
>  
> -	VM_BUG_ON(PageLocked(page) || PageSwapBacked(page));
> +	VM_BUG_ON(PageSwapBacked(page));
> +	VM_BUG_ON(PageLocked(page));
>  	__SetPageLocked(page);
>  	__SetPageSwapBacked(page);
>  	__SetPageUptodate(page);
>  
>  	ret = -EFAULT;
> -	offset = linear_page_index(dst_vma, dst_addr);
>  	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> -	if (unlikely(offset >= max_off))
> +	if (unlikely(pgoff >= max_off))

Yes, that's better.

>  		goto out_release;
>  
>  	ret = shmem_add_to_page_cache(page, mapping, pgoff, NULL,
> @@ -2439,7 +2447,7 @@ static int shmem_mfill_atomic_pte(struct
>  
>  	ret = -EFAULT;
>  	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> -	if (unlikely(offset >= max_off))
> +	if (unlikely(pgoff >= max_off))
>  		goto out_release_unlock;
>  
>  	ret = -EEXIST;
> @@ -2476,28 +2484,7 @@ out_unacct_blocks:
>  	shmem_inode_unacct_blocks(inode, 1);
>  	goto out;
>  }
> -
> -int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
> -			   pmd_t *dst_pmd,
> -			   struct vm_area_struct *dst_vma,
> -			   unsigned long dst_addr,
> -			   unsigned long src_addr,
> -			   struct page **pagep)
> -{
> -	return shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
> -				      dst_addr, src_addr, false, pagep);
> -}
> -
> -int shmem_mfill_zeropage_pte(struct mm_struct *dst_mm,
> -			     pmd_t *dst_pmd,
> -			     struct vm_area_struct *dst_vma,
> -			     unsigned long dst_addr)
> -{
> -	struct page *page = NULL;
> -
> -	return shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
> -				      dst_addr, 0, true, &page);
> -}

Yes, I like the way you have combined them into one.

> +#endif /* CONFIG_USERFAULTFD */
>  
>  #ifdef CONFIG_TMPFS
>  static const struct inode_operations shmem_symlink_inode_operations;
> diff -purN 5125m243/mm/userfaultfd.c 5125m247/mm/userfaultfd.c
> --- 5125m243/mm/userfaultfd.c	2021-04-04 22:32:32.102245048 -0700
> +++ 5125m247/mm/userfaultfd.c	2021-04-04 22:34:15.022860799 -0700
> @@ -48,21 +48,103 @@ struct vm_area_struct *find_dst_vma(stru
>  	return dst_vma;
>  }
>  
> +/*
> + * Install PTEs, to map dst_addr (within dst_vma) to page.
> + *
> + * This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),
> + * whether or not dst_vma is VM_SHARED. It also handles the more general
> + * MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file
> + * backed, or not).
> + *
> + * Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by
> + * shmem_mcopy_atomic_pte instead.
> + */
> +static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> +				     struct vm_area_struct *dst_vma,
> +				     unsigned long dst_addr, struct page *page,
> +				     enum mcopy_atomic_mode mode, bool wp_copy)
> +{
> +	int ret;
> +	pte_t _dst_pte, *dst_pte;
> +	bool is_continue = mode == MCOPY_ATOMIC_CONTINUE;
> +	int writable;
> +	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
> +	bool is_file_backed = dst_vma->vm_file;
> +	spinlock_t *ptl;
> +	struct inode *inode;
> +	pgoff_t offset, max_off;
> +
> +	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
> +	writable = dst_vma->vm_flags & VM_WRITE;
> +	/* For CONTINUE on a non-shared VMA, don't pte_mkwrite for CoW. */
> +	if (is_continue && !vm_shared)
> +		writable = 0;

Indeed, there is nothing more important than keeping pte_write() off
page cache mapped privately.

> +
> +	if (writable) {
> +		_dst_pte = pte_mkdirty(_dst_pte);
> +		if (wp_copy)
> +			_dst_pte = pte_mkuffd_wp(_dst_pte);
> +		else
> +			_dst_pte = pte_mkwrite(_dst_pte);
> +	} else if (vm_shared) {
> +		/*
> +		 * Since we didn't pte_mkdirty(), mark the page dirty or it
> +		 * could be freed from under us. We could do this
> +		 * unconditionally, but doing it only if !writable is faster.
> +		 */
> +		set_page_dirty(page);

I do not remember why Andrea or I preferred set_page_dirty() here to
pte_mkdirty(); but I suppose there might somewhere be a BUG_ON(pte_dirty)
which this would avoid.  Risky to change it, though it does look odd.

> +	}
> +
> +	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> +
> +	if (is_file_backed) {
> +		/* The shmem MAP_PRIVATE case requires checking the i_size */
> +		inode = dst_vma->vm_file->f_inode;
> +		offset = linear_page_index(dst_vma, dst_addr);
> +		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> +		ret = -EFAULT;
> +		if (unlikely(offset >= max_off))
> +			goto out_unlock;
> +	}
> +
> +	ret = -EEXIST;
> +	if (!pte_none(*dst_pte))
> +		goto out_unlock;
> +
> +	inc_mm_counter(dst_mm, mm_counter(page));
> +	if (is_file_backed)
> +		page_add_file_rmap(page, false);
> +	else
> +		page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
> +
> +	if (!is_continue)
> +		lru_cache_add_inactive_or_unevictable(page, dst_vma);

I'm beginning to think that you took a wrong direction in v4 and v5,
sending MINOR/CONTINUE this way instead of into shmem.c.  I haven't
spotted any mistakes in this function, but it's not easy.

If shmem_mcopy_atomic_pte() ended up using this mcopy_atomic_install_ptes()
(and so mm/shmem.c contain no pte manipulation at all), that would be one
kind of nice outcome.

Or if shmem_mcopy_atomic_pte() handled all of the shmem page cache issues,
and this just did the anonymous, that would be a different nice outcome.

But here we have mcopy_atomic_install_ptes(), with a host of at-first-sight
independent booleans (is_continue, is_file_backed, vm_shared, writable,
wp_copy), doing too much yet not enough.

I think it needs to shift in one direction or another: maybe revert
back towards something more like v3, or maybe go further with
mcopy_atomic_install_ptes().

> +
> +	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> +
> +	/* No need to invalidate - it was non-present before */
> +	update_mmu_cache(dst_vma, dst_addr, dst_pte);
> +	pte_unmap_unlock(dst_pte, ptl);
> +	ret = 0;

	ret =0;
out_unlock:
	pte_unmap_unlock(dst_pte, ptl);

would avoid the goto out contortions at out_unlock below.

> +out:
> +	return ret;
> +out_unlock:
> +	pte_unmap_unlock(dst_pte, ptl);
> +	goto out;
> +}
> +
>  static int mcopy_atomic_pte(struct mm_struct *dst_mm,
>  			    pmd_t *dst_pmd,
>  			    struct vm_area_struct *dst_vma,
>  			    unsigned long dst_addr,
>  			    unsigned long src_addr,
>  			    struct page **pagep,
> +			    enum mcopy_atomic_mode mode,
>  			    bool wp_copy)
>  {
> -	pte_t _dst_pte, *dst_pte;
> -	spinlock_t *ptl;
>  	void *page_kaddr;
>  	int ret;
>  	struct page *page;
> -	pgoff_t offset, max_off;
> -	struct inode *inode;
>  
>  	if (!*pagep) {
>  		ret = -ENOMEM;
> @@ -99,43 +181,12 @@ static int mcopy_atomic_pte(struct mm_st
>  	if (mem_cgroup_charge(page, dst_mm, GFP_KERNEL))
>  		goto out_release;
>  
> -	_dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));
> -	if (dst_vma->vm_flags & VM_WRITE) {
> -		if (wp_copy)
> -			_dst_pte = pte_mkuffd_wp(_dst_pte);
> -		else
> -			_dst_pte = pte_mkwrite(_dst_pte);
> -	}
> -
> -	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> -	if (dst_vma->vm_file) {
> -		/* the shmem MAP_PRIVATE case requires checking the i_size */
> -		inode = dst_vma->vm_file->f_inode;
> -		offset = linear_page_index(dst_vma, dst_addr);
> -		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> -		ret = -EFAULT;
> -		if (unlikely(offset >= max_off))
> -			goto out_release_uncharge_unlock;
> -	}
> -	ret = -EEXIST;
> -	if (!pte_none(*dst_pte))
> -		goto out_release_uncharge_unlock;
> -
> -	inc_mm_counter(dst_mm, MM_ANONPAGES);
> -	page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
> -	lru_cache_add_inactive_or_unevictable(page, dst_vma);
> -
> -	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> -
> -	/* No need to invalidate - it was non-present before */
> -	update_mmu_cache(dst_vma, dst_addr, dst_pte);
> -
> -	pte_unmap_unlock(dst_pte, ptl);
> -	ret = 0;
> +	ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,
> +					page, mode, wp_copy);
> +	if (ret)
> +		goto out_release;
>  out:
>  	return ret;
> -out_release_uncharge_unlock:
> -	pte_unmap_unlock(dst_pte, ptl);
>  out_release:
>  	put_page(page);
>  	goto out;
> @@ -176,6 +227,38 @@ out_unlock:
>  	return ret;
>  }
>  
> +static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
> +				pmd_t *dst_pmd,
> +				struct vm_area_struct *dst_vma,
> +				unsigned long dst_addr,
> +				bool wp_copy)
> +{
> +	struct inode *inode = file_inode(dst_vma->vm_file);
> +	struct address_space *mapping = inode->i_mapping;
> +	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
> +	struct page *page;
> +	int ret;
> +
> +	ret = -EFAULT;
> +	page = find_lock_page(mapping, pgoff);
> +	if (!page)
> +		goto out;

No. That looks nice and generic, but IIUC only shmem gets to come this way,
and find_lock_page() is not allowing for swap. Agreed that it will be
unlikely for the page (vetted by userspace before ioctl'ing to here) to
have gotten swapped out meanwhile, but we must allow for that unlikelihood.

You will need to use shmem_getpage(SGP_CACHE) instead: NULL vma so you
don't get trapped into endless userfaultfd_minor() breakouts.  (If someone
else punched a hole in the file meanwhile, shmem_getpage() will allocate
a new page you don't particularly want: but that is not a case we need to
care about, beyond getting the accounting right and not crashing.)

> +
> +	ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,
> +					page, MCOPY_ATOMIC_CONTINUE, wp_copy);
> +	if (ret)
> +		goto out_release;
> +
> +	unlock_page(page);
> +	ret = 0;
> +out:
> +	return ret;
> +out_release:
> +	unlock_page(page);
> +	put_page(page);
> +	goto out;
> +}
> +
>  static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
>  {
>  	pgd_t *pgd;
> @@ -415,10 +498,16 @@ static __always_inline ssize_t mfill_ato
>  						unsigned long dst_addr,
>  						unsigned long src_addr,
>  						struct page **page,
> -						bool zeropage,
> +						enum mcopy_atomic_mode mode,
>  						bool wp_copy)
>  {
> -	ssize_t err;
> +	ssize_t err = 0;
> +
> +	if (mode == MCOPY_ATOMIC_CONTINUE) {
> +		err = mcontinue_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> +					   wp_copy);
> +		goto out;
> +	}
>  
>  	/*
>  	 * The normal page fault path for a shmem will invoke the
> @@ -431,24 +520,20 @@ static __always_inline ssize_t mfill_ato
>  	 * and not in the radix tree.
>  	 */
>  	if (!(dst_vma->vm_flags & VM_SHARED)) {
> -		if (!zeropage)
> +		if (mode == MCOPY_ATOMIC_NORMAL)
>  			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
>  					       dst_addr, src_addr, page,
> -					       wp_copy);
> -		else
> +					       mode, wp_copy);
> +		else if (mode == MCOPY_ATOMIC_ZEROPAGE)
>  			err = mfill_zeropage_pte(dst_mm, dst_pmd,
>  						 dst_vma, dst_addr);
>  	} else {
>  		VM_WARN_ON_ONCE(wp_copy);
> -		if (!zeropage)
> -			err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
> -						     dst_vma, dst_addr,
> -						     src_addr, page);
> -		else
> -			err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
> -						       dst_vma, dst_addr);
> +		err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> +					     src_addr, mode, page);
>  	}
>  
> +out:
>  	return err;
>  }
>  
> @@ -467,7 +552,6 @@ static __always_inline ssize_t __mcopy_a
>  	long copied;
>  	struct page *page;
>  	bool wp_copy;
> -	bool zeropage = (mcopy_mode == MCOPY_ATOMIC_ZEROPAGE);
>  
>  	/*
>  	 * Sanitize the command parameters:
> @@ -530,7 +614,7 @@ retry:
>  
>  	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
>  		goto out_unlock;
> -	if (mcopy_mode == MCOPY_ATOMIC_CONTINUE)
> +	if (!vma_is_shmem(dst_vma) && mcopy_mode == MCOPY_ATOMIC_CONTINUE)
>  		goto out_unlock;
>  
>  	/*
> @@ -578,7 +662,7 @@ retry:
>  		BUG_ON(pmd_trans_huge(*dst_pmd));
>  
>  		err = mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> -				       src_addr, &page, zeropage, wp_copy);
> +				       src_addr, &page, mcopy_mode, wp_copy);
>  		cond_resched();
>  
>  		if (unlikely(err == -ENOENT)) {

I didn't pay a great deal of attention to the remainder above: I think it's
right, but it would have been easier to review if mcopy_mode and refactoring
had been introduced in a preliminary patch, before going on to implement
MINOR/CONTINUE on shmem.

Hugh

^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
  2021-04-07  6:14 ` Hugh Dickins
@ 2021-04-07 20:33   ` Axel Rasmussen
  2021-04-07 22:50     ` Hugh Dickins
  2021-04-12 21:54   ` Peter Xu
  1 sibling, 1 reply; 7+ messages in thread
From: Axel Rasmussen @ 2021-04-07 20:33 UTC (permalink / raw)
  To: Hugh Dickins
  Cc: Alexander Viro, Andrea Arcangeli, Andrew Morton, Jerome Glisse,
	Joe Perches, Lokesh Gidra, Mike Rapoport, Peter Xu, Shaohua Li,
	Shuah Khan, Stephen Rothwell, Wang Qing, LKML, linux-fsdevel,
	Linux MM, linux-kselftest, Brian Geffon, Cannon Matthews,
	Dr . David Alan Gilbert, David Rientjes, Michel Lespinasse,
	Mina Almasry, Oliver Upton

Thanks for the thorough and insightful review, Hugh!

On Tue, Apr 6, 2021 at 11:14 PM Hugh Dickins <hughd@google.com> wrote:
>
> [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
> was a significant rework, so here I'm reviewing a synthetic patch
> merged from 5.12-rc5's 2021-03-31 mmotm patches:
>   userfaultfd-support-minor-fault-handling-for-shmem.patch
>   userfaultfd-support-minor-fault-handling-for-shmem-fix.patch
>   userfaultfd-support-minor-fault-handling-for-shmem-fix-2.patch
> Plus the PATCH v4 which akpm added the next day as fix-3:
>   userfaultfd-support-minor-fault-handling-for-shmem-fix-3.patch
>
> [PATCH v5] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
> was the same as v4, except for adding a change in selftests, which
> would not apply at this stage of the series: so I've ignored it.

Makes sense. :) Sorry about the confusing diff, a significant portion
of this patch is just undoing what we did in the series that added
this feature in the first place, due to the change in direction.

So, I'm planning to follow Peter's suggestion to roughly squash this
together, and send it as a full series. That ought to be much easier
to review.

So, the process of applying that series (to Andrew's tree) then
becomes, dropping some of the patches that are already in Andrew's
tree, and then applying the full new series. I'll include a precise
list of patches the new series replaces in its cover letter.

>
> >  fs/userfaultfd.c                 |    6
> >  include/linux/shmem_fs.h         |   26 +--
> >  include/uapi/linux/userfaultfd.h |    4
> >  mm/memory.c                      |    8 -
> >  mm/shmem.c                       |   65 +++------
> >  mm/userfaultfd.c                 |  192 ++++++++++++++++++++---------
> >  6 files changed, 186 insertions(+), 115 deletions(-)
> >
> > diff -purN 5125m243/fs/userfaultfd.c 5125m247/fs/userfaultfd.c
> > --- 5125m243/fs/userfaultfd.c 2021-04-04 22:32:32.018244547 -0700
> > +++ 5125m247/fs/userfaultfd.c 2021-04-04 22:34:14.946860343 -0700
> > @@ -1267,8 +1267,7 @@ static inline bool vma_can_userfault(str
> >       }
> >
> >       if (vm_flags & VM_UFFD_MINOR) {
> > -             /* FIXME: Add minor fault interception for shmem. */
> > -             if (!is_vm_hugetlb_page(vma))
> > +             if (!(is_vm_hugetlb_page(vma) || vma_is_shmem(vma)))
> >                       return false;
> >       }
> >
> > @@ -1941,7 +1940,8 @@ static int userfaultfd_api(struct userfa
> >       /* report all available features and ioctls to userland */
> >       uffdio_api.features = UFFD_API_FEATURES;
> >  #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
> > -     uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS;
> > +     uffdio_api.features &=
> > +             ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
> >  #endif
> >       uffdio_api.ioctls = UFFD_API_IOCTLS;
> >       ret = -EFAULT;
> > diff -purN 5125m243/include/linux/shmem_fs.h 5125m247/include/linux/shmem_fs.h
> > --- 5125m243/include/linux/shmem_fs.h 2021-02-14 14:32:24.000000000 -0800
> > +++ 5125m247/include/linux/shmem_fs.h 2021-04-04 22:34:14.958860415 -0700
> > @@ -9,6 +9,7 @@
> >  #include <linux/percpu_counter.h>
> >  #include <linux/xattr.h>
> >  #include <linux/fs_parser.h>
> > +#include <linux/userfaultfd_k.h>
>
> I'd much rather not include userfaultfd_k.h in shmem_fs.h, and go back
> to including it in mm/shmem.c: it's better to minimize everyone's header
> file inclusion, where reasonably possible.  A small change below for that.
>
> I advise the same for include/linux/hugetlb.h and mm/hugetlb.c,
> but those are outside the scope of this userfaultfd/shmem patch.
>
> >
> >  /* inode in-kernel data */
> >
> > @@ -122,21 +123,16 @@ static inline bool shmem_file(struct fil
> >  extern bool shmem_charge(struct inode *inode, long pages);
> >  extern void shmem_uncharge(struct inode *inode, long pages);
> >
> > +#ifdef CONFIG_USERFAULTFD
> >  #ifdef CONFIG_SHMEM
> > -extern int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > -                               struct vm_area_struct *dst_vma,
> > -                               unsigned long dst_addr,
> > -                               unsigned long src_addr,
> > -                               struct page **pagep);
> > -extern int shmem_mfill_zeropage_pte(struct mm_struct *dst_mm,
> > -                                 pmd_t *dst_pmd,
> > -                                 struct vm_area_struct *dst_vma,
> > -                                 unsigned long dst_addr);
> > -#else
> > -#define shmem_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma, dst_addr, \
> > -                            src_addr, pagep)        ({ BUG(); 0; })
> > -#define shmem_mfill_zeropage_pte(dst_mm, dst_pmd, dst_vma, \
> > -                              dst_addr)      ({ BUG(); 0; })
> > -#endif
>
> Please add
> enum mcopy_atomic_mode;
> here, so the compiler can understand it without needing userfaultfd_k.h.
>
> > +int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > +                        struct vm_area_struct *dst_vma,
> > +                        unsigned long dst_addr, unsigned long src_addr,
> > +                        enum mcopy_atomic_mode mode, struct page **pagep);
> > +#else /* !CONFIG_SHMEM */
> > +#define shmem_mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr, \
> > +                            src_addr, mode, pagep)        ({ BUG(); 0; })
> > +#endif /* CONFIG_SHMEM */
> > +#endif /* CONFIG_USERFAULTFD */
> >
> >  #endif
> > diff -purN 5125m243/include/uapi/linux/userfaultfd.h 5125m247/include/uapi/linux/userfaultfd.h
> > --- 5125m243/include/uapi/linux/userfaultfd.h 2021-04-04 22:32:32.042244690 -0700
> > +++ 5125m247/include/uapi/linux/userfaultfd.h 2021-04-04 22:34:14.962860439 -0700
> > @@ -31,7 +31,8 @@
> >                          UFFD_FEATURE_MISSING_SHMEM |         \
> >                          UFFD_FEATURE_SIGBUS |                \
> >                          UFFD_FEATURE_THREAD_ID |             \
> > -                        UFFD_FEATURE_MINOR_HUGETLBFS)
> > +                        UFFD_FEATURE_MINOR_HUGETLBFS |       \
> > +                        UFFD_FEATURE_MINOR_SHMEM)
> >  #define UFFD_API_IOCTLS                              \
> >       ((__u64)1 << _UFFDIO_REGISTER |         \
> >        (__u64)1 << _UFFDIO_UNREGISTER |       \
> > @@ -196,6 +197,7 @@ struct uffdio_api {
> >  #define UFFD_FEATURE_SIGBUS                  (1<<7)
> >  #define UFFD_FEATURE_THREAD_ID                       (1<<8)
> >  #define UFFD_FEATURE_MINOR_HUGETLBFS         (1<<9)
> > +#define UFFD_FEATURE_MINOR_SHMEM             (1<<10)
>
> That's fine, but looking at the file itself, UFFD_FEATURE_MINOR_HUGETLBFS
> has been given a comment above this list, so UFFD_FEATURE_MINOR_SHMEM is
> feeling lonely without one.
>
> >       __u64 features;
> >
> >       __u64 ioctls;
> > diff -purN 5125m243/mm/memory.c 5125m247/mm/memory.c
> > --- 5125m243/mm/memory.c      2021-04-04 22:32:32.082244929 -0700
> > +++ 5125m247/mm/memory.c      2021-04-04 22:34:15.002860678 -0700
> > @@ -3972,9 +3972,11 @@ static vm_fault_t do_read_fault(struct v
> >        * something).
> >        */
> >       if (vma->vm_ops->map_pages && fault_around_bytes >> PAGE_SHIFT > 1) {
> > -             ret = do_fault_around(vmf);
> > -             if (ret)
> > -                     return ret;
> > +             if (likely(!userfaultfd_minor(vmf->vma))) {
> > +                     ret = do_fault_around(vmf);
> > +                     if (ret)
> > +                             return ret;
> > +             }
>
> Ah yes, that's important, well spotted.
>
> >       }
> >
> >       ret = __do_fault(vmf);
> > diff -purN 5125m243/mm/shmem.c 5125m247/mm/shmem.c
> > --- 5125m243/mm/shmem.c       2021-02-28 18:30:29.692414467 -0800
> > +++ 5125m247/mm/shmem.c       2021-04-04 22:34:15.014860751 -0700
> > @@ -77,7 +77,6 @@ static struct vfsmount *shm_mnt;
> >  #include <linux/syscalls.h>
> >  #include <linux/fcntl.h>
> >  #include <uapi/linux/memfd.h>
> > -#include <linux/userfaultfd_k.h>
> >  #include <linux/rmap.h>
> >  #include <linux/uuid.h>
> >
> > @@ -1785,8 +1784,8 @@ unlock:
> >   * vm. If we swap it in we mark it dirty since we also free the swap
> >   * entry since a page cannot live in both the swap and page cache.
> >   *
> > - * vmf and fault_type are only supplied by shmem_fault:
> > - * otherwise they are NULL.
> > + * vma, vmf, and fault_type are only supplied by shmem_fault: otherwise they
>
> Yes, you're right (though I did prefer the newline after ":" as before).
>
> > + * are NULL.
> >   */
> >  static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> >       struct page **pagep, enum sgp_type sgp, gfp_t gfp,
> > @@ -1830,6 +1829,13 @@ repeat:
> >               return error;
> >       }
> >
> > +     if (page && vma && userfaultfd_minor(vma)) {
> > +             unlock_page(page);
> > +             put_page(page);
> > +             *fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
> > +             return 0;
> > +     }
> > +
>
> Yes and no. The problem here is that just above is an early return
> from the xa_is_value() shmem_swapin_page() case: if the page had been
> out on swap, VM_UFFD_MINOR must return there too.
>
> (I haven't looked into the neatest way of coding that: it would
> be preferable to have just one userfaultfd_minor() check and one
> handle_userfault() call for both swapped-out and in-cache cases;
> but maybe doing it that way would turn out to uglify the flow.)
>
> (Should shmem_getpage_gfp() return before doing shmem_swapin_page(),
> if swap is found when VM_UFFD_MINOR? Leaving it to userspace to touch
> the page and swap it in? That could be more efficient, letting userspace
> do it without the mmap_lock; but that would not be a robust interface,
> and the exceptional swap path does not need such optmization.)
>
> >       if (page)
> >               hindex = page->index;
> >       if (page && sgp == SGP_WRITE)
> > @@ -2354,13 +2360,11 @@ static struct inode *shmem_get_inode(str
> >       return inode;
> >  }
> >
> > -static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> > -                               pmd_t *dst_pmd,
> > -                               struct vm_area_struct *dst_vma,
> > -                               unsigned long dst_addr,
> > -                               unsigned long src_addr,
> > -                               bool zeropage,
> > -                               struct page **pagep)
> > +#ifdef CONFIG_USERFAULTFD
>
> Thank you! It's particularly helpful, given that "shmem_mcopy_atomic_pte"
> gives no hint that it's for userfaultfd. (And I had to read Documentation
> to understand why it likes to say "atomic" here.)  Okay, it's not your job
> to change userfaultfd naming and organization; but on these rare occasions
> that I have to revisit it, I do wish it were easier to follow.
>
> > +int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > +                        struct vm_area_struct *dst_vma,
> > +                        unsigned long dst_addr, unsigned long src_addr,
> > +                        enum mcopy_atomic_mode mode, struct page **pagep)
> >  {
> >       struct inode *inode = file_inode(dst_vma->vm_file);
> >       struct shmem_inode_info *info = SHMEM_I(inode);
> > @@ -2372,7 +2376,11 @@ static int shmem_mfill_atomic_pte(struct
> >       struct page *page;
> >       pte_t _dst_pte, *dst_pte;
> >       int ret;
> > -     pgoff_t offset, max_off;
> > +     pgoff_t max_off;
> > +
> > +     /* Handled by mcontinue_atomic_pte instead. */
> > +     if (WARN_ON_ONCE(mode == MCOPY_ATOMIC_CONTINUE))
> > +             return -EINVAL;
> >
> >       ret = -ENOMEM;
> >       if (!shmem_inode_acct_block(inode, 1))
> > @@ -2383,7 +2391,7 @@ static int shmem_mfill_atomic_pte(struct
> >               if (!page)
> >                       goto out_unacct_blocks;
> >
> > -             if (!zeropage) {        /* mcopy_atomic */
> > +             if (mode == MCOPY_ATOMIC_NORMAL) {      /* mcopy_atomic */
> >                       page_kaddr = kmap_atomic(page);
> >                       ret = copy_from_user(page_kaddr,
> >                                            (const void __user *)src_addr,
> > @@ -2397,7 +2405,7 @@ static int shmem_mfill_atomic_pte(struct
> >                               /* don't free the page */
> >                               return -ENOENT;
> >                       }
> > -             } else {                /* mfill_zeropage_atomic */
> > +             } else {                /* zeropage */
> >                       clear_highpage(page);
> >               }
> >       } else {
> > @@ -2405,15 +2413,15 @@ static int shmem_mfill_atomic_pte(struct
> >               *pagep = NULL;
> >       }
> >
> > -     VM_BUG_ON(PageLocked(page) || PageSwapBacked(page));
> > +     VM_BUG_ON(PageSwapBacked(page));
> > +     VM_BUG_ON(PageLocked(page));
> >       __SetPageLocked(page);
> >       __SetPageSwapBacked(page);
> >       __SetPageUptodate(page);
> >
> >       ret = -EFAULT;
> > -     offset = linear_page_index(dst_vma, dst_addr);
> >       max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> > -     if (unlikely(offset >= max_off))
> > +     if (unlikely(pgoff >= max_off))
>
> Yes, that's better.
>
> >               goto out_release;
> >
> >       ret = shmem_add_to_page_cache(page, mapping, pgoff, NULL,
> > @@ -2439,7 +2447,7 @@ static int shmem_mfill_atomic_pte(struct
> >
> >       ret = -EFAULT;
> >       max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> > -     if (unlikely(offset >= max_off))
> > +     if (unlikely(pgoff >= max_off))
> >               goto out_release_unlock;
> >
> >       ret = -EEXIST;
> > @@ -2476,28 +2484,7 @@ out_unacct_blocks:
> >       shmem_inode_unacct_blocks(inode, 1);
> >       goto out;
> >  }
> > -
> > -int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > -                        pmd_t *dst_pmd,
> > -                        struct vm_area_struct *dst_vma,
> > -                        unsigned long dst_addr,
> > -                        unsigned long src_addr,
> > -                        struct page **pagep)
> > -{
> > -     return shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
> > -                                   dst_addr, src_addr, false, pagep);
> > -}
> > -
> > -int shmem_mfill_zeropage_pte(struct mm_struct *dst_mm,
> > -                          pmd_t *dst_pmd,
> > -                          struct vm_area_struct *dst_vma,
> > -                          unsigned long dst_addr)
> > -{
> > -     struct page *page = NULL;
> > -
> > -     return shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
> > -                                   dst_addr, 0, true, &page);
> > -}
>
> Yes, I like the way you have combined them into one.
>
> > +#endif /* CONFIG_USERFAULTFD */
> >
> >  #ifdef CONFIG_TMPFS
> >  static const struct inode_operations shmem_symlink_inode_operations;
> > diff -purN 5125m243/mm/userfaultfd.c 5125m247/mm/userfaultfd.c
> > --- 5125m243/mm/userfaultfd.c 2021-04-04 22:32:32.102245048 -0700
> > +++ 5125m247/mm/userfaultfd.c 2021-04-04 22:34:15.022860799 -0700
> > @@ -48,21 +48,103 @@ struct vm_area_struct *find_dst_vma(stru
> >       return dst_vma;
> >  }
> >
> > +/*
> > + * Install PTEs, to map dst_addr (within dst_vma) to page.
> > + *
> > + * This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),
> > + * whether or not dst_vma is VM_SHARED. It also handles the more general
> > + * MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file
> > + * backed, or not).
> > + *
> > + * Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by
> > + * shmem_mcopy_atomic_pte instead.
> > + */
> > +static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > +                                  struct vm_area_struct *dst_vma,
> > +                                  unsigned long dst_addr, struct page *page,
> > +                                  enum mcopy_atomic_mode mode, bool wp_copy)
> > +{
> > +     int ret;
> > +     pte_t _dst_pte, *dst_pte;
> > +     bool is_continue = mode == MCOPY_ATOMIC_CONTINUE;
> > +     int writable;
> > +     bool vm_shared = dst_vma->vm_flags & VM_SHARED;
> > +     bool is_file_backed = dst_vma->vm_file;
> > +     spinlock_t *ptl;
> > +     struct inode *inode;
> > +     pgoff_t offset, max_off;
> > +
> > +     _dst_pte = mk_pte(page, dst_vma->vm_page_prot);
> > +     writable = dst_vma->vm_flags & VM_WRITE;
> > +     /* For CONTINUE on a non-shared VMA, don't pte_mkwrite for CoW. */
> > +     if (is_continue && !vm_shared)
> > +             writable = 0;
>
> Indeed, there is nothing more important than keeping pte_write() off
> page cache mapped privately.
>
> > +
> > +     if (writable) {
> > +             _dst_pte = pte_mkdirty(_dst_pte);
> > +             if (wp_copy)
> > +                     _dst_pte = pte_mkuffd_wp(_dst_pte);
> > +             else
> > +                     _dst_pte = pte_mkwrite(_dst_pte);
> > +     } else if (vm_shared) {
> > +             /*
> > +              * Since we didn't pte_mkdirty(), mark the page dirty or it
> > +              * could be freed from under us. We could do this
> > +              * unconditionally, but doing it only if !writable is faster.
> > +              */
> > +             set_page_dirty(page);
>
> I do not remember why Andrea or I preferred set_page_dirty() here to
> pte_mkdirty(); but I suppose there might somewhere be a BUG_ON(pte_dirty)
> which this would avoid.  Risky to change it, though it does look odd.
>
> > +     }
> > +
> > +     dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> > +
> > +     if (is_file_backed) {
> > +             /* The shmem MAP_PRIVATE case requires checking the i_size */
> > +             inode = dst_vma->vm_file->f_inode;
> > +             offset = linear_page_index(dst_vma, dst_addr);
> > +             max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> > +             ret = -EFAULT;
> > +             if (unlikely(offset >= max_off))
> > +                     goto out_unlock;
> > +     }
> > +
> > +     ret = -EEXIST;
> > +     if (!pte_none(*dst_pte))
> > +             goto out_unlock;
> > +
> > +     inc_mm_counter(dst_mm, mm_counter(page));
> > +     if (is_file_backed)
> > +             page_add_file_rmap(page, false);
> > +     else
> > +             page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
> > +
> > +     if (!is_continue)
> > +             lru_cache_add_inactive_or_unevictable(page, dst_vma);
>
> I'm beginning to think that you took a wrong direction in v4 and v5,
> sending MINOR/CONTINUE this way instead of into shmem.c.  I haven't
> spotted any mistakes in this function, but it's not easy.
>
> If shmem_mcopy_atomic_pte() ended up using this mcopy_atomic_install_ptes()
> (and so mm/shmem.c contain no pte manipulation at all), that would be one
> kind of nice outcome.
>
> Or if shmem_mcopy_atomic_pte() handled all of the shmem page cache issues,
> and this just did the anonymous, that would be a different nice outcome.
>
> But here we have mcopy_atomic_install_ptes(), with a host of at-first-sight
> independent booleans (is_continue, is_file_backed, vm_shared, writable,
> wp_copy), doing too much yet not enough.
>
> I think it needs to shift in one direction or another: maybe revert
> back towards something more like v3, or maybe go further with
> mcopy_atomic_install_ptes().

Agreed about taking one direction or the other further.

I get the sense that Peter prefers the mcopy_atomic_install_ptes()
version, and would thus prefer to just expose that and let
shmem_mcopy_atomic_pte() use it.

But, I get the sense that you (Hugh) slightly prefer the other way -
just letting shmem_mcopy_atomic_pte() deal with both the VM_SHARED and
!VM_SHARED cases.

I was planning to write "I prefer option X because (reasons), and
objections?" but I'm realizing that it isn't really clear to me which
route would end up being cleaner. I think I have to just pick one,
write it out, and see where I end up. If it ends up gross, I don't
mind backtracking and taking the other route. :) To that end, I'll
proceed by having shmem_mcopy_atomic_pte() call the new
mcopy_atomic_install_ptes() helper, and see how it looks (unless there
are objections).


>
> > +
> > +     set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> > +
> > +     /* No need to invalidate - it was non-present before */
> > +     update_mmu_cache(dst_vma, dst_addr, dst_pte);
> > +     pte_unmap_unlock(dst_pte, ptl);
> > +     ret = 0;
>
>         ret =0;
> out_unlock:
>         pte_unmap_unlock(dst_pte, ptl);
>
> would avoid the goto out contortions at out_unlock below.
>
> > +out:
> > +     return ret;
> > +out_unlock:
> > +     pte_unmap_unlock(dst_pte, ptl);
> > +     goto out;
> > +}
> > +
> >  static int mcopy_atomic_pte(struct mm_struct *dst_mm,
> >                           pmd_t *dst_pmd,
> >                           struct vm_area_struct *dst_vma,
> >                           unsigned long dst_addr,
> >                           unsigned long src_addr,
> >                           struct page **pagep,
> > +                         enum mcopy_atomic_mode mode,
> >                           bool wp_copy)
> >  {
> > -     pte_t _dst_pte, *dst_pte;
> > -     spinlock_t *ptl;
> >       void *page_kaddr;
> >       int ret;
> >       struct page *page;
> > -     pgoff_t offset, max_off;
> > -     struct inode *inode;
> >
> >       if (!*pagep) {
> >               ret = -ENOMEM;
> > @@ -99,43 +181,12 @@ static int mcopy_atomic_pte(struct mm_st
> >       if (mem_cgroup_charge(page, dst_mm, GFP_KERNEL))
> >               goto out_release;
> >
> > -     _dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));
> > -     if (dst_vma->vm_flags & VM_WRITE) {
> > -             if (wp_copy)
> > -                     _dst_pte = pte_mkuffd_wp(_dst_pte);
> > -             else
> > -                     _dst_pte = pte_mkwrite(_dst_pte);
> > -     }
> > -
> > -     dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
> > -     if (dst_vma->vm_file) {
> > -             /* the shmem MAP_PRIVATE case requires checking the i_size */
> > -             inode = dst_vma->vm_file->f_inode;
> > -             offset = linear_page_index(dst_vma, dst_addr);
> > -             max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> > -             ret = -EFAULT;
> > -             if (unlikely(offset >= max_off))
> > -                     goto out_release_uncharge_unlock;
> > -     }
> > -     ret = -EEXIST;
> > -     if (!pte_none(*dst_pte))
> > -             goto out_release_uncharge_unlock;
> > -
> > -     inc_mm_counter(dst_mm, MM_ANONPAGES);
> > -     page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
> > -     lru_cache_add_inactive_or_unevictable(page, dst_vma);
> > -
> > -     set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> > -
> > -     /* No need to invalidate - it was non-present before */
> > -     update_mmu_cache(dst_vma, dst_addr, dst_pte);
> > -
> > -     pte_unmap_unlock(dst_pte, ptl);
> > -     ret = 0;
> > +     ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,
> > +                                     page, mode, wp_copy);
> > +     if (ret)
> > +             goto out_release;
> >  out:
> >       return ret;
> > -out_release_uncharge_unlock:
> > -     pte_unmap_unlock(dst_pte, ptl);
> >  out_release:
> >       put_page(page);
> >       goto out;
> > @@ -176,6 +227,38 @@ out_unlock:
> >       return ret;
> >  }
> >
> > +static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
> > +                             pmd_t *dst_pmd,
> > +                             struct vm_area_struct *dst_vma,
> > +                             unsigned long dst_addr,
> > +                             bool wp_copy)
> > +{
> > +     struct inode *inode = file_inode(dst_vma->vm_file);
> > +     struct address_space *mapping = inode->i_mapping;
> > +     pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
> > +     struct page *page;
> > +     int ret;
> > +
> > +     ret = -EFAULT;
> > +     page = find_lock_page(mapping, pgoff);
> > +     if (!page)
> > +             goto out;
>
> No. That looks nice and generic, but IIUC only shmem gets to come this way,
> and find_lock_page() is not allowing for swap. Agreed that it will be
> unlikely for the page (vetted by userspace before ioctl'ing to here) to
> have gotten swapped out meanwhile, but we must allow for that unlikelihood.
>
> You will need to use shmem_getpage(SGP_CACHE) instead: NULL vma so you
> don't get trapped into endless userfaultfd_minor() breakouts.  (If someone
> else punched a hole in the file meanwhile, shmem_getpage() will allocate
> a new page you don't particularly want: but that is not a case we need to
> care about, beyond getting the accounting right and not crashing.)
>
> > +
> > +     ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,
> > +                                     page, MCOPY_ATOMIC_CONTINUE, wp_copy);
> > +     if (ret)
> > +             goto out_release;
> > +
> > +     unlock_page(page);
> > +     ret = 0;
> > +out:
> > +     return ret;
> > +out_release:
> > +     unlock_page(page);
> > +     put_page(page);
> > +     goto out;
> > +}
> > +
> >  static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
> >  {
> >       pgd_t *pgd;
> > @@ -415,10 +498,16 @@ static __always_inline ssize_t mfill_ato
> >                                               unsigned long dst_addr,
> >                                               unsigned long src_addr,
> >                                               struct page **page,
> > -                                             bool zeropage,
> > +                                             enum mcopy_atomic_mode mode,
> >                                               bool wp_copy)
> >  {
> > -     ssize_t err;
> > +     ssize_t err = 0;
> > +
> > +     if (mode == MCOPY_ATOMIC_CONTINUE) {
> > +             err = mcontinue_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> > +                                        wp_copy);
> > +             goto out;
> > +     }
> >
> >       /*
> >        * The normal page fault path for a shmem will invoke the
> > @@ -431,24 +520,20 @@ static __always_inline ssize_t mfill_ato
> >        * and not in the radix tree.
> >        */
> >       if (!(dst_vma->vm_flags & VM_SHARED)) {
> > -             if (!zeropage)
> > +             if (mode == MCOPY_ATOMIC_NORMAL)
> >                       err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
> >                                              dst_addr, src_addr, page,
> > -                                            wp_copy);
> > -             else
> > +                                            mode, wp_copy);
> > +             else if (mode == MCOPY_ATOMIC_ZEROPAGE)
> >                       err = mfill_zeropage_pte(dst_mm, dst_pmd,
> >                                                dst_vma, dst_addr);
> >       } else {
> >               VM_WARN_ON_ONCE(wp_copy);
> > -             if (!zeropage)
> > -                     err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
> > -                                                  dst_vma, dst_addr,
> > -                                                  src_addr, page);
> > -             else
> > -                     err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
> > -                                                    dst_vma, dst_addr);
> > +             err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> > +                                          src_addr, mode, page);
> >       }
> >
> > +out:
> >       return err;
> >  }
> >
> > @@ -467,7 +552,6 @@ static __always_inline ssize_t __mcopy_a
> >       long copied;
> >       struct page *page;
> >       bool wp_copy;
> > -     bool zeropage = (mcopy_mode == MCOPY_ATOMIC_ZEROPAGE);
> >
> >       /*
> >        * Sanitize the command parameters:
> > @@ -530,7 +614,7 @@ retry:
> >
> >       if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
> >               goto out_unlock;
> > -     if (mcopy_mode == MCOPY_ATOMIC_CONTINUE)
> > +     if (!vma_is_shmem(dst_vma) && mcopy_mode == MCOPY_ATOMIC_CONTINUE)
> >               goto out_unlock;
> >
> >       /*
> > @@ -578,7 +662,7 @@ retry:
> >               BUG_ON(pmd_trans_huge(*dst_pmd));
> >
> >               err = mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> > -                                    src_addr, &page, zeropage, wp_copy);
> > +                                    src_addr, &page, mcopy_mode, wp_copy);
> >               cond_resched();
> >
> >               if (unlikely(err == -ENOENT)) {
>
> I didn't pay a great deal of attention to the remainder above: I think it's
> right, but it would have been easier to review if mcopy_mode and refactoring
> had been introduced in a preliminary patch, before going on to implement
> MINOR/CONTINUE on shmem.
>
> Hugh

^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
  2021-04-07 20:33   ` Axel Rasmussen
@ 2021-04-07 22:50     ` Hugh Dickins
  0 siblings, 0 replies; 7+ messages in thread
From: Hugh Dickins @ 2021-04-07 22:50 UTC (permalink / raw)
  To: Axel Rasmussen
  Cc: Hugh Dickins, Alexander Viro, Andrea Arcangeli, Andrew Morton,
	Jerome Glisse, Joe Perches, Lokesh Gidra, Mike Rapoport,
	Peter Xu, Shaohua Li, Shuah Khan, Stephen Rothwell, Wang Qing,
	LKML, linux-fsdevel, Linux MM, linux-kselftest, Brian Geffon,
	Cannon Matthews, Dr . David Alan Gilbert, David Rientjes,
	Michel Lespinasse, Mina Almasry, Oliver Upton

On Wed, 7 Apr 2021, Axel Rasmussen wrote:
> Agreed about taking one direction or the other further.
> 
> I get the sense that Peter prefers the mcopy_atomic_install_ptes()
> version, and would thus prefer to just expose that and let
> shmem_mcopy_atomic_pte() use it.
> 
> But, I get the sense that you (Hugh) slightly prefer the other way -
> just letting shmem_mcopy_atomic_pte() deal with both the VM_SHARED and
> !VM_SHARED cases.

No, either direction seems plausible to me: start from whichever
end you prefer.

> 
> I was planning to write "I prefer option X because (reasons), and
> objections?" but I'm realizing that it isn't really clear to me which
> route would end up being cleaner. I think I have to just pick one,
> write it out, and see where I end up. If it ends up gross, I don't
> mind backtracking and taking the other route. :) To that end, I'll
> proceed by having shmem_mcopy_atomic_pte() call the new
> mcopy_atomic_install_ptes() helper, and see how it looks (unless there
> are objections).

I am pleased to read that: it's exactly how I would approach it -
so it must be right :-)

Hugh

^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
  2021-04-07  6:14 ` Hugh Dickins
  2021-04-07 20:33   ` Axel Rasmussen
@ 2021-04-12 21:54   ` Peter Xu
  2021-04-13  0:51     ` Hugh Dickins
  1 sibling, 1 reply; 7+ messages in thread
From: Peter Xu @ 2021-04-12 21:54 UTC (permalink / raw)
  To: Hugh Dickins
  Cc: Axel Rasmussen, Alexander Viro, Andrea Arcangeli, Andrew Morton,
	Jerome Glisse, Joe Perches, Lokesh Gidra, Mike Rapoport,
	Shaohua Li, Shuah Khan, Stephen Rothwell, Wang Qing,
	linux-kernel, linux-fsdevel, linux-mm, linux-kselftest,
	Brian Geffon, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Michel Lespinasse, Mina Almasry, Oliver Upton

Hi, Hugh,

On Tue, Apr 06, 2021 at 11:14:30PM -0700, Hugh Dickins wrote:
> > +static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > +				     struct vm_area_struct *dst_vma,
> > +				     unsigned long dst_addr, struct page *page,
> > +				     enum mcopy_atomic_mode mode, bool wp_copy)
> > +{

[...]

> > +	if (writable) {
> > +		_dst_pte = pte_mkdirty(_dst_pte);
> > +		if (wp_copy)
> > +			_dst_pte = pte_mkuffd_wp(_dst_pte);
> > +		else
> > +			_dst_pte = pte_mkwrite(_dst_pte);
> > +	} else if (vm_shared) {
> > +		/*
> > +		 * Since we didn't pte_mkdirty(), mark the page dirty or it
> > +		 * could be freed from under us. We could do this
> > +		 * unconditionally, but doing it only if !writable is faster.
> > +		 */
> > +		set_page_dirty(page);
> 
> I do not remember why Andrea or I preferred set_page_dirty() here to
> pte_mkdirty(); but I suppose there might somewhere be a BUG_ON(pte_dirty)
> which this would avoid.  Risky to change it, though it does look odd.

Is any of the possible BUG_ON(pte_dirty) going to trigger because the pte has
write bit cleared?  That's one question I was not very sure, e.g., whether one
pte is allowed to be "dirty" if it's not writable.

To me it's okay, it's actually very suitable for UFFDIO_COPY case, where it is
definitely dirty data (so we must never drop it) even if it's installed as RO,
however to achieve that we can still set the dirty on the page rather than the
pte as what we do here.  It's just a bit awkward as you said.

Meanwhile today I just noticed this in arm64 code:

static inline pte_t pte_wrprotect(pte_t pte)
{
	/*
	 * If hardware-dirty (PTE_WRITE/DBM bit set and PTE_RDONLY
	 * clear), set the PTE_DIRTY bit.
	 */
	if (pte_hw_dirty(pte))
		pte = pte_mkdirty(pte);

	pte = clear_pte_bit(pte, __pgprot(PTE_WRITE));
	pte = set_pte_bit(pte, __pgprot(PTE_RDONLY));
	return pte;
}

So arm64 will explicitly set the dirty bit (from the HW dirty bit) when
wr-protect.  It seems to prove that at least for arm64 it's very valid to have
!write && dirty pte.

Thanks,

-- 
Peter Xu


^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
  2021-04-12 21:54   ` Peter Xu
@ 2021-04-13  0:51     ` Hugh Dickins
  2021-04-13  1:21       ` Peter Xu
  0 siblings, 1 reply; 7+ messages in thread
From: Hugh Dickins @ 2021-04-13  0:51 UTC (permalink / raw)
  To: Peter Xu
  Cc: Hugh Dickins, Axel Rasmussen, Alexander Viro, Andrea Arcangeli,
	Andrew Morton, Jerome Glisse, Joe Perches, Lokesh Gidra,
	Mike Rapoport, Shaohua Li, Shuah Khan, Stephen Rothwell,
	Wang Qing, linux-kernel, linux-fsdevel, linux-mm,
	linux-kselftest, Brian Geffon, Cannon Matthews,
	Dr . David Alan Gilbert, David Rientjes, Michel Lespinasse,
	Mina Almasry, Oliver Upton

On Mon, 12 Apr 2021, Peter Xu wrote:
> On Tue, Apr 06, 2021 at 11:14:30PM -0700, Hugh Dickins wrote:
> > > +static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > > +				     struct vm_area_struct *dst_vma,
> > > +				     unsigned long dst_addr, struct page *page,
> > > +				     enum mcopy_atomic_mode mode, bool wp_copy)
> > > +{
> 
> [...]
> 
> > > +	if (writable) {
> > > +		_dst_pte = pte_mkdirty(_dst_pte);
> > > +		if (wp_copy)
> > > +			_dst_pte = pte_mkuffd_wp(_dst_pte);
> > > +		else
> > > +			_dst_pte = pte_mkwrite(_dst_pte);
> > > +	} else if (vm_shared) {
> > > +		/*
> > > +		 * Since we didn't pte_mkdirty(), mark the page dirty or it
> > > +		 * could be freed from under us. We could do this
> > > +		 * unconditionally, but doing it only if !writable is faster.
> > > +		 */
> > > +		set_page_dirty(page);
> > 
> > I do not remember why Andrea or I preferred set_page_dirty() here to
> > pte_mkdirty(); but I suppose there might somewhere be a BUG_ON(pte_dirty)
> > which this would avoid.  Risky to change it, though it does look odd.
> 
> Is any of the possible BUG_ON(pte_dirty) going to trigger because the pte has
> write bit cleared?  That's one question I was not very sure, e.g., whether one
> pte is allowed to be "dirty" if it's not writable.
> 
> To me it's okay, it's actually very suitable for UFFDIO_COPY case, where it is
> definitely dirty data (so we must never drop it) even if it's installed as RO,
> however to achieve that we can still set the dirty on the page rather than the
> pte as what we do here.  It's just a bit awkward as you said.
> 
> Meanwhile today I just noticed this in arm64 code:
> 
> static inline pte_t pte_wrprotect(pte_t pte)
> {
> 	/*
> 	 * If hardware-dirty (PTE_WRITE/DBM bit set and PTE_RDONLY
> 	 * clear), set the PTE_DIRTY bit.
> 	 */
> 	if (pte_hw_dirty(pte))
> 		pte = pte_mkdirty(pte);
> 
> 	pte = clear_pte_bit(pte, __pgprot(PTE_WRITE));
> 	pte = set_pte_bit(pte, __pgprot(PTE_RDONLY));
> 	return pte;
> }
> 
> So arm64 will explicitly set the dirty bit (from the HW dirty bit) when
> wr-protect.  It seems to prove that at least for arm64 it's very valid to have
> !write && dirty pte.

I did not mean to imply that it's wrong to have pte_dirty without
pte_write: no, I agree with you, I believe that there are accepted
and generic ways in which we can have pte_dirty without pte_write
(and we could each probably add a warning somewhere which would
very quickly prove that - but those would not prove that there
are not BUG_ONs on some other path, which had been my fear).

I wanted now to demonstrate that by pointing to change_pte_range() in
mm/mprotect.c, showing that it does not clear pte_dirty when it clears
pte_write. But alarmingly found rather the reverse: that it appears to
set pte_write when it finds pte_dirty - if dirty_accountable.

That looks very wrong, but if I spent long enough following up
dirty_accountable in detail, I think I would be reassured to find that
it is only adding the pte_write there when it had removed it from the
prot passed down, for dirty accounting reasons (which apply !VM_SHARED
protections in the VM_SHARED case, so that page_mkwrite() is called
and dirty accounting done when necessary).

What I did mean to imply is that changing set_page_dirty to pte_mkdirty,
to make that userfaultfd code block look nicer, is not a change to be
done lightly: by all means try it out, test it, and send a patch after
Axel's series is in, but please do not ask Axel to make that change as
a part of his series - it would be taking a risk, just for a cleanup.

Now, I have also looked up the mail exchange with Andrea which led to
his dcf7fe9d8976 ("userfaultfd: shmem: UFFDIO_COPY: set the page dirty
if VM_WRITE is not set") - it had to be off-list at the time.  And he
was rather led to that set_page_dirty by following old patterns left
over in shmem_getpage_gfp(); but when I said "or it could be done with
pte_mkdirty without pte_mkwrite", he answered "I explicitly avoided
that because pte_dirty then has side effects on mprotect to decide
pte_write. It looks safer to do set_page_dirty and not set dirty bits
in not writable ptes unnecessarily".

Haha: I think Andrea is referring to exactly the dirty_accountable code
in change_pte_protection() which worried me above. Now, I think that
will turn out okay (shmem does not have a page_mkwrite(), and does not
participate in dirty accounting), but you will have to do some work to
assure us all of that, before sending in a cleanup patch.

Hugh

^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior
  2021-04-13  0:51     ` Hugh Dickins
@ 2021-04-13  1:21       ` Peter Xu
  0 siblings, 0 replies; 7+ messages in thread
From: Peter Xu @ 2021-04-13  1:21 UTC (permalink / raw)
  To: Hugh Dickins
  Cc: Axel Rasmussen, Alexander Viro, Andrea Arcangeli, Andrew Morton,
	Jerome Glisse, Joe Perches, Lokesh Gidra, Mike Rapoport,
	Shaohua Li, Shuah Khan, Stephen Rothwell, Wang Qing,
	linux-kernel, linux-fsdevel, linux-mm, linux-kselftest,
	Brian Geffon, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Michel Lespinasse, Mina Almasry, Oliver Upton

On Mon, Apr 12, 2021 at 05:51:14PM -0700, Hugh Dickins wrote:
> On Mon, 12 Apr 2021, Peter Xu wrote:
> > On Tue, Apr 06, 2021 at 11:14:30PM -0700, Hugh Dickins wrote:
> > > > +static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> > > > +				     struct vm_area_struct *dst_vma,
> > > > +				     unsigned long dst_addr, struct page *page,
> > > > +				     enum mcopy_atomic_mode mode, bool wp_copy)
> > > > +{
> > 
> > [...]
> > 
> > > > +	if (writable) {
> > > > +		_dst_pte = pte_mkdirty(_dst_pte);
> > > > +		if (wp_copy)
> > > > +			_dst_pte = pte_mkuffd_wp(_dst_pte);
> > > > +		else
> > > > +			_dst_pte = pte_mkwrite(_dst_pte);
> > > > +	} else if (vm_shared) {
> > > > +		/*
> > > > +		 * Since we didn't pte_mkdirty(), mark the page dirty or it
> > > > +		 * could be freed from under us. We could do this
> > > > +		 * unconditionally, but doing it only if !writable is faster.
> > > > +		 */
> > > > +		set_page_dirty(page);
> > > 
> > > I do not remember why Andrea or I preferred set_page_dirty() here to
> > > pte_mkdirty(); but I suppose there might somewhere be a BUG_ON(pte_dirty)
> > > which this would avoid.  Risky to change it, though it does look odd.
> > 
> > Is any of the possible BUG_ON(pte_dirty) going to trigger because the pte has
> > write bit cleared?  That's one question I was not very sure, e.g., whether one
> > pte is allowed to be "dirty" if it's not writable.
> > 
> > To me it's okay, it's actually very suitable for UFFDIO_COPY case, where it is
> > definitely dirty data (so we must never drop it) even if it's installed as RO,
> > however to achieve that we can still set the dirty on the page rather than the
> > pte as what we do here.  It's just a bit awkward as you said.
> > 
> > Meanwhile today I just noticed this in arm64 code:
> > 
> > static inline pte_t pte_wrprotect(pte_t pte)
> > {
> > 	/*
> > 	 * If hardware-dirty (PTE_WRITE/DBM bit set and PTE_RDONLY
> > 	 * clear), set the PTE_DIRTY bit.
> > 	 */
> > 	if (pte_hw_dirty(pte))
> > 		pte = pte_mkdirty(pte);
> > 
> > 	pte = clear_pte_bit(pte, __pgprot(PTE_WRITE));
> > 	pte = set_pte_bit(pte, __pgprot(PTE_RDONLY));
> > 	return pte;
> > }
> > 
> > So arm64 will explicitly set the dirty bit (from the HW dirty bit) when
> > wr-protect.  It seems to prove that at least for arm64 it's very valid to have
> > !write && dirty pte.
> 
> I did not mean to imply that it's wrong to have pte_dirty without
> pte_write: no, I agree with you, I believe that there are accepted
> and generic ways in which we can have pte_dirty without pte_write
> (and we could each probably add a warning somewhere which would
> very quickly prove that - but those would not prove that there
> are not BUG_ONs on some other path, which had been my fear).
> 
> I wanted now to demonstrate that by pointing to change_pte_range() in
> mm/mprotect.c, showing that it does not clear pte_dirty when it clears
> pte_write. But alarmingly found rather the reverse: that it appears to
> set pte_write when it finds pte_dirty - if dirty_accountable.
> 
> That looks very wrong, but if I spent long enough following up
> dirty_accountable in detail, I think I would be reassured to find that
> it is only adding the pte_write there when it had removed it from the
> prot passed down, for dirty accounting reasons (which apply !VM_SHARED
> protections in the VM_SHARED case, so that page_mkwrite() is called
> and dirty accounting done when necessary).
> 
> What I did mean to imply is that changing set_page_dirty to pte_mkdirty,
> to make that userfaultfd code block look nicer, is not a change to be
> done lightly: by all means try it out, test it, and send a patch after
> Axel's series is in, but please do not ask Axel to make that change as
> a part of his series - it would be taking a risk, just for a cleanup.

Fair enough.  Sorry if I wasn't clear when asking, the reason to ask was that I
wanted to be clear on these differences.  For example, in my uffd-wp shmem
series I'll have to make sure dirty bit always there, I used pte_mkdirty()
unconditionally but I wanted to make sure I'm not overlooking anything..

Though this case is slightly special here, since if without the cleaning up the
logic will definitely be harder to review (with the cleanup, it'll be as simple
as one unconditional pte_mkdirty() and that's all).  However you're definitely
right, it's not a reason to overload Axel with this patchset, especially if
such a cleanup is seen to be risky.

> 
> Now, I have also looked up the mail exchange with Andrea which led to
> his dcf7fe9d8976 ("userfaultfd: shmem: UFFDIO_COPY: set the page dirty
> if VM_WRITE is not set") - it had to be off-list at the time.  And he
> was rather led to that set_page_dirty by following old patterns left
> over in shmem_getpage_gfp(); but when I said "or it could be done with
> pte_mkdirty without pte_mkwrite", he answered "I explicitly avoided
> that because pte_dirty then has side effects on mprotect to decide
> pte_write. It looks safer to do set_page_dirty and not set dirty bits
> in not writable ptes unnecessarily".
> 
> Haha: I think Andrea is referring to exactly the dirty_accountable code
> in change_pte_protection() which worried me above. Now, I think that
> will turn out okay (shmem does not have a page_mkwrite(), and does not
> participate in dirty accounting), but you will have to do some work to
> assure us all of that, before sending in a cleanup patch.

OK. Maybe I can clean this after Axel's work.

Thanks a lot for all these details, Hugh!

-- 
Peter Xu


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Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
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2021-04-01 18:37 [PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior Axel Rasmussen
2021-04-07  6:14 ` Hugh Dickins
2021-04-07 20:33   ` Axel Rasmussen
2021-04-07 22:50     ` Hugh Dickins
2021-04-12 21:54   ` Peter Xu
2021-04-13  0:51     ` Hugh Dickins
2021-04-13  1:21       ` Peter Xu

Linux-kselftest Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-kselftest/0 linux-kselftest/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-kselftest linux-kselftest/ https://lore.kernel.org/linux-kselftest \
		linux-kselftest@vger.kernel.org
	public-inbox-index linux-kselftest

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kselftest


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git