[PATCH v3 0/4] page table check fixes and cleanups

[PATCH v3 0/4] page table check fixes and cleanups

[Pasha Tatashin]

From: Pasha Tatashin <tatashin@google.com>

Changelog:
v3:	- Resolved a regression introduced in previous version, where
	  page collapse in khugepaged would cause crash on boot.
	- Addressed comments from Anshuman Khandual regarding commit
	  log.
v2:	- Addressed simplification comments from Wei Xu
	- Added Review-by/Tested-by's from Zi Yan and Wei Xu


Two fixes:

  mm/debug_vm_pgtable: remove pte entry from the page table
	- remove a pte entry from the page table at the end of
	  debug_vm_pgtable pte test

  mm/khugepaged: unify collapse pmd clear, flush and free
  mm/page_table_check: check entries at pmd levels
	- check pmd level in page_table_check for PTE regular entries
	  prior to freeing.
	  repro.c: https://gist.github.com/soleen/fdcd501d5df103976245fe84e9535087
	  config: https://gist.github.com/soleen/8a56f923c2fea9ce9c75b4e2517d4162
	  qemu_script: https://gist.github.com/soleen/f4be4795826b7ab1a51ae659582e179c
	  base image:
	  https://storage.googleapis.com/syzkaller/wheezy.img
	  https://storage.googleapis.com/syzkaller/wheezy.img.key

Small cleanup:
  mm/page_table_check: use unsigned long for page counters and cleanup

Pasha Tatashin (4):
  mm/debug_vm_pgtable: remove pte entry from the page table
  mm/page_table_check: use unsigned long for page counters and cleanup
  mm/khugepaged: unify collapse pmd clear, flush and free
  mm/page_table_check: check entries at pmd levels

 include/linux/page_table_check.h | 18 ++++++++++
 mm/debug_vm_pgtable.c            |  2 ++
 mm/khugepaged.c                  | 34 ++++++++++---------
 mm/page_table_check.c            | 56 ++++++++++++++++----------------
 4 files changed, 66 insertions(+), 44 deletions(-)

-- 
2.35.0.rc0.227.g00780c9af4-goog

[PATCH v3 1/4] mm/debug_vm_pgtable: remove pte entry from the page table

[Pasha Tatashin]

The pte entry that is used in pte_advanced_tests() is never removed from
the page table at the end of the test.

The issue is detected by page_table_check, to repro compile kernel with
the following configs:

CONFIG_DEBUG_VM_PGTABLE=y
CONFIG_PAGE_TABLE_CHECK=y
CONFIG_PAGE_TABLE_CHECK_ENFORCED=y

During the boot the following BUG is printed:

[    2.262821] debug_vm_pgtable: [debug_vm_pgtable         ]: Validating
               architecture page table helpers
[    2.276826] ------------[ cut here ]------------
[    2.280426] kernel BUG at mm/page_table_check.c:162!
[    2.284118] invalid opcode: 0000 [#1] PREEMPT SMP PTI
[    2.287787] CPU: 0 PID: 1 Comm: swapper/0 Not tainted
               5.16.0-11413-g2c271fe77d52 #3
[    2.293226] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
               BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org
               04/01/2014
...

The entry should be properly removed from the page table before the page
is released to the free list.

Fixes: a5c3b9ffb0f4 ("mm/debug_vm_pgtable: add tests validating advanced arch page table helpers")

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Tested-by: Zi Yan <ziy@nvidia.com>
---
 mm/debug_vm_pgtable.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index a7ac97c76762..db2abd9e415b 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c
@@ -171,6 +171,8 @@ static void __init pte_advanced_tests(struct pgtable_debug_args *args)
 	ptep_test_and_clear_young(args->vma, args->vaddr, args->ptep);
 	pte = ptep_get(args->ptep);
 	WARN_ON(pte_young(pte));
+
+	ptep_get_and_clear_full(args->mm, args->vaddr, args->ptep, 1);
 }
 
 static void __init pte_savedwrite_tests(struct pgtable_debug_args *args)
-- 
2.35.0.rc0.227.g00780c9af4-goog

[PATCH v3 2/4] mm/page_table_check: use unsigned long for page counters and cleanup

[Pasha Tatashin]

For the consistency, use "unsigned long" for all page counters.

Also, reduce code duplication by calling
__page_table_check_*_clear() from __page_table_check_*_set() functions.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Wei Xu <weixugc@google.com>
---
 mm/page_table_check.c | 35 +++++++----------------------------
 1 file changed, 7 insertions(+), 28 deletions(-)

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 7504e7caa2a1..c61d7ebe13b1 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -86,8 +86,8 @@ static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
 {
 	struct page_ext *page_ext;
 	struct page *page;
+	unsigned long i;
 	bool anon;
-	int i;
 
 	if (!pfn_valid(pfn))
 		return;
@@ -121,8 +121,8 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
 {
 	struct page_ext *page_ext;
 	struct page *page;
+	unsigned long i;
 	bool anon;
-	int i;
 
 	if (!pfn_valid(pfn))
 		return;
@@ -152,10 +152,10 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
 void __page_table_check_zero(struct page *page, unsigned int order)
 {
 	struct page_ext *page_ext = lookup_page_ext(page);
-	int i;
+	unsigned long i;
 
 	BUG_ON(!page_ext);
-	for (i = 0; i < (1 << order); i++) {
+	for (i = 0; i < (1ul << order); i++) {
 		struct page_table_check *ptc = get_page_table_check(page_ext);
 
 		BUG_ON(atomic_read(&ptc->anon_map_count));
@@ -206,17 +206,10 @@ EXPORT_SYMBOL(__page_table_check_pud_clear);
 void __page_table_check_pte_set(struct mm_struct *mm, unsigned long addr,
 				pte_t *ptep, pte_t pte)
 {
-	pte_t old_pte;
-
 	if (&init_mm == mm)
 		return;
 
-	old_pte = *ptep;
-	if (pte_user_accessible_page(old_pte)) {
-		page_table_check_clear(mm, addr, pte_pfn(old_pte),
-				       PAGE_SIZE >> PAGE_SHIFT);
-	}
-
+	__page_table_check_pte_clear(mm, addr, *ptep);
 	if (pte_user_accessible_page(pte)) {
 		page_table_check_set(mm, addr, pte_pfn(pte),
 				     PAGE_SIZE >> PAGE_SHIFT,
@@ -228,17 +221,10 @@ EXPORT_SYMBOL(__page_table_check_pte_set);
 void __page_table_check_pmd_set(struct mm_struct *mm, unsigned long addr,
 				pmd_t *pmdp, pmd_t pmd)
 {
-	pmd_t old_pmd;
-
 	if (&init_mm == mm)
 		return;
 
-	old_pmd = *pmdp;
-	if (pmd_user_accessible_page(old_pmd)) {
-		page_table_check_clear(mm, addr, pmd_pfn(old_pmd),
-				       PMD_PAGE_SIZE >> PAGE_SHIFT);
-	}
-
+	__page_table_check_pmd_clear(mm, addr, *pmdp);
 	if (pmd_user_accessible_page(pmd)) {
 		page_table_check_set(mm, addr, pmd_pfn(pmd),
 				     PMD_PAGE_SIZE >> PAGE_SHIFT,
@@ -250,17 +236,10 @@ EXPORT_SYMBOL(__page_table_check_pmd_set);
 void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
 				pud_t *pudp, pud_t pud)
 {
-	pud_t old_pud;
-
 	if (&init_mm == mm)
 		return;
 
-	old_pud = *pudp;
-	if (pud_user_accessible_page(old_pud)) {
-		page_table_check_clear(mm, addr, pud_pfn(old_pud),
-				       PUD_PAGE_SIZE >> PAGE_SHIFT);
-	}
-
+	__page_table_check_pud_clear(mm, addr, *pudp);
 	if (pud_user_accessible_page(pud)) {
 		page_table_check_set(mm, addr, pud_pfn(pud),
 				     PUD_PAGE_SIZE >> PAGE_SHIFT,
-- 
2.35.0.rc0.227.g00780c9af4-goog

[PATCH v3 3/4] mm/khugepaged: unify collapse pmd clear, flush and free

[Pasha Tatashin]

Unify the code that flushes, clears pmd entry, and frees the PTE table
level into a new function collapse_and_free_pmd().

This clean-up is useful as in the next patch we will add another call to
this function to iterate through PTE prior to freeing the level for page
table check.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 mm/khugepaged.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 35f14d0a00a6..440112355ffe 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1416,6 +1416,17 @@ static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
 	return 0;
 }
 
+static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
+				  unsigned long addr, pmd_t *pmdp)
+{
+	spinlock_t *ptl = pmd_lock(vma->vm_mm, pmdp);
+	pmd_t pmd = pmdp_collapse_flush(vma, addr, pmdp);
+
+	spin_unlock(ptl);
+	mm_dec_nr_ptes(mm);
+	pte_free(mm, pmd_pgtable(pmd));
+}
+
 /**
  * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at
  * address haddr.
@@ -1433,7 +1444,7 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
 	struct vm_area_struct *vma = find_vma(mm, haddr);
 	struct page *hpage;
 	pte_t *start_pte, *pte;
-	pmd_t *pmd, _pmd;
+	pmd_t *pmd;
 	spinlock_t *ptl;
 	int count = 0;
 	int i;
@@ -1509,12 +1520,7 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
 	}
 
 	/* step 4: collapse pmd */
-	ptl = pmd_lock(vma->vm_mm, pmd);
-	_pmd = pmdp_collapse_flush(vma, haddr, pmd);
-	spin_unlock(ptl);
-	mm_dec_nr_ptes(mm);
-	pte_free(mm, pmd_pgtable(_pmd));
-
+	collapse_and_free_pmd(mm, vma, haddr, pmd);
 drop_hpage:
 	unlock_page(hpage);
 	put_page(hpage);
@@ -1552,7 +1558,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 	struct vm_area_struct *vma;
 	struct mm_struct *mm;
 	unsigned long addr;
-	pmd_t *pmd, _pmd;
+	pmd_t *pmd;
 
 	i_mmap_lock_write(mapping);
 	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
@@ -1591,14 +1597,8 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 		 * reverse order. Trylock is a way to avoid deadlock.
 		 */
 		if (mmap_write_trylock(mm)) {
-			if (!khugepaged_test_exit(mm)) {
-				spinlock_t *ptl = pmd_lock(mm, pmd);
-				/* assume page table is clear */
-				_pmd = pmdp_collapse_flush(vma, addr, pmd);
-				spin_unlock(ptl);
-				mm_dec_nr_ptes(mm);
-				pte_free(mm, pmd_pgtable(_pmd));
-			}
+			if (!khugepaged_test_exit(mm))
+				collapse_and_free_pmd(mm, vma, addr, pmd);
 			mmap_write_unlock(mm);
 		} else {
 			/* Try again later */
-- 
2.35.0.rc0.227.g00780c9af4-goog

[PATCH v3 4/4] mm/page_table_check: check entries at pmd levels

[Pasha Tatashin]

syzbot detected a case where the page table counters were not properly
updated.

syzkaller login:  ------------[ cut here ]------------
kernel BUG at mm/page_table_check.c:162!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 3099 Comm: pasha Not tainted 5.16.0+ #48
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIO4
RIP: 0010:__page_table_check_zero+0x159/0x1a0
Code: 7d 3a b2 ff 45 39 f5 74 2a e8 43 38 b2 ff 4d 85 e4 01
RSP: 0018:ffff888010667418 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000
RDX: ffff88800cea8680 RSI: ffffffff81becaf9 RDI: 0000000003
RBP: ffff888010667450 R08: 0000000000000001 R09: 0000000000
R10: ffffffff81becaab R11: 0000000000000001 R12: ffff888008
R13: 0000000000000001 R14: 0000000000000200 R15: dffffc0000
FS:  0000000000000000(0000) GS:ffff888035e00000(0000) knlG0
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffd875cad00 CR3: 00000000094ce000 CR4: 0000000000
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000
Call Trace:
 <TASK>
 free_pcp_prepare+0x3be/0xaa0
 free_unref_page+0x1c/0x650
 ? trace_hardirqs_on+0x6a/0x1d0
 free_compound_page+0xec/0x130
 free_transhuge_page+0x1be/0x260
 __put_compound_page+0x90/0xd0
 release_pages+0x54c/0x1060
 ? filemap_remove_folio+0x161/0x210
 ? lock_downgrade+0x720/0x720
 ? __put_page+0x150/0x150
 ? filemap_free_folio+0x164/0x350
 __pagevec_release+0x7c/0x110
 shmem_undo_range+0x85e/0x1250
...

The repro involved having a huge page that is split due to uprobe event
temporarily replacing one of the pages in the huge page. Later the huge
page was combined again, but the counters were off, as the PTE level
was not properly updated.

Make sure that when PMD is cleared and prior to freeing the level the
PTEs are updated.

Fixes: df4e817b7108 ("mm: page table check")

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/page_table_check.h | 18 ++++++++++++++++++
 mm/khugepaged.c                  |  2 ++
 mm/page_table_check.c            | 21 +++++++++++++++++++++
 3 files changed, 41 insertions(+)

diff --git a/include/linux/page_table_check.h b/include/linux/page_table_check.h
index 38cace1da7b6..e88bbe37727b 100644
--- a/include/linux/page_table_check.h
+++ b/include/linux/page_table_check.h
@@ -26,6 +26,8 @@ void __page_table_check_pmd_set(struct mm_struct *mm, unsigned long addr,
 				pmd_t *pmdp, pmd_t pmd);
 void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
 				pud_t *pudp, pud_t pud);
+void __page_table_check_pmd_clear_full(struct mm_struct *mm, unsigned long addr,
+				       pmd_t pmd);
 
 static inline void page_table_check_alloc(struct page *page, unsigned int order)
 {
@@ -100,6 +102,16 @@ static inline void page_table_check_pud_set(struct mm_struct *mm,
 	__page_table_check_pud_set(mm, addr, pudp, pud);
 }
 
+static inline void page_table_check_pmd_clear_full(struct mm_struct *mm,
+						   unsigned long addr,
+						   pmd_t pmd)
+{
+	if (static_branch_likely(&page_table_check_disabled))
+		return;
+
+	__page_table_check_pmd_clear_full(mm, addr, pmd);
+}
+
 #else
 
 static inline void page_table_check_alloc(struct page *page, unsigned int order)
@@ -143,5 +155,11 @@ static inline void page_table_check_pud_set(struct mm_struct *mm,
 {
 }
 
+static inline void page_table_check_pmd_clear_full(struct mm_struct *mm,
+						   unsigned long addr,
+						   pmd_t pmd)
+{
+}
+
 #endif /* CONFIG_PAGE_TABLE_CHECK */
 #endif /* __LINUX_PAGE_TABLE_CHECK_H */
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 440112355ffe..eefe3706f6c2 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -16,6 +16,7 @@
 #include <linux/hashtable.h>
 #include <linux/userfaultfd_k.h>
 #include <linux/page_idle.h>
+#include <linux/page_table_check.h>
 #include <linux/swapops.h>
 #include <linux/shmem_fs.h>
 
@@ -1424,6 +1425,7 @@ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *v
 
 	spin_unlock(ptl);
 	mm_dec_nr_ptes(mm);
+	page_table_check_pmd_clear_full(mm, addr, pmd);
 	pte_free(mm, pmd_pgtable(pmd));
 }
 
diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index c61d7ebe13b1..251f95a808b4 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -247,3 +247,24 @@ void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
 	}
 }
 EXPORT_SYMBOL(__page_table_check_pud_set);
+
+void __page_table_check_pmd_clear_full(struct mm_struct *mm, unsigned long addr,
+				       pmd_t pmd)
+{
+	if (&init_mm == mm)
+		return;
+
+	if (!pmd_bad(pmd) && !pmd_leaf(pmd)) {
+		pte_t *ptep = pte_offset_map(&pmd, addr);
+		unsigned long i;
+
+		pte_unmap(ptep);
+		for (i = 0; i < PTRS_PER_PTE; i++) {
+			__page_table_check_pte_clear(mm, addr, *ptep);
+			addr += PAGE_SIZE;
+			ptep++;
+		}
+	} else {
+		__page_table_check_pmd_clear(mm, addr, pmd);
+	}
+}
-- 
2.35.0.rc0.227.g00780c9af4-goog

Re: [PATCH v3 1/4] mm/debug_vm_pgtable: remove pte entry from the page table

[David Rientjes]

On Wed, 26 Jan 2022, Pasha Tatashin wrote:

> The pte entry that is used in pte_advanced_tests() is never removed from
> the page table at the end of the test.
> 
> The issue is detected by page_table_check, to repro compile kernel with
> the following configs:
> 
> CONFIG_DEBUG_VM_PGTABLE=y
> CONFIG_PAGE_TABLE_CHECK=y
> CONFIG_PAGE_TABLE_CHECK_ENFORCED=y
> 
> During the boot the following BUG is printed:
> 
> [    2.262821] debug_vm_pgtable: [debug_vm_pgtable         ]: Validating
>                architecture page table helpers
> [    2.276826] ------------[ cut here ]------------
> [    2.280426] kernel BUG at mm/page_table_check.c:162!
> [    2.284118] invalid opcode: 0000 [#1] PREEMPT SMP PTI
> [    2.287787] CPU: 0 PID: 1 Comm: swapper/0 Not tainted
>                5.16.0-11413-g2c271fe77d52 #3
> [    2.293226] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
>                BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org
>                04/01/2014
> ...
> 
> The entry should be properly removed from the page table before the page
> is released to the free list.
> 
> Fixes: a5c3b9ffb0f4 ("mm/debug_vm_pgtable: add tests validating advanced arch page table helpers")
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Tested-by: Zi Yan <ziy@nvidia.com>

Acked-by: David Rientjes <rientjes@google.com>

I think we also likely want:

Cc: stable@vger.kernel.org # 5.9+

Re: [PATCH v3 2/4] mm/page_table_check: use unsigned long for page counters and cleanup

[David Rientjes]

On Wed, 26 Jan 2022, Pasha Tatashin wrote:

> For the consistency, use "unsigned long" for all page counters.
> 
> Also, reduce code duplication by calling
> __page_table_check_*_clear() from __page_table_check_*_set() functions.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Reviewed-by: Wei Xu <weixugc@google.com>

Acked-by: David Rientjes <rientjes@google.com>

Re: [PATCH v3 3/4] mm/khugepaged: unify collapse pmd clear, flush and free

[David Rientjes]

On Wed, 26 Jan 2022, Pasha Tatashin wrote:

> Unify the code that flushes, clears pmd entry, and frees the PTE table
> level into a new function collapse_and_free_pmd().
> 
> This clean-up is useful as in the next patch we will add another call to
> this function to iterate through PTE prior to freeing the level for page
> table check.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>

Acked-by: David Rientjes <rientjes@google.com>

One nit below.

> ---
>  mm/khugepaged.c | 32 ++++++++++++++++----------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 35f14d0a00a6..440112355ffe 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1416,6 +1416,17 @@ static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
>  	return 0;
>  }
>  
> +static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
> +				  unsigned long addr, pmd_t *pmdp)
> +{
> +	spinlock_t *ptl = pmd_lock(vma->vm_mm, pmdp);
> +	pmd_t pmd = pmdp_collapse_flush(vma, addr, pmdp);
> +
> +	spin_unlock(ptl);

No strong objection, but I think the typical style would be to declare the 
local variables separately and avoid mixing the code, especially in cases 
when taking locks (and not just initializing the local variables).

> +	mm_dec_nr_ptes(mm);
> +	pte_free(mm, pmd_pgtable(pmd));
> +}
> +
>  /**
>   * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at
>   * address haddr.
> @@ -1433,7 +1444,7 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
>  	struct vm_area_struct *vma = find_vma(mm, haddr);
>  	struct page *hpage;
>  	pte_t *start_pte, *pte;
> -	pmd_t *pmd, _pmd;
> +	pmd_t *pmd;
>  	spinlock_t *ptl;
>  	int count = 0;
>  	int i;
> @@ -1509,12 +1520,7 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
>  	}
>  
>  	/* step 4: collapse pmd */
> -	ptl = pmd_lock(vma->vm_mm, pmd);
> -	_pmd = pmdp_collapse_flush(vma, haddr, pmd);
> -	spin_unlock(ptl);
> -	mm_dec_nr_ptes(mm);
> -	pte_free(mm, pmd_pgtable(_pmd));
> -
> +	collapse_and_free_pmd(mm, vma, haddr, pmd);
>  drop_hpage:
>  	unlock_page(hpage);
>  	put_page(hpage);
> @@ -1552,7 +1558,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
>  	struct vm_area_struct *vma;
>  	struct mm_struct *mm;
>  	unsigned long addr;
> -	pmd_t *pmd, _pmd;
> +	pmd_t *pmd;
>  
>  	i_mmap_lock_write(mapping);
>  	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
> @@ -1591,14 +1597,8 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
>  		 * reverse order. Trylock is a way to avoid deadlock.
>  		 */
>  		if (mmap_write_trylock(mm)) {
> -			if (!khugepaged_test_exit(mm)) {
> -				spinlock_t *ptl = pmd_lock(mm, pmd);
> -				/* assume page table is clear */
> -				_pmd = pmdp_collapse_flush(vma, addr, pmd);
> -				spin_unlock(ptl);
> -				mm_dec_nr_ptes(mm);
> -				pte_free(mm, pmd_pgtable(_pmd));
> -			}
> +			if (!khugepaged_test_exit(mm))
> +				collapse_and_free_pmd(mm, vma, addr, pmd);
>  			mmap_write_unlock(mm);
>  		} else {
>  			/* Try again later */
> -- 
> 2.35.0.rc0.227.g00780c9af4-goog
> 
> 

Re: [PATCH v3 4/4] mm/page_table_check: check entries at pmd levels

[David Rientjes]

On Wed, 26 Jan 2022, Pasha Tatashin wrote:

> syzbot detected a case where the page table counters were not properly
> updated.
> 

Is there a Reported-by tag that syzbot wants us to use to track this?

> syzkaller login:  ------------[ cut here ]------------
> kernel BUG at mm/page_table_check.c:162!
> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> CPU: 0 PID: 3099 Comm: pasha Not tainted 5.16.0+ #48
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIO4
> RIP: 0010:__page_table_check_zero+0x159/0x1a0
> Code: 7d 3a b2 ff 45 39 f5 74 2a e8 43 38 b2 ff 4d 85 e4 01
> RSP: 0018:ffff888010667418 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000
> RDX: ffff88800cea8680 RSI: ffffffff81becaf9 RDI: 0000000003
> RBP: ffff888010667450 R08: 0000000000000001 R09: 0000000000
> R10: ffffffff81becaab R11: 0000000000000001 R12: ffff888008
> R13: 0000000000000001 R14: 0000000000000200 R15: dffffc0000
> FS:  0000000000000000(0000) GS:ffff888035e00000(0000) knlG0
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007ffd875cad00 CR3: 00000000094ce000 CR4: 0000000000
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000
> Call Trace:
>  <TASK>
>  free_pcp_prepare+0x3be/0xaa0
>  free_unref_page+0x1c/0x650
>  ? trace_hardirqs_on+0x6a/0x1d0
>  free_compound_page+0xec/0x130
>  free_transhuge_page+0x1be/0x260
>  __put_compound_page+0x90/0xd0
>  release_pages+0x54c/0x1060
>  ? filemap_remove_folio+0x161/0x210
>  ? lock_downgrade+0x720/0x720
>  ? __put_page+0x150/0x150
>  ? filemap_free_folio+0x164/0x350
>  __pagevec_release+0x7c/0x110
>  shmem_undo_range+0x85e/0x1250
> ...
> 
> The repro involved having a huge page that is split due to uprobe event
> temporarily replacing one of the pages in the huge page. Later the huge
> page was combined again, but the counters were off, as the PTE level
> was not properly updated.
> 
> Make sure that when PMD is cleared and prior to freeing the level the
> PTEs are updated.
> 
> Fixes: df4e817b7108 ("mm: page table check")
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  include/linux/page_table_check.h | 18 ++++++++++++++++++
>  mm/khugepaged.c                  |  2 ++
>  mm/page_table_check.c            | 21 +++++++++++++++++++++
>  3 files changed, 41 insertions(+)
> 
> diff --git a/include/linux/page_table_check.h b/include/linux/page_table_check.h
> index 38cace1da7b6..e88bbe37727b 100644
> --- a/include/linux/page_table_check.h
> +++ b/include/linux/page_table_check.h
> @@ -26,6 +26,8 @@ void __page_table_check_pmd_set(struct mm_struct *mm, unsigned long addr,
>  				pmd_t *pmdp, pmd_t pmd);
>  void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
>  				pud_t *pudp, pud_t pud);
> +void __page_table_check_pmd_clear_full(struct mm_struct *mm, unsigned long addr,
> +				       pmd_t pmd);
>  
>  static inline void page_table_check_alloc(struct page *page, unsigned int order)
>  {
> @@ -100,6 +102,16 @@ static inline void page_table_check_pud_set(struct mm_struct *mm,
>  	__page_table_check_pud_set(mm, addr, pudp, pud);
>  }
>  
> +static inline void page_table_check_pmd_clear_full(struct mm_struct *mm,
> +						   unsigned long addr,
> +						   pmd_t pmd)
> +{
> +	if (static_branch_likely(&page_table_check_disabled))
> +		return;
> +
> +	__page_table_check_pmd_clear_full(mm, addr, pmd);
> +}
> +
>  #else
>  
>  static inline void page_table_check_alloc(struct page *page, unsigned int order)
> @@ -143,5 +155,11 @@ static inline void page_table_check_pud_set(struct mm_struct *mm,
>  {
>  }
>  
> +static inline void page_table_check_pmd_clear_full(struct mm_struct *mm,
> +						   unsigned long addr,
> +						   pmd_t pmd)
> +{
> +}
> +
>  #endif /* CONFIG_PAGE_TABLE_CHECK */
>  #endif /* __LINUX_PAGE_TABLE_CHECK_H */
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 440112355ffe..eefe3706f6c2 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -16,6 +16,7 @@
>  #include <linux/hashtable.h>
>  #include <linux/userfaultfd_k.h>
>  #include <linux/page_idle.h>
> +#include <linux/page_table_check.h>
>  #include <linux/swapops.h>
>  #include <linux/shmem_fs.h>
>  
> @@ -1424,6 +1425,7 @@ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *v
>  
>  	spin_unlock(ptl);
>  	mm_dec_nr_ptes(mm);
> +	page_table_check_pmd_clear_full(mm, addr, pmd);
>  	pte_free(mm, pmd_pgtable(pmd));
>  }

This looks right, I'm wondering if we want to add a 
mmap_assert_write_locked(mm) to collapse_and_free_pmd().

>  
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index c61d7ebe13b1..251f95a808b4 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -247,3 +247,24 @@ void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
>  	}
>  }
>  EXPORT_SYMBOL(__page_table_check_pud_set);
> +
> +void __page_table_check_pmd_clear_full(struct mm_struct *mm, unsigned long addr,
> +				       pmd_t pmd)
> +{
> +	if (&init_mm == mm)
> +		return;
> +
> +	if (!pmd_bad(pmd) && !pmd_leaf(pmd)) {
> +		pte_t *ptep = pte_offset_map(&pmd, addr);
> +		unsigned long i;
> +
> +		pte_unmap(ptep);
> +		for (i = 0; i < PTRS_PER_PTE; i++) {
> +			__page_table_check_pte_clear(mm, addr, *ptep);
> +			addr += PAGE_SIZE;
> +			ptep++;
> +		}
> +	} else {
> +		__page_table_check_pmd_clear(mm, addr, pmd);
> +	}
> +}
> -- 
> 2.35.0.rc0.227.g00780c9af4-goog
> 
> 

Re: [PATCH v3 3/4] mm/khugepaged: unify collapse pmd clear, flush and free

[Pasha Tatashin]

On Wed, Jan 26, 2022 at 1:34 AM David Rientjes <rientjes@google.com> wrote:
>
> On Wed, 26 Jan 2022, Pasha Tatashin wrote:
>
> > Unify the code that flushes, clears pmd entry, and frees the PTE table
> > level into a new function collapse_and_free_pmd().
> >
> > This clean-up is useful as in the next patch we will add another call to
> > this function to iterate through PTE prior to freeing the level for page
> > table check.
> >
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
>
> Acked-by: David Rientjes <rientjes@google.com>

Thank you, David.

>
> One nit below.
>
> > ---
> >  mm/khugepaged.c | 32 ++++++++++++++++----------------
> >  1 file changed, 16 insertions(+), 16 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 35f14d0a00a6..440112355ffe 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1416,6 +1416,17 @@ static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
> >       return 0;
> >  }
> >
> > +static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
> > +                               unsigned long addr, pmd_t *pmdp)
> > +{
> > +     spinlock_t *ptl = pmd_lock(vma->vm_mm, pmdp);
> > +     pmd_t pmd = pmdp_collapse_flush(vma, addr, pmdp);
> > +
> > +     spin_unlock(ptl);
>
> No strong objection, but I think the typical style would be to declare the
> local variables separately and avoid mixing the code, especially in cases
> when taking locks (and not just initializing the local variables).

I will address this in the next revision.

Thank you,
Pasha

Re: [PATCH v3 4/4] mm/page_table_check: check entries at pmd levels

[Pasha Tatashin]

On Wed, Jan 26, 2022 at 1:45 AM David Rientjes <rientjes@google.com> wrote:
>
> On Wed, 26 Jan 2022, Pasha Tatashin wrote:
>
> > syzbot detected a case where the page table counters were not properly
> > updated.
> >
>
> Is there a Reported-by tag that syzbot wants us to use to track this?

Google internal syzbot found it, so I did not add Reported-by as there
is no  public e-mail to reference. However, I added all the necessary
steps to repro this problem to the cover letter.

>
>
> This looks right, I'm wondering if we want to add a
> mmap_assert_write_locked(mm) to collapse_and_free_pmd().

Good idea, I will add it in the next revision.

Thanks,
Pasha