[PATCH mm-unstable v2 3/9] mm/madvise: add file and shmem support to MADV_COLLAPSE

["Zach O'Keefe" <zokeefe@google.com>]

Add support for MADV_COLLAPSE to collapse shmem-backed and file-backed
memory into THPs (requires CONFIG_READ_ONLY_THP_FOR_FS=y).

On success, the backing memory will be a hugepage.  For the memory range
and process provided, the page tables will synchronously have a huge pmd
installed, mapping the THP.  Other mappings of the file extent mapped by
the memory range may be added to a set of entries that khugepaged will
later process and attempt update their page tables to map the THP by a pmd.

This functionality unlocks two important uses:

(1)	Immediately back executable text by THPs.  Current support provided
	by CONFIG_READ_ONLY_THP_FOR_FS may take a long time on a large
	system which might impair services from serving at their full rated
	load after (re)starting.  Tricks like mremap(2)'ing text onto
	anonymous memory to immediately realize iTLB performance prevents
	page sharing and demand paging, both of which increase steady state
	memory footprint.  Now, we can have the best of both worlds: Peak
	upfront performance and lower RAM footprints.

(2)	userfaultfd-based live migration of virtual machines satisfy UFFD
	faults by fetching native-sized pages over the network (to avoid
	latency of transferring an entire hugepage).  However, after guest
	memory has been fully copied to the new host, MADV_COLLAPSE can
	be used to immediately increase guest performance.

Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
 include/linux/khugepaged.h         |  13 +-
 include/trace/events/huge_memory.h |   1 +
 kernel/events/uprobes.c            |   2 +-
 mm/khugepaged.c                    | 241 ++++++++++++++++++++++-------
 4 files changed, 198 insertions(+), 59 deletions(-)

diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
index 384f034ae947..70162d707caf 100644
--- a/include/linux/khugepaged.h
+++ b/include/linux/khugepaged.h
@@ -16,11 +16,13 @@ extern void khugepaged_enter_vma(struct vm_area_struct *vma,
 				 unsigned long vm_flags);
 extern void khugepaged_min_free_kbytes_update(void);
 #ifdef CONFIG_SHMEM
-extern void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr);
+extern int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
+				   bool install_pmd);
 #else
-static inline void collapse_pte_mapped_thp(struct mm_struct *mm,
-					   unsigned long addr)
+static inline int collapse_pte_mapped_thp(struct mm_struct *mm,
+					  unsigned long addr, bool install_pmd)
 {
+	return 0;
 }
 #endif
 
@@ -46,9 +48,10 @@ static inline void khugepaged_enter_vma(struct vm_area_struct *vma,
 					unsigned long vm_flags)
 {
 }
-static inline void collapse_pte_mapped_thp(struct mm_struct *mm,
-					   unsigned long addr)
+static inline int collapse_pte_mapped_thp(struct mm_struct *mm,
+					  unsigned long addr, bool install_pmd)
 {
+	return 0;
 }
 
 static inline void khugepaged_min_free_kbytes_update(void)
diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index fbbb25494d60..a8db658e99e9 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -11,6 +11,7 @@
 	EM( SCAN_FAIL,			"failed")			\
 	EM( SCAN_SUCCEED,		"succeeded")			\
 	EM( SCAN_PMD_NULL,		"pmd_null")			\
+	EM( SCAN_PMD_NON_PRESENT,	"pmd_non_present")		\
 	EM( SCAN_PMD_MAPPED,		"page_pmd_mapped")		\
 	EM( SCAN_EXCEED_NONE_PTE,	"exceed_none_pte")		\
 	EM( SCAN_EXCEED_SWAP_PTE,	"exceed_swap_pte")		\
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 401bc2d24ce0..d48c47811e45 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -553,7 +553,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 
 	/* try collapse pmd for compound page */
 	if (!ret && orig_page_huge)
-		collapse_pte_mapped_thp(mm, vaddr);
+		collapse_pte_mapped_thp(mm, vaddr, false);
 
 	return ret;
 }
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6022a08db1cd..34c0c74b3839 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -28,6 +28,7 @@ enum scan_result {
 	SCAN_FAIL,
 	SCAN_SUCCEED,
 	SCAN_PMD_NULL,
+	SCAN_PMD_NON_PRESENT,
 	SCAN_PMD_MAPPED,
 	SCAN_EXCEED_NONE_PTE,
 	SCAN_EXCEED_SWAP_PTE,
@@ -870,6 +871,18 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 	if (!hugepage_vma_check(vma, vma->vm_flags, false, false,
 				cc->is_khugepaged))
 		return SCAN_VMA_CHECK;
+	return SCAN_SUCCEED;
+}
+
+static int hugepage_vma_revalidate_anon(struct mm_struct *mm,
+					unsigned long address,
+					struct vm_area_struct **vmap,
+					struct collapse_control *cc)
+{
+	int ret = hugepage_vma_revalidate(mm, address, vmap, cc);
+
+	if (ret != SCAN_SUCCEED)
+		return ret;
 	/*
 	 * Anon VMA expected, the address may be unmapped then
 	 * remapped to file after khugepaged reaquired the mmap_lock.
@@ -877,8 +890,8 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 	 * hugepage_vma_check may return true for qualified file
 	 * vmas.
 	 */
-	if (!vma->anon_vma || !vma_is_anonymous(vma))
-		return SCAN_VMA_CHECK;
+	if (!(*vmap)->anon_vma || !vma_is_anonymous(*vmap))
+		return SCAN_PAGE_ANON;
 	return SCAN_SUCCEED;
 }
 
@@ -899,7 +912,7 @@ static int find_pmd_or_thp_or_none(struct mm_struct *mm,
 	barrier();
 #endif
 	if (!pmd_present(pmde))
-		return SCAN_PMD_NULL;
+		return SCAN_PMD_NON_PRESENT;
 	if (pmd_trans_huge(pmde))
 		return SCAN_PMD_MAPPED;
 	if (pmd_bad(pmde))
@@ -1027,7 +1040,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 		goto out_nolock;
 
 	mmap_read_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, &vma, cc);
+	result = hugepage_vma_revalidate_anon(mm, address, &vma, cc);
 	if (result != SCAN_SUCCEED) {
 		mmap_read_unlock(mm);
 		goto out_nolock;
@@ -1058,7 +1071,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 * handled by the anon_vma lock + PG_lock.
 	 */
 	mmap_write_lock(mm);
-	result = hugepage_vma_revalidate(mm, address, &vma, cc);
+	result = hugepage_vma_revalidate_anon(mm, address, &vma, cc);
 	if (result != SCAN_SUCCEED)
 		goto out_up_write;
 	/* check if the pmd is still valid */
@@ -1361,13 +1374,44 @@ static bool khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
 	spin_lock(&khugepaged_mm_lock);
 	mm_slot = get_mm_slot(mm);
 	if (likely(mm_slot && mm_slot->nr_pte_mapped_thp < MAX_PTE_MAPPED_THP)) {
+		int i;
+		/*
+		 * Multiple callers may be adding entries here.  Do a quick
+		 * check to see the entry hasn't already been added by someone
+		 * else.
+		 */
+		for (i = 0; i < mm_slot->nr_pte_mapped_thp; ++i)
+			if (mm_slot->pte_mapped_thp[i] == addr)
+				goto out;
 		mm_slot->pte_mapped_thp[mm_slot->nr_pte_mapped_thp++] = addr;
 		ret = true;
 	}
+out:
 	spin_unlock(&khugepaged_mm_lock);
 	return ret;
 }
 
+/* hpage must be locked, and mmap_lock must be held in write */
+static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,
+			pmd_t *pmdp, struct page *hpage)
+{
+	struct vm_fault vmf = {
+		.vma = vma,
+		.address = addr,
+		.flags = 0,
+		.pmd = pmdp,
+	};
+
+	VM_BUG_ON(!PageTransHuge(hpage));
+	mmap_assert_write_locked(vma->vm_mm);
+
+	if (do_set_pmd(&vmf, hpage))
+		return SCAN_FAIL;
+
+	get_page(hpage);
+	return SCAN_SUCCEED;
+}
+
 static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
 				  unsigned long addr, pmd_t *pmdp)
 {
@@ -1389,12 +1433,14 @@ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *v
  *
  * @mm: process address space where collapse happens
  * @addr: THP collapse address
+ * @install_pmd: If a huge PMD should be installed
  *
  * This function checks whether all the PTEs in the PMD are pointing to the
  * right THP. If so, retract the page table so the THP can refault in with
- * as pmd-mapped.
+ * as pmd-mapped. Possibly install a huge PMD mapping the THP.
  */
-void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
+int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
+			    bool install_pmd)
 {
 	unsigned long haddr = addr & HPAGE_PMD_MASK;
 	struct vm_area_struct *vma = vma_lookup(mm, haddr);
@@ -1409,12 +1455,12 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
 
 	/* Fast check before locking page if already PMD-mapped  */
 	result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
-	if (result != SCAN_SUCCEED)
-		return;
+	if (result == SCAN_PMD_MAPPED)
+		return result;
 
 	if (!vma || !vma->vm_file ||
 	    !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
-		return;
+		return SCAN_VMA_CHECK;
 
 	/*
 	 * If we are here, we've succeeded in replacing all the native pages
@@ -1424,24 +1470,44 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
 	 * analogously elide sysfs THP settings here.
 	 */
 	if (!hugepage_vma_check(vma, vma->vm_flags, false, false, false))
-		return;
+		return SCAN_VMA_CHECK;
 
 	/* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */
 	if (userfaultfd_wp(vma))
-		return;
+		return SCAN_PTE_UFFD_WP;
 
 	hpage = find_lock_page(vma->vm_file->f_mapping,
 			       linear_page_index(vma, haddr));
 	if (!hpage)
-		return;
+		return SCAN_PAGE_NULL;
 
-	if (!PageHead(hpage))
+	if (!PageHead(hpage)) {
+		result = SCAN_FAIL;
 		goto drop_hpage;
+	}
 
-	if (find_pmd_or_thp_or_none(mm, haddr, &pmd) != SCAN_SUCCEED)
+	if (!PageTransCompound(hpage)) {
+		result = SCAN_FAIL;
 		goto drop_hpage;
+	}
+
+	result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
+	switch (result) {
+	case SCAN_SUCCEED:
+		break;
+	case SCAN_PMD_NON_PRESENT:
+		/*
+		 * In MADV_COLLAPSE path, possible race with khugepaged where
+		 * all pte entries have been removed and pmd cleared.  If so,
+		 * skip all the pte checks and just update the pmd mapping.
+		 */
+		goto install_pmd;
+	default:
+		goto drop_hpage;
+	}
 
 	start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
+	result = SCAN_FAIL;
 
 	/* step 1: check all mapped PTEs are to the right huge page */
 	for (i = 0, addr = haddr, pte = start_pte;
@@ -1453,8 +1519,10 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
 			continue;
 
 		/* page swapped out, abort */
-		if (!pte_present(*pte))
+		if (!pte_present(*pte)) {
+			result = SCAN_PTE_NON_PRESENT;
 			goto abort;
+		}
 
 		page = vm_normal_page(vma, addr, *pte);
 		if (WARN_ON_ONCE(page && is_zone_device_page(page)))
@@ -1489,12 +1557,19 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
 		add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count);
 	}
 
-	/* step 4: collapse pmd */
+	/* step 4: remove pte entries */
 	collapse_and_free_pmd(mm, vma, haddr, pmd);
+
+install_pmd:
+	/* step 5: install pmd entry */
+	result = install_pmd
+			? set_huge_pmd(vma, haddr, pmd, hpage)
+			: SCAN_SUCCEED;
+
 drop_hpage:
 	unlock_page(hpage);
 	put_page(hpage);
-	return;
+	return result;
 
 abort:
 	pte_unmap_unlock(start_pte, ptl);
@@ -1516,22 +1591,29 @@ static void khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot)
 		goto out;
 
 	for (i = 0; i < mm_slot->nr_pte_mapped_thp; i++)
-		collapse_pte_mapped_thp(mm, mm_slot->pte_mapped_thp[i]);
+		collapse_pte_mapped_thp(mm, mm_slot->pte_mapped_thp[i], false);
 
 out:
 	mm_slot->nr_pte_mapped_thp = 0;
 	mmap_write_unlock(mm);
 }
 
-static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
+static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
+			       struct mm_struct *target_mm,
+			       unsigned long target_addr, struct page *hpage,
+			       struct collapse_control *cc)
 {
 	struct vm_area_struct *vma;
-	struct mm_struct *mm;
-	unsigned long addr;
-	pmd_t *pmd;
+	int target_result = SCAN_FAIL;
 
 	i_mmap_lock_write(mapping);
 	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
+		int result = SCAN_FAIL;
+		struct mm_struct *mm = NULL;
+		unsigned long addr = 0;
+		pmd_t *pmd;
+		bool is_target = false;
+
 		/*
 		 * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
 		 * got written to. These VMAs are likely not worth investing
@@ -1548,24 +1630,34 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 		 * ptl. It has higher chance to recover THP for the VMA, but
 		 * has higher cost too.
 		 */
-		if (vma->anon_vma)
-			continue;
+		if (vma->anon_vma) {
+			result = SCAN_PAGE_ANON;
+			goto next;
+		}
 		addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
-		if (addr & ~HPAGE_PMD_MASK)
-			continue;
-		if (vma->vm_end < addr + HPAGE_PMD_SIZE)
-			continue;
+		if (addr & ~HPAGE_PMD_MASK ||
+		    vma->vm_end < addr + HPAGE_PMD_SIZE) {
+			result = SCAN_VMA_CHECK;
+			goto next;
+		}
 		mm = vma->vm_mm;
-		if (find_pmd_or_thp_or_none(mm, addr, &pmd) != SCAN_SUCCEED)
-			continue;
+		is_target = mm == target_mm && addr == target_addr;
+		result = find_pmd_or_thp_or_none(mm, addr, &pmd);
+		if (result != SCAN_SUCCEED)
+			goto next;
 		/*
 		 * We need exclusive mmap_lock to retract page table.
 		 *
 		 * We use trylock due to lock inversion: we need to acquire
 		 * mmap_lock while holding page lock. Fault path does it in
 		 * reverse order. Trylock is a way to avoid deadlock.
+		 *
+		 * Also, it's not MADV_COLLAPSE's job to collapse other
+		 * mappings - let khugepaged take care of them later.
 		 */
-		if (mmap_write_trylock(mm)) {
+		result = SCAN_PTE_MAPPED_HUGEPAGE;
+		if ((cc->is_khugepaged || is_target) &&
+		    mmap_write_trylock(mm)) {
 			/*
 			 * When a vma is registered with uffd-wp, we can't
 			 * recycle the pmd pgtable because there can be pte
@@ -1574,22 +1666,45 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 			 * it'll always mapped in small page size for uffd-wp
 			 * registered ranges.
 			 */
-			if (!hpage_collapse_test_exit(mm) &&
-			    !userfaultfd_wp(vma))
-				collapse_and_free_pmd(mm, vma, addr, pmd);
+			if (hpage_collapse_test_exit(mm)) {
+				result = SCAN_ANY_PROCESS;
+				goto unlock_next;
+			}
+			if (userfaultfd_wp(vma)) {
+				result = SCAN_PTE_UFFD_WP;
+				goto unlock_next;
+			}
+			collapse_and_free_pmd(mm, vma, addr, pmd);
+			if (!cc->is_khugepaged && is_target)
+				result = set_huge_pmd(vma, addr, pmd, hpage);
+			else
+				result = SCAN_SUCCEED;
+
+unlock_next:
 			mmap_write_unlock(mm);
-		} else {
-			/* Try again later */
+			goto next;
+		}
+		/*
+		 * Calling context will handle target mm/addr. Otherwise, let
+		 * khugepaged try again later.
+		 */
+		if (!is_target) {
 			khugepaged_add_pte_mapped_thp(mm, addr);
+			continue;
 		}
+next:
+		if (is_target)
+			target_result = result;
 	}
 	i_mmap_unlock_write(mapping);
+	return target_result;
 }
 
 /**
  * collapse_file - collapse filemap/tmpfs/shmem pages into huge one.
  *
  * @mm: process address space where collapse happens
+ * @addr: virtual collapse start address
  * @file: file that collapse on
  * @start: collapse start address
  * @cc: collapse context and scratchpad
@@ -1609,8 +1724,9 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
  *    + restore gaps in the page cache;
  *    + unlock and free huge page;
  */
-static int collapse_file(struct mm_struct *mm, struct file *file,
-			 pgoff_t start, struct collapse_control *cc)
+static int collapse_file(struct mm_struct *mm, unsigned long addr,
+			 struct file *file, pgoff_t start,
+			 struct collapse_control *cc)
 {
 	struct address_space *mapping = file->f_mapping;
 	struct page *hpage;
@@ -1912,7 +2028,8 @@ static int collapse_file(struct mm_struct *mm, struct file *file,
 		/*
 		 * Remove pte page tables, so we can re-fault the page as huge.
 		 */
-		retract_page_tables(mapping, start);
+		result = retract_page_tables(mapping, start, mm, addr, hpage,
+					     cc);
 		unlock_page(hpage);
 		hpage = NULL;
 	} else {
@@ -1968,8 +2085,9 @@ static int collapse_file(struct mm_struct *mm, struct file *file,
 	return result;
 }
 
-static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
-				pgoff_t start, struct collapse_control *cc)
+static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
+				    struct file *file, pgoff_t start,
+				    struct collapse_control *cc)
 {
 	struct page *page = NULL;
 	struct address_space *mapping = file->f_mapping;
@@ -2049,7 +2167,7 @@ static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
 			result = SCAN_EXCEED_NONE_PTE;
 			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
 		} else {
-			result = collapse_file(mm, file, start, cc);
+			result = collapse_file(mm, addr, file, start, cc);
 		}
 	}
 
@@ -2057,8 +2175,9 @@ static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
 	return result;
 }
 #else
-static int khugepaged_scan_file(struct mm_struct *mm, struct file *file,
-				pgoff_t start, struct collapse_control *cc)
+static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
+				    struct file *file, pgoff_t start,
+				    struct collapse_control *cc)
 {
 	BUILD_BUG();
 }
@@ -2153,8 +2272,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 						khugepaged_scan.address);
 
 				mmap_read_unlock(mm);
-				*result = khugepaged_scan_file(mm, file, pgoff,
-							       cc);
+				*result = hpage_collapse_scan_file(mm,
+								   khugepaged_scan.address,
+								   file, pgoff, cc);
 				mmap_locked = false;
 				fput(file);
 			} else {
@@ -2452,10 +2572,6 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
 
 	*prev = vma;
 
-	/* TODO: Support file/shmem */
-	if (!vma->anon_vma || !vma_is_anonymous(vma))
-		return -EINVAL;
-
 	if (!hugepage_vma_check(vma, vma->vm_flags, false, false, false))
 		return -EINVAL;
 
@@ -2486,16 +2602,35 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
 		}
 		mmap_assert_locked(mm);
 		memset(cc->node_load, 0, sizeof(cc->node_load));
-		result = hpage_collapse_scan_pmd(mm, vma, addr, &mmap_locked,
-						 cc);
+		if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {
+			struct file *file = get_file(vma->vm_file);
+			pgoff_t pgoff = linear_page_index(vma, addr);
+
+			mmap_read_unlock(mm);
+			mmap_locked = false;
+			result = hpage_collapse_scan_file(mm, addr, file, pgoff,
+							  cc);
+			fput(file);
+		} else {
+			result = hpage_collapse_scan_pmd(mm, vma, addr,
+							 &mmap_locked, cc);
+		}
 		if (!mmap_locked)
 			*prev = NULL;  /* Tell caller we dropped mmap_lock */
 
+handle_result:
 		switch (result) {
 		case SCAN_SUCCEED:
 		case SCAN_PMD_MAPPED:
 			++thps;
 			break;
+		case SCAN_PTE_MAPPED_HUGEPAGE:
+			BUG_ON(mmap_locked);
+			BUG_ON(*prev);
+			mmap_write_lock(mm);
+			result = collapse_pte_mapped_thp(mm, addr, true);
+			mmap_write_unlock(mm);
+			goto handle_result;
 		/* Whitelisted set of results where continuing OK */
 		case SCAN_PMD_NULL:
 		case SCAN_PTE_NON_PRESENT:
-- 
2.37.2.672.g94769d06f0-goog