[PATCH v3] mm/THP: use hugepage_vma_check() in khugepaged_enter_vma_merge()

[Song Liu <songliubraving@fb.com>]

khugepaged_enter_vma_merge() is using a different approach to check
whether a vma is valid for khugepaged_enter():

    if (!vma->anon_vma)
            /*
             * Not yet faulted in so we will register later in the
             * page fault if needed.
             */
            return 0;
    if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
            /* khugepaged not yet working on file or special mappings */
            return 0;

This check has some problems. One of the obvious problems is that
it doesn't check shmem_file(), so that vma backed with shmem files
will not call khugepaged_enter(). Here is an example of failed madvise():

   /* mount /dev/shm with huge=advise:
    *     mount -o remount,huge=advise /dev/shm */
   /* create file /dev/shm/huge */
   #define HUGE_FILE "/dev/shm/huge"

   fd = open(HUGE_FILE, O_RDONLY);
   ptr = mmap(NULL, FILE_SIZE, PROT_READ, MAP_PRIVATE, fd, 0);
   ret = madvise(ptr, FILE_SIZE, MADV_HUGEPAGE);

madvise() will return 0, but this memory region is never put in huge
page (check from /proc/meminfo: ShmemHugePages).

This patch fixes these problems by reusing hugepage_vma_check() in
khugepaged_enter_vma_merge().

vma->vm_flags is not yet updated in khugepaged_enter_vma_merge(),
so we need to pass the new vm_flags to hugepage_vma_check() through
a separate argument.

Signed-off-by: Song Liu <songliubraving@fb.com>
---
 mm/khugepaged.c | 53 ++++++++++++++++++++++++-----------------------------
 1 file changed, 24 insertions(+), 29 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d7b2a4b..e0a3bdf 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -430,18 +430,32 @@ int __khugepaged_enter(struct mm_struct *mm)
 	return 0;
 }
 
+static bool hugepage_vma_check(struct vm_area_struct *vma,
+			       unsigned long vm_flags)
+{
+	if ((!(vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
+	    (vm_flags & VM_NOHUGEPAGE) ||
+	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
+		return false;
+	if (shmem_file(vma->vm_file)) {
+		if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
+			return false;
+		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
+				HPAGE_PMD_NR);
+	}
+	if (!vma->anon_vma || vma->vm_ops)
+		return false;
+	if (is_vma_temporary_stack(vma))
+		return false;
+	return !(vm_flags & VM_NO_KHUGEPAGED);
+}
+
 int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
 			       unsigned long vm_flags)
 {
 	unsigned long hstart, hend;
-	if (!vma->anon_vma)
-		/*
-		 * Not yet faulted in so we will register later in the
-		 * page fault if needed.
-		 */
-		return 0;
-	if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
-		/* khugepaged not yet working on file or special mappings */
+
+	if (!hugepage_vma_check(vma, vm_flags))
 		return 0;
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 	hend = vma->vm_end & HPAGE_PMD_MASK;
@@ -819,25 +833,6 @@ khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
 }
 #endif
 
-static bool hugepage_vma_check(struct vm_area_struct *vma)
-{
-	if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
-	    (vma->vm_flags & VM_NOHUGEPAGE) ||
-	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
-		return false;
-	if (shmem_file(vma->vm_file)) {
-		if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
-			return false;
-		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
-				HPAGE_PMD_NR);
-	}
-	if (!vma->anon_vma || vma->vm_ops)
-		return false;
-	if (is_vma_temporary_stack(vma))
-		return false;
-	return !(vma->vm_flags & VM_NO_KHUGEPAGED);
-}
-
 /*
  * If mmap_sem temporarily dropped, revalidate vma
  * before taking mmap_sem.
@@ -862,7 +857,7 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 	hend = vma->vm_end & HPAGE_PMD_MASK;
 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
 		return SCAN_ADDRESS_RANGE;
-	if (!hugepage_vma_check(vma))
+	if (!hugepage_vma_check(vma, vma->vm_flags))
 		return SCAN_VMA_CHECK;
 	return 0;
 }
@@ -1694,7 +1689,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
 			progress++;
 			break;
 		}
-		if (!hugepage_vma_check(vma)) {
+		if (!hugepage_vma_check(vma, vma->vm_flags)) {
 skip:
 			progress++;
 			continue;
-- 
2.9.5