[v3 PATCH 8/8] mm: mmap: register suitable readonly file vmas for khugepaged

[Yang Shi]

The readonly FS THP relies on khugepaged to collapse THP for suitable
vmas.  But it is kind of "random luck" for khugepaged to see the
readonly FS vmas (https://lore.kernel.org/linux-mm/00f195d4-d039-3cf2-d3a1-a2c88de397a0@xxxxxxx/)
since currently the vmas are registered to khugepaged when:
  - Anon huge pmd page fault
  - VMA merge
  - MADV_HUGEPAGE
  - Shmem mmap

If the above conditions are not met, even though khugepaged is enabled
it won't see readonly FS vmas at all.  MADV_HUGEPAGE could be specified
explicitly to tell khugepaged to collapse this area, but when khugepaged
mode is "always" it should scan suitable vmas as long as VM_NOHUGEPAGE
is not set.

So make sure readonly FS vmas are registered to khugepaged to make the
behavior more consistent.

Registering suitable vmas in common mmap path, that could cover both
readonly FS vmas and shmem vmas, so removed the khugepaged calls in
shmem.c.

Still need to keep the khugepaged call in vma_merge() since vma_merge()
is called in a lot of places, for example, madvise, mprotect, etc.

Reported-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
 mm/mmap.c  | 6 ++++++
 mm/shmem.c | 4 ----
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 604c8dece5dd..616ebbc2d052 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1842,6 +1842,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	}
 
 	vma_link(mm, vma, prev, rb_link, rb_parent);
+
+	/*
+	 * vma_merge() calls khugepaged_enter_vma() either, the below
+	 * call covers the non-merge case.
+	 */
+	khugepaged_enter_vma(vma, vma->vm_flags);
 	/* Once vma denies write, undo our temporary denial count */
 unmap_writable:
 	if (file && vm_flags & VM_SHARED)
diff --git a/mm/shmem.c b/mm/shmem.c
index 92eca974771d..0c448080d210 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -34,7 +34,6 @@
 #include <linux/export.h>
 #include <linux/swap.h>
 #include <linux/uio.h>
-#include <linux/khugepaged.h>
 #include <linux/hugetlb.h>
 #include <linux/fs_parser.h>
 #include <linux/swapfile.h>
@@ -2239,7 +2238,6 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 
 	file_accessed(file);
 	vma->vm_ops = &shmem_vm_ops;
-	khugepaged_enter_vma(vma, vma->vm_flags);
 	return 0;
 }
 
@@ -4132,8 +4130,6 @@ int shmem_zero_setup(struct vm_area_struct *vma)
 	vma->vm_file = file;
 	vma->vm_ops = &shmem_vm_ops;
 
-	khugepaged_enter_vma(vma, vma->vm_flags);
-
 	return 0;
 }
 
-- 
2.26.3