linux-kernel - [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility

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Date:   Wed, 24 Apr 2019 00:43:01 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     mhocko@...e.com, vbabka@...e.cz, rientjes@...gle.com,
        kirill@...temov.name, akpm@...ux-foundation.org
Cc:     yang.shi@...ux.alibaba.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility

The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each
vma") introduced THPeligible bit for processes' smaps. But, when checking
the eligibility for shmem vma, __transparent_hugepage_enabled() is
called to override the result from shmem_huge_enabled().  It may result
in the anonymous vma's THP flag override shmem's.  For example, running a
simple test which create THP for shmem, but with anonymous THP disabled,
when reading the process's smaps, it may show:

7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test
Size:               4096 kB
...
[snip]
...
ShmemPmdMapped:     4096 kB
...
[snip]
...
THPeligible:    0

And, /proc/meminfo does show THP allocated and PMD mapped too:

ShmemHugePages:     4096 kB
ShmemPmdMapped:     4096 kB

This doesn't make too much sense.  The anonymous THP flag should not
intervene shmem THP.  Calling shmem_huge_enabled() with checking
MMF_DISABLE_THP sounds good enough.  And, we could skip stack and
dax vma check since we already checked if the vma is shmem already.

Fixes: 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each vma")
Cc: Michal Hocko <mhocko@...e.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Kirill A. Shutemov <kirill@...temov.name>
Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
---
v2: Check VM_NOHUGEPAGE per Michal Hocko

 mm/huge_memory.c | 4 ++--
 mm/shmem.c       | 3 +++
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 165ea46..5881e82 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -67,8 +67,8 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
 {
 	if (vma_is_anonymous(vma))
 		return __transparent_hugepage_enabled(vma);
-	if (vma_is_shmem(vma) && shmem_huge_enabled(vma))
-		return __transparent_hugepage_enabled(vma);
+	if (vma_is_shmem(vma))
+		return shmem_huge_enabled(vma);
 
 	return false;
 }
diff --git a/mm/shmem.c b/mm/shmem.c
index 2275a0f..6f09a31 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3873,6 +3873,9 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
 	loff_t i_size;
 	pgoff_t off;
 
+	if ((vma->vm_flags & VM_NOHUGEPAGE) ||
+	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
+		return false;
 	if (shmem_huge == SHMEM_HUGE_FORCE)
 		return true;
 	if (shmem_huge == SHMEM_HUGE_DENY)
-- 
1.8.3.1