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

From: Yang Shi
Date: Tue Apr 23 2019 - 20:22:55 EST




On 4/23/19 11:34 AM, Yang Shi wrote:


On 4/23/19 10:52 AM, Michal Hocko wrote:
On Wed 24-04-19 00:43:01, Yang Shi wrote:
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:ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 4096 kB
...
[snip]
...
ShmemPmdMapped:ÂÂÂÂ 4096 kB
...
[snip]
...
THPeligible:ÂÂÂ 0

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

ShmemHugePages:ÂÂÂÂ 4096 kB
ShmemPmdMapped:ÂÂÂÂ 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.
Kirill, can we get a confirmation that this is really intended behavior
rather than an omission please? Is this documented? What is a global
knob to simply disable THP system wise?

I have to say that the THP tuning API is one giant mess :/

Btw. this patch also seem to fix khugepaged behavior because it previously
ignored both VM_NOHUGEPAGE and MMF_DISABLE_THP.

Second look shows this is not ignored. hugepage_vma_check() would check this for both anonymous vma and shmem vma before scanning. It is called before shmem_huge_enabled().


Aha, I didn't notice this. It looks we need separate the patch to fix that khugepaged problem for both 5.1-rc and LTS.


Fixes: 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each vma")
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
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)
 {
ÂÂÂÂÂ if (vma_is_anonymous(vma))
ÂÂÂÂÂÂÂÂÂ return __transparent_hugepage_enabled(vma);
-ÂÂÂ if (vma_is_shmem(vma) && shmem_huge_enabled(vma))
-ÂÂÂÂÂÂÂ return __transparent_hugepage_enabled(vma);
+ÂÂÂ if (vma_is_shmem(vma))
+ÂÂÂÂÂÂÂ return shmem_huge_enabled(vma);
 Â 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)
ÂÂÂÂÂ loff_t i_size;
ÂÂÂÂÂ pgoff_t off;
 + if ((vma->vm_flags & VM_NOHUGEPAGE) ||
+ÂÂÂÂÂÂÂ test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
+ÂÂÂÂÂÂÂ return false;
ÂÂÂÂÂ if (shmem_huge == SHMEM_HUGE_FORCE)
ÂÂÂÂÂÂÂÂÂ return true;
ÂÂÂÂÂ if (shmem_huge == SHMEM_HUGE_DENY)
--
1.8.3.1