[v3,0/2] Fix false negative of shmem vma's THP eligibility

Message

Yang Shi June 13, 2019, 4:43 a.m. UTC

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 shmem objects should be treated
separately from anonymous 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.

The transhuge_vma_suitable() is needed to check vma, but it was only
available for shmem THP.  The patch 1/2 makes it available for all kind of
THPs and does some code duplication cleanup, so it is made a separate patch.


Changelog:
v3: * Check if vma is suitable for allocating THP per Hugh Dickins
    * Fixed smaps output alignment and documentation per Hugh Dickins
v2: * Check VM_NOHUGEPAGE per Michal Hocko


Yang Shi (2):
      mm: thp: make transhuge_vma_suitable available for anonymous THP
      mm: thp: fix false negative of shmem vma's THP eligibility

 Documentation/filesystems/proc.txt |  4 ++--
 fs/proc/task_mmu.c                 |  3 ++-
 mm/huge_memory.c                   | 11 ++++++++---
 mm/internal.h                      | 25 +++++++++++++++++++++++++
 mm/memory.c                        | 13 -------------
 mm/shmem.c                         |  3 +++
 6 files changed, 40 insertions(+), 19 deletions(-)

Comments

Yang Shi July 15, 2019, 7:49 p.m. UTC | #1

Hi Hugh,


Any comments for this version? Although they have been in -mm tree, they 
didn't make in 5.3 merge window, I'm supposed Andrew needs ack from you 
or someone else.


Thanks,

Yang



On 6/12/19 9:43 PM, 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:               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 shmem objects should be treated
> separately from anonymous 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.
>
> The transhuge_vma_suitable() is needed to check vma, but it was only
> available for shmem THP.  The patch 1/2 makes it available for all kind of
> THPs and does some code duplication cleanup, so it is made a separate patch.
>
>
> Changelog:
> v3: * Check if vma is suitable for allocating THP per Hugh Dickins
>      * Fixed smaps output alignment and documentation per Hugh Dickins
> v2: * Check VM_NOHUGEPAGE per Michal Hocko
>
>
> Yang Shi (2):
>        mm: thp: make transhuge_vma_suitable available for anonymous THP
>        mm: thp: fix false negative of shmem vma's THP eligibility
>
>   Documentation/filesystems/proc.txt |  4 ++--
>   fs/proc/task_mmu.c                 |  3 ++-
>   mm/huge_memory.c                   | 11 ++++++++---
>   mm/internal.h                      | 25 +++++++++++++++++++++++++
>   mm/memory.c                        | 13 -------------
>   mm/shmem.c                         |  3 +++
>   6 files changed, 40 insertions(+), 19 deletions(-)