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SubjectRe: [PATCH] mm: thp: fix false negative of shmem vma's THP eligibility
On Tue 23-04-19 06:24:53, 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.

Hmm, I was under impression that thw global sysfs is not anonymous
memory specific and it overrides whatever sysfs comes with. Isn't
ignoring the global setting a bug in the shmemfs allocation paths?
Kirill what is the actual semantic here?

> 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.

Even if I am wrong about the /sys/kernel/mm/transparent_hugepage/enabled
being the global setting for _all_ THP then this patch is not sufficient
because it doesn't reflect VM_NOHUGEPAGE.
>
> Fixes: 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each vma")
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> ---
> mm/huge_memory.c | 4 ++--
> mm/shmem.c | 2 ++
> 2 files changed, 4 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..be15e9b 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3873,6 +3873,8 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
> loff_t i_size;
> pgoff_t off;
>
> + if (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
>

--
Michal Hocko
SUSE Labs

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