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

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On 4/23/19 6:43 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

But how does this happen in the first place?
In __handle_mm_fault() we do:

        if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) {
                ret = create_huge_pmd(&vmf);
                if (!(ret & VM_FAULT_FALLBACK))
                        return ret;

And __transparent_hugepage_enabled() checks the global THP settings.
If THP is not enabled / is only for madvise and the vma is not madvised,
then this should fail, and also khugepaged shouldn't either run at all,
or don't do its job for such non-madvised vma.

What am I missing?

> ...
> [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@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)
>  {
>  	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)
> 




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