Re: [v3 PATCH 2/2] mm: thp: fix false negative of shmem vma's THP eligibility

[Hugh Dickins]

On Thu, 13 Jun 2019, 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.
> 
> Also check if vma is suitable for THP by calling
> transhuge_vma_suitable().
> 
> And minor fix to smaps output format and documentation.
> 
> Fixes: 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each vma")
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>

Thanks,
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
>  Documentation/filesystems/proc.txt | 4 ++--
>  fs/proc/task_mmu.c                 | 3 ++-
>  mm/huge_memory.c                   | 9 +++++++--
>  mm/shmem.c                         | 3 +++
>  4 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 66cad5c..b0ded06 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -477,8 +477,8 @@ replaced by copy-on-write) part of the underlying shmem object out on swap.
>  "SwapPss" shows proportional swap share of this mapping. Unlike "Swap", this
>  does not take into account swapped out page of underlying shmem objects.
>  "Locked" indicates whether the mapping is locked in memory or not.
> -"THPeligible" indicates whether the mapping is eligible for THP pages - 1 if
> -true, 0 otherwise.
> +"THPeligible" indicates whether the mapping is eligible for allocating THP
> +pages - 1 if true, 0 otherwise. It just shows the current status.
>  
>  "VmFlags" field deserves a separate description. This member represents the kernel
>  flags associated with the particular virtual memory area in two letter encoded
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 01d4eb0..6a13882 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -796,7 +796,8 @@ static int show_smap(struct seq_file *m, void *v)
>  
>  	__show_smap(m, &mss);
>  
> -	seq_printf(m, "THPeligible:    %d\n", transparent_hugepage_enabled(vma));
> +	seq_printf(m, "THPeligible:		%d\n",
> +		   transparent_hugepage_enabled(vma));
>  
>  	if (arch_pkeys_enabled())
>  		seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 4bc2552..36f0225 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -65,10 +65,15 @@
>  
>  bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>  {
> +	/* The addr is used to check if the vma size fits */
> +	unsigned long addr = (vma->vm_end & HPAGE_PMD_MASK) - HPAGE_PMD_SIZE;
> +
> +	if (!transhuge_vma_suitable(vma, addr))
> +		return false;
>  	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 1bb3b8d..a807712 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3872,6 +3872,9 @@ 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