Re: [PATCH v2] fix judgment error in shmem_is_huge()

[Hugh Dickins]

On Thu, 9 Sep 2021, Liu Yuntao wrote:

> In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded
> up correctly. When the page index points to the first page in a huge
> page, round_up() cannot bring it to the end of the huge page, but
> to the end of the previous one.
> 
> an example:
> HPAGE_PMD_NR on my machine is 512(2 MB huge page size).
> After allcoating a 3000 KB buffer, I access it at location 2050 KB.

Your example is certainly helpful, but weird!  It's not impossible,
but wouldn't it be easier to understand if you said "2048 KB" there?

> In shmem_is_huge(), the corresponding index happens to be 512.
> After rounded up by HPAGE_PMD_NR, it will still be 512 which is
> smaller than i_size, and shmem_is_huge() will return true.
> As a result, my buffer takes an additional huge page, and that
> shouldn't happen when shmem_enabled is set to within_size.

A colleague very recently opened my eyes to within_size on shmem_enabled:
I've always been dubious of both, but they can work quite well together.

> 
> Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> Signed-off-by: Liu Yuntao <liuyuntao10@xxxxxxxxxx>

Thanks, with a nice simplification from Kirill.

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

Ignore the comment I've added below - it's not worth worrying about.

> ---
> V1->V2:
> add simplification of the condition after round_up()
> ---
>  mm/shmem.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 88742953532c..b5860f4a2738 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -490,9 +490,9 @@ bool shmem_is_huge(struct vm_area_struct *vma,
>  	case SHMEM_HUGE_ALWAYS:
>  		return true;
>  	case SHMEM_HUGE_WITHIN_SIZE:
> -		index = round_up(index, HPAGE_PMD_NR);
> +		index = round_up(index + 1, HPAGE_PMD_NR);

Even without your change, I notice now that there's a possibility of
index wrapping to 0 on 32-bit architecture here.  But nothing goes
terribly wrong in that case: it is not worth worrying about here.

>  		i_size = round_up(i_size_read(inode), PAGE_SIZE);
> -		if (i_size >= HPAGE_PMD_SIZE && (i_size >> PAGE_SHIFT) >= index)
> +		if (i_size >> PAGE_SHIFT >= index)
>  			return true;
>  		fallthrough;
>  	case SHMEM_HUGE_ADVISE:
> -- 
> 2.23.0