On Wed, Sep 08, 2021 at 06:26:48PM +0800, 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.
> 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.
>
> Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> Signed-off-by: Liu Yuntao <liuyuntao10@xxxxxxxxxx>
> ---
> mm/shmem.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 88742953532c..5747572859d1 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -490,7 +490,7 @@ 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);
> i_size = round_up(i_size_read(inode), PAGE_SIZE);
> if (i_size >= HPAGE_PMD_SIZE && (i_size >> PAGE_SHIFT) >= index)
With the change, the condition can be simplified to
if (i_size >> PAGE_SHIFT >= index)
right?
> return true;
> --
> 2.23.0
>
--
Kirill A. Shutemov
