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 <[email protected]>
---
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)
return true;
--
2.23.0
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 <[email protected]>
> ---
> 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
On Wed, 8 Sep 2021 17:58:44 +0300, Kirill A. Shutemov wrote:
> 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 <[email protected]>
> > ---
> > 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?
Yes, will add it.
>
> > return true;
> > --
> > 2.23.0
> >
>
> --
> Kirill A. Shutemov