Re: [PATCH] mm/huge_memory: Clean up typo when copying user highpage

Johannes Weiner
2011-10-12T15:56:52Z

On Wed, Oct 12, 2011 at 10:39:36PM +0800, Hillf Danton wrote:
> Hi Andrea
>
> When copying user highpage, the PAGE_SHIFT in the third parameter is a typo,
> I think, and is replaced with PAGE_SIZE.

This is a pretty nasty data corruption bug, so 'clean up' might be a
bit of an understatement ;-)

Nice catch.

Would you mind extending the changelog to include a problem
description? Feel free to steal from this:

The THP copy-on-write handler falls back to regular-sized
pages for a huge page replacement upon allocation failure or
if THP has been individually disabled in the target VMA. The
loop responsible for copying page-sized chunks accidentally
uses multiples of PAGE_SHIFT instead of PAGE_SIZE as the byte
offset into the original huge page, though, and the
COW-breaking task ends up with a corrupt copy of the data.

> Signed-off-by: Hillf Danton <dhillf@gmail.com>

Acked-by: Johannes Weiner <jweiner@redhat.com>

> --- a/mm/huge_memory.c Sat Aug 13 11:45:14 2011
> +++ b/mm/huge_memory.c Wed Oct 12 22:26:15 2011
> @@ -829,7 +829,7 @@ static int do_huge_pmd_wp_page_fallback(
>
> for (i = 0; i < HPAGE_PMD_NR; i++) {
> copy_user_highpage(pages[i], page + i,
> - haddr + PAGE_SHIFT*i, vma);
> + haddr + PAGE_SIZE * i, vma);
> __SetPageUptodate(pages[i]);
> cond_resched();
> }