On Thu, 11 Nov 2010 18:54:55 +1100 Nick Piggin <npiggin@xxxxxxxxx> wrote:
> Testing ->mapping and ->index without a ref is not stable as the page
> may have been reused at this point.
>
> Signed-off-by: Nick Piggin <npiggin@xxxxxxxxx>
> ---
> mm/filemap.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c 2010-11-11 18:51:51.000000000 +1100
> +++ linux-2.6/mm/filemap.c 2010-11-11 18:51:52.000000000 +1100
> @@ -835,9 +835,6 @@ unsigned find_get_pages_contig(struct ad
> if (radix_tree_deref_retry(page))
> goto restart;
>
> - if (page->mapping == NULL || page->index != index)
> - break;
> -
> if (!page_cache_get_speculative(page))
> goto repeat;
>
> @@ -847,6 +844,16 @@ unsigned find_get_pages_contig(struct ad
> goto repeat;
> }
>
> + /*
> + * must check mapping and index after taking the ref.
> + * otherwise we can get both false positives and false
> + * negatives, which is just confusing to the caller.
> + */
> + if (page->mapping == NULL || page->index != index) {
> + page_cache_release(page);
> + break;
> + }
> +
Dumb question: if it's been "reused" then what prevents the page from
having a non-NULL ->mapping and a matching index?
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