The patch titled
Subject: mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED
has been added to the -mm tree. Its filename is
mm-do-not-discard-partial-pages-with-posix_fadv_dontneed.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-do-not-discard-partial-pages-with-posix_fadv_dontneed.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-do-not-discard-partial-pages-with-posix_fadv_dontneed.patch
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------------------------------------------------------
From: Oleg Drokin <green@xxxxxxxxxxxxxx>
Subject: mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED
I noticed that the logic in the fadvise64_64 syscall is incorrect for
partial pages. While first page of the region is correctly skipped if it
is partial, the last page of the region is mistakenly discarded. This
leads to problems for applications that read data in non-page-aligned
chunks discarding already processed data between the reads.
A somewhat misguided application that does something like write(XX bytes
(non-page-alligned)); drop the data it just wrote; repeat gets a
significant penalty in performance as a result.
Link: http://lkml.kernel.org/r/1464917140-1506698-1-git-send-email-green@xxxxxxxxxxxxxx
Signed-off-by: Oleg Drokin <green@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/fadvise.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff -puN mm/fadvise.c~mm-do-not-discard-partial-pages-with-posix_fadv_dontneed mm/fadvise.c
--- a/mm/fadvise.c~mm-do-not-discard-partial-pages-with-posix_fadv_dontneed
+++ a/mm/fadvise.c
@@ -126,6 +126,17 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, l
*/
start_index = (offset+(PAGE_SIZE-1)) >> PAGE_SHIFT;
end_index = (endbyte >> PAGE_SHIFT);
+ if ((endbyte & ~PAGE_MASK) != ~PAGE_MASK) {
+ /* First page is tricky as 0 - 1 = -1, but pgoff_t
+ * is unsigned, so the end_index >= start_index
+ * check below would be true and we'll discard the whole
+ * file cache which is not what was asked.
+ */
+ if (end_index == 0)
+ break;
+
+ end_index--;
+ }
if (end_index >= start_index) {
unsigned long count = invalidate_mapping_pages(mapping,
_
Patches currently in -mm which might be from green@xxxxxxxxxxxxxx are
mm-do-not-discard-partial-pages-with-posix_fadv_dontneed.patch
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