[PATCH v2] mm/filemap: don't allow partially uptodate page for pipes

[Eryu Guan <guaneryu@gmail.com>]

Starting from 4.9-rc1 kernel, I started noticing some test failures
of sendfile(2) and splice(2) (sendfile0N and splice01 from LTP) when
testing on sub-page block size filesystems (tested both XFS and
ext4), these syscalls start to return EIO in the tests. e.g.

sendfile02    1  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 26, got: -1
sendfile02    2  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 24, got: -1
sendfile02    3  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 22, got: -1
sendfile02    4  TFAIL  :  sendfile02.c:133: sendfile(2) failed to return expected value, expected: 20, got: -1

This is because that in sub-page block size cases, we don't need the
whole page to be uptodate, only the part we care about is uptodate
is OK (if fs has ->is_partially_uptodate defined). But
page_cache_pipe_buf_confirm() doesn't have the ability to check the
partially-uptodate case, it needs the whole page to be uptodate. So
it returns EIO in this case.

This is a regression introduced by commit 82c156f85384 ("switch
generic_file_splice_read() to use of ->read_iter()"). Prior to the
change, generic_file_splice_read() doesn't allow partially-uptodate
page either, so it worked fine.

Fix it by skipping the partially-uptodate check if we're working on
a pipe in do_generic_file_read(), so we read the whole page from
disk as long as the page is not uptodate.

Signed-off-by: Eryu Guan <guaneryu@gmail.com>
---

I think the other way to fix it is to add the ability to check & allow
partially-uptodate page to page_cache_pipe_buf_confirm(), but that is much
harder to do and seems gain little.

v2:
- Update summary a little bit
- Update commit log
- Add comment to the code
- Add more people/list to cc

v1: http://marc.info/?l=linux-mm&m=147756897431777&w=2

 mm/filemap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 849f459..670264d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1734,6 +1734,9 @@ static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
 			if (inode->i_blkbits == PAGE_SHIFT ||
 					!mapping->a_ops->is_partially_uptodate)
 				goto page_not_up_to_date;
+			/* pipes can't handle partially uptodate pages */
+			if (unlikely(iter->type & ITER_PIPE))
+				goto page_not_up_to_date;
 			if (!trylock_page(page))
 				goto page_not_up_to_date;
 			/* Did it get truncated before we got the lock? */
-- 
2.7.4

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