[PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O

[Vlastimil Babka]

From: Jiri Kosina <jkosina@xxxxxxx>

preadv2(RWF_NOWAIT) can be used to open a side-channel to pagecache contents, as
it reveals metadata about residency of pages in pagecache.

If preadv2(RWF_NOWAIT) returns immediately, it provides a clear "page not
resident" information, and vice versa.

Close that sidechannel by always initiating readahead on the cache if we
encounter a cache miss for preadv2(RWF_NOWAIT); with that in place, probing
the pagecache residency itself will actually populate the cache, making the
sidechannel useless.

Originally-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Kevin Easton <kevin@xxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Cyril Hrubis <chrubis@xxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Cc: Daniel Gruss <daniel@xxxxxxxx>
Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/filemap.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 9f5e323e883e..7bcdd36e629d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2075,8 +2075,6 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 		page = find_get_page(mapping, index);
 		if (!page) {
-			if (iocb->ki_flags & IOCB_NOWAIT)
-				goto would_block;
 			page_cache_sync_readahead(mapping,
 					ra, filp,
 					index, last_index - index);
-- 
2.20.1