linux-kernel - [PATCH 2/3] mm/filemap: initiate readahead even if IOCB

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Date:   Wed, 30 Jan 2019 13:44:19 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>, Jiri Kosina <jkosina@...e.cz>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Chinner <david@...morbit.com>,
        Kevin Easton <kevin@...rana.org>,
        Matthew Wilcox <willy@...radead.org>,
        Cyril Hrubis <chrubis@...e.cz>, Tejun Heo <tj@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Daniel Gruss <daniel@...ss.cc>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jiri Kosina <jikos@...nel.org>
Subject: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O

From: Jiri Kosina <jkosina@...e.cz>

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@...ux-foundation.org>
Cc: Dominique Martinet <asmadeus@...ewreck.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Dave Chinner <david@...morbit.com>
Cc: Kevin Easton <kevin@...rana.org>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Cyril Hrubis <chrubis@...e.cz>
Cc: Tejun Heo <tj@...nel.org>
Cc: Kirill A. Shutemov <kirill@...temov.name>
Cc: Daniel Gruss <daniel@...ss.cc>
Signed-off-by: Jiri Kosina <jkosina@...e.cz>
Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 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