Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O
From: Michal Hocko <mhocko@kernel.org>
Date: 2019-01-31 09:56:52
Also in:
linux-fsdevel, linux-mm, lkml
[Cc fs-devel] On Wed 30-01-19 13:44:19, Vlastimil Babka wrote:
From: Jiri Kosina <redacted> 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.
I guess the current wording doesn't disallow background IO to be triggered for EAGAIN case. I am not sure whether that breaks clever applications which try to perform larger IO for those cases though.
quoted hunk ↗ jump to hunk
Originally-by: Linus Torvalds [off-list ref] Cc: Dominique Martinet <asmadeus@codewreck.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Dave Chinner <david@fromorbit.com> Cc: Kevin Easton <redacted> Cc: Matthew Wilcox <willy@infradead.org> Cc: Cyril Hrubis <chrubis@suse.cz> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill A. Shutemov <redacted> Cc: Daniel Gruss <redacted> Signed-off-by: Jiri Kosina <redacted> Signed-off-by: Vlastimil Babka <redacted> --- 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);
Maybe a stupid question but I am not really familiar with this path but what exactly does prevent a sync read down page_cache_sync_readahead path? -- Michal Hocko SUSE Labs