Messages in this thread | | | Date | Thu, 31 Jan 2019 10:56:44 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O |
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[Cc fs-devel]
On Wed 30-01-19 13:44:19, Vlastimil Babka wrote: > From: Jiri Kosina <jkosina@suse.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.
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.
> Originally-by: Linus Torvalds <torvalds@linux-foundation.org> > Cc: Dominique Martinet <asmadeus@codewreck.org> > Cc: Andy Lutomirski <luto@amacapital.net> > Cc: Dave Chinner <david@fromorbit.com> > Cc: Kevin Easton <kevin@guarana.org> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Cyril Hrubis <chrubis@suse.cz> > Cc: Tejun Heo <tj@kernel.org> > Cc: Kirill A. Shutemov <kirill@shutemov.name> > Cc: Daniel Gruss <daniel@gruss.cc> > Signed-off-by: Jiri Kosina <jkosina@suse.cz> > Signed-off-by: Vlastimil Babka <vbabka@suse.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);
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
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