[Cc fs-devel] On Wed 30-01-19 13:44:19, Vlastimil Babka wrote: > From: Jiri Kosina <[email protected]> > > 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 <[email protected]> > Cc: Dominique Martinet <[email protected]> > Cc: Andy Lutomirski <[email protected]> > Cc: Dave Chinner <[email protected]> > Cc: Kevin Easton <[email protected]> > Cc: Matthew Wilcox <[email protected]> > Cc: Cyril Hrubis <[email protected]> > Cc: Tejun Heo <[email protected]> > Cc: Kirill A. Shutemov <[email protected]> > Cc: Daniel Gruss <[email protected]> > Signed-off-by: Jiri Kosina <[email protected]> > Signed-off-by: Vlastimil Babka <[email protected]> > --- > 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
