On Fri, Jun 19, 2020 at 6:52 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > This patch lifts the IOCB_CACHED idea expressed by Andreas to the VFS. > The advantage of this patch is that we can avoid taking any filesystem > lock, as long as the pages being accessed are in the cache (and we don't > need to readahead any pages into the cache). We also avoid an indirect > function call in these cases. > > I'm sure reusing the name call_read_iter() is the wrong way to go about > this, but renaming all the callers would make this a larger patch. > I'm happy to do it if something like this stands a chance of being > accepted. > > Compared to Andreas' patch, I removed the -ECANCELED return value. > We can happily return 0 from generic_file_buffered_read() and it's less > code to handle that. I bypass the attempt to read from the page cache > for O_DIRECT reads, and for inodes which have no cached pages. Hopefully > this will avoid calling generic_file_buffered_read() for drivers which > implement read_iter() (although I haven't audited them all to check that > > This could go horribly wrong if filesystems rely on doing work in their > ->read_iter implementation (eg checking i_size after acquiring their > lock) instead of keeping the page cache uptodate. On the other hand, > the ->map_pages() method is already called without locks, so filesystems > should already be prepared for this. > XFS is taking i_rwsem lock in read_iter() for a surprising reason: https://lore.kernel.org/linux-xfs/CAOQ4uxjpqDQP2AKA8Hrt4jDC65cTo4QdYDOKFE-C3cLxBBa6pQ@xxxxxxxxxxxxxx/ In that post I claim that ocfs2 and cifs also do some work in read_iter(). I didn't go back to check what, but it sounds like cache coherence among nodes. So filesystems will need to opt-in to this behavior. I wonder if we should make this behavior also opt-in by userspace, for example, RWF_OPPORTUNISTIC_CACHED. Because if I am not mistaken, even though this change has a potential to improve many workloads, it may also degrade some workloads in cases where case readahead is not properly tuned. Imagine reading a large file and getting only a few pages worth of data read on every syscall. Or did I misunderstand your patch's behavior in that case? Another up side of user opt-in flag - it can be used to mitigate the objection of XFS developers against changing the "atomic write vs. read" behavior. New flag - no commitment to an XFS specific behavior that nobody knows if any application out there relies on. Thanks, Amir.
