Messages in this thread | | | Date | Fri, 1 Feb 2019 16:13:55 +1100 | From | Dave Chinner <> | Subject | Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O |
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On Thu, Jan 31, 2019 at 09:54:16AM -0800, Linus Torvalds wrote: > On Thu, Jan 31, 2019 at 2:23 AM Michal Hocko <mhocko@kernel.org> wrote: > > > > OK, I guess my question was not precise. What does prevent taking fs > > locks down the path? > > IOCB_NOWAIT has never meant that, and will never mean it.
I think you're wrong, Linus. IOCB_NOWAIT was specifically designed to prevent blocking on filesystem locks during AIO submission. The initial commits spell that out pretty clearly:
commit b745fafaf70c0a98a2e1e7ac8cb14542889ceb0e Author: Goldwyn Rodrigues <rgoldwyn@suse.com> Date: Tue Jun 20 07:05:43 2017 -0500
fs: Introduce RWF_NOWAIT and FMODE_AIO_NOWAIT
RWF_NOWAIT informs kernel to bail out if an AIO request will block for reasons such as file allocations, or a writeback triggered, or would block while allocating requests while performing direct I/O.
RWF_NOWAIT is translated to IOCB_NOWAIT for iocb->ki_flags.
FMODE_AIO_NOWAIT is a flag which identifies the file opened is capable of returning -EAGAIN if the AIO call will block. This must be set by supporting filesystems in the ->open() call.
Filesystems xfs, btrfs and ext4 would be supported in the following patches.
Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
commit 29a5d29ec181ebdc98a26cedbd76ce9870248892 Author: Goldwyn Rodrigues <rgoldwyn@suse.com> Date: Tue Jun 20 07:05:48 2017 -0500
xfs: nowait aio support
If IOCB_NOWAIT is set, bail if the i_rwsem is not lockable immediately.
IF IOMAP_NOWAIT is set, return EAGAIN in xfs_file_iomap_begin if it needs allocation either due to file extension, writing to a hole, or COW or waiting for other DIOs to finish.
Return -EAGAIN if we don't have extent list in memory.
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
commit 728fbc0e10b7f3ce2ee043b32e3453fd5201c055 Author: Goldwyn Rodrigues <rgoldwyn@suse.com> Date: Tue Jun 20 07:05:47 2017 -0500
ext4: nowait aio support
Return EAGAIN if any of the following checks fail for direct I/O: + i_rwsem is lockable + Writing beyond end of file (will trigger allocation) + Blocks are not allocated at the write location
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com> Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> We will never give user space those kinds of guarantees. We do locking > for various reasons. For example, we'll do the mm lock just when > fetching/storing data from/to user space if there's a page fault.
You are conflating "best effort non-blocking operation" with "atomic guarantee". RWF_NOWAIT/IOCB_NOWAIT is the former, not the latter.
i.e. RWF_NOWAIT addresses the "every second IO submission blocks" problems that AIO submission suffered from due to filesystem lock contention, not the rare and unusual things like "page fault during get_user_pages in direct IO submission". Maybe one day, but right now those rare cases are not pain points for applications that require nonblock AIO submission via RWF_NOWAIT.
> Or - > more obviously - we'll also check for - and sleep on - mandatory locks > in rw_verify_area().
Well, only if you don't use fcntl(O_NONBLOCK) on the file to tell mandatory locking to fail with -EAGAIN instead of sleeping.
-Dave. -- Dave Chinner david@fromorbit.com
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