Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O

[Dave Chinner]

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@xxxxxxxxxx> 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@xxxxxxxx>
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@xxxxxx>
    Reviewed-by: Jan Kara <jack@xxxxxxx>
    Signed-off-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>
    Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

commit 29a5d29ec181ebdc98a26cedbd76ce9870248892
Author: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>
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@xxxxxxxx>
    Reviewed-by: Christoph Hellwig <hch@xxxxxx>
    Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
    Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

commit 728fbc0e10b7f3ce2ee043b32e3453fd5201c055
Author: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>
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@xxxxxxxx>
    Reviewed-by: Jan Kara <jack@xxxxxxx>
    Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

> 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@xxxxxxxxxxxxx