[lkml]   [2014]   [Dec]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 2/8] swap: lock i_mutex for swap_writepage direct_IO
On Mon, Dec 15, 2014 at 08:56:15AM -0800, Christoph Hellwig wrote:
> On Mon, Dec 15, 2014 at 05:27:05PM +0100, Jan Kara wrote:
> > On Sun 14-12-14 21:26:56, Omar Sandoval wrote:
> > > The generic write code locks i_mutex for a direct_IO. Swap-over-NFS
> > > doesn't grab the mutex because nfs_direct_IO doesn't expect i_mutex to
> > > be held, but most direct_IO implementations do.
> > I think you are speaking about direct IO writes only, aren't you? For DIO
> > reads we don't hold i_mutex AFAICS. And also for DIO writes we don't
> > necessarily hold i_mutex - see for example XFS which doesn't take i_mutex
> > for direct IO writes. It uses it's internal rwlock for this (see
> > xfs_file_dio_aio_write()). So I think this is just wrong.
> The problem is that the use of ->direct_IO by the swap code is a gross
> layering violation. ->direct_IO is a callback for the filesystem, and
> the swap code need to call ->read_iter instead of ->readpage and
> ->write_tier instead of ->direct_IO, and leave the locking to the
> filesystem.

The thing is, ->read_iter() and ->write_iter() might decide to fall back to
buffered IO path. XFS is unusual in that respect - there O_DIRECT ends up
with short write in such case. Other filesystems, OTOH...

 \ /
  Last update: 2014-12-20 08:21    [W:1.135 / U:0.340 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site