linux-kernel - Re: [PATCH 14/23] NFS: swap IO handling is slightly different for O

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Date:   Thu, 27 Jan 2022 09:51:41 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Mark Hemment" <markhemm@...glemail.com>
Cc:     "Trond Myklebust" <trond.myklebust@...merspace.com>,
        "Anna Schumaker" <anna.schumaker@...app.com>,
        "Chuck Lever" <chuck.lever@...cle.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Mel Gorman" <mgorman@...e.de>,
        "Christoph Hellwig" <hch@...radead.org>,
        "David Howells" <dhowells@...hat.com>, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/23] NFS: swap IO handling is slightly different for O_DIRECT IO

On Tue, 25 Jan 2022, Mark Hemment wrote:
> On Mon, 24 Jan 2022 at 03:53, NeilBrown <neilb@...e.de> wrote:
> >
> > 1/ Taking the i_rwsem for swap IO triggers lockdep warnings regarding
> >    possible deadlocks with "fs_reclaim".  These deadlocks could, I believe,
> >    eventuate if a buffered read on the swapfile was attempted.
> >
> >    We don't need coherence with the page cache for a swap file, and
> >    buffered writes are forbidden anyway.  There is no other need for
> >    i_rwsem during direct IO.  So never take it for swap_rw()
> >
> > 2/ generic_write_checks() explicitly forbids writes to swap, and
> >    performs checks that are not needed for swap.  So bypass it
> >    for swap_rw().
> >
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > ---
> >  fs/nfs/direct.c        |   30 +++++++++++++++++++++---------
> >  fs/nfs/file.c          |    4 ++--
> >  include/linux/nfs_fs.h |    4 ++--
> >  3 files changed, 25 insertions(+), 13 deletions(-)
> >
> ...
> > @@ -943,7 +954,8 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, struct iov_iter *iter)
> >                                               pos >> PAGE_SHIFT, end);
> >         }
> >
> > -       nfs_end_io_direct(inode);
> > +       if (!swap)
> > +               nfs_end_io_direct(inode);
> 
> Just above this code diff, there is;
>     if (mapping->nrpages) {
>         invalidate_inode_pages2_range(mapping,
>              pos >> PAGE_SHIFT, end);
>     }
> 
> This invalidation looks strange/wrong for a NFS swap write.  Should it
> be disabled for the swap case?

Yes, I think it should - particularly as we don't take the mutex in the
swap case.  Thanks!
This change improves the look of the code too :-)

Thanks,
NeilBrown