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

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Date:   Mon, 24 Jan 2022 13:22:15 +0000
From:   Mark Hemment <markhemm@...glemail.com>
To:     NeilBrown <neilb@...e.de>
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 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?

Cheers,
Mark