Re: [PATCH 4/4] NFS: Fix fscache read from NFS after cache error

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2021-06-28 at 17:12 -0400, David Wysochanski wrote:
> On Mon, Jun 28, 2021 at 3:09 PM Trond Myklebust
> <trondmy@hammerspace.com> wrote:
> > 
> > On Mon, 2021-06-28 at 13:39 -0400, Dave Wysochanski wrote:
> > > Earlier commits refactored some NFS read code and removed
> > > nfs_readpage_async(), but neglected to properly fixup
> > > nfs_readpage_from_fscache_complete().  The code path is
> > > only hit when something unusual occurs with the cachefiles
> > > backing filesystem, such as an IO error or while a cookie
> > > is being invalidated.
> > > 
> > > Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
> > > ---
> > >  fs/nfs/fscache.c | 14 ++++++++++++--
> > >  1 file changed, 12 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/nfs/fscache.c b/fs/nfs/fscache.c
> > > index c4c021c6ebbd..d308cb7e1dd4 100644
> > > --- a/fs/nfs/fscache.c
> > > +++ b/fs/nfs/fscache.c
> > > @@ -381,15 +381,25 @@ static void
> > > nfs_readpage_from_fscache_complete(struct page *page,
> > >                                                void *context,
> > >                                                int error)
> > >  {
> > > +       struct nfs_readdesc desc;
> > > +       struct inode *inode = page->mapping->host;
> > > +
> > >         dfprintk(FSCACHE,
> > >                  "NFS: readpage_from_fscache_complete
> > > (0x%p/0x%p/%d)\n",
> > >                  page, context, error);
> > > 
> > > -       /* if the read completes with an error, we just unlock
> > > the
> > > page and let
> > > -        * the VM reissue the readpage */
> > >         if (!error) {
> > >                 SetPageUptodate(page);
> > >                 unlock_page(page);
> > > +       } else {
> > > +               desc.ctx = context;
> > > +               nfs_pageio_init_read(&desc.pgio, inode, false,
> > > +                                   
> > > &nfs_async_read_completion_ops);
> > > +               error = readpage_async_filler(&desc, page);
> > > +               if (error)
> > > +                       return;
> > 
> > This code path can clearly fail too. Why can we not fix this code
> > to
> > allow it to return that reported error so that we can handle the
> > failure case in nfs_readpage() instead of dead-ending here?
> > 
> 
> Maybe the below patch is what you had in mind?  That way if fscache
> is enabled, nfs_readpage() should behave the same way as if it's not,
> for the case where an IO error occurs in the NFS read completion
> path.
> 
> If we call into fscache and we get back that the IO has been
> submitted,
> wait until it is completed, so we'll catch any IO errors in the read
> completion
> path.  This does not solve the "catch the internal errors", IOW, the
> ones that show up as pg_error, that will probably require copying
> pg_error into nfs_open_context.error field.
> 
> diff --git a/fs/nfs/read.c b/fs/nfs/read.c
> index 78b9181e94ba..28e3318080e0 100644
> --- a/fs/nfs/read.c
> +++ b/fs/nfs/read.c
> @@ -357,13 +357,13 @@ int nfs_readpage(struct file *file, struct page
> *page)
>         } else
>                 desc.ctx =
> get_nfs_open_context(nfs_file_open_context(file));
> 
> +       xchg(&desc.ctx->error, 0);
>         if (!IS_SYNC(inode)) {
>                 ret = nfs_readpage_from_fscache(desc.ctx, inode,
> page);
>                 if (ret == 0)
> -                       goto out;
> +                       goto out_wait;
>         }
> 
> -       xchg(&desc.ctx->error, 0);
>         nfs_pageio_init_read(&desc.pgio, inode, false,
>                              &nfs_async_read_completion_ops);
> 
> @@ -373,6 +373,7 @@ int nfs_readpage(struct file *file, struct page
> *page)
> 
>         nfs_pageio_complete_read(&desc.pgio);
>         ret = desc.pgio.pg_error < 0 ? desc.pgio.pg_error : 0;
> +out_wait:
>         if (!ret) {
>                 ret = wait_on_page_locked_killable(page);
>                 if (!PageUptodate(page) && !ret)
> 
> 
> 
> 
> > > +
> > > +               nfs_pageio_complete_read(&desc.pgio);
> > >         }
> > >  }
> > > 
> > 
> > --
> > Trond Myklebust
> > Linux NFS client maintainer, Hammerspace
> > trond.myklebust@hammerspace.com
> > 
> > 
> 

Yes, please. This avoids that duplication of NFS read code in the
fscache layer.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com