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

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2021-06-28 at 19:46 -0400, David Wysochanski wrote:
> On Mon, Jun 28, 2021 at 5:59 PM Trond Myklebust
> <trondmy@hammerspace.com> wrote:
> > 
> > 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.
> > 
> 
> If you mean patch 4 we still need that - I don't see anyway to
> avoid it.  The above just will make the fscache enabled
> path waits for the IO to complete, same as the non-fscache case.
> 

With the above, you can simplify patch 4/4 to just make the page unlock
unconditional on the error, no?

i.e.
	if (!error)
		SetPageUptodate(page);
	unlock_page(page);

End result: the client just does the same check as before and let's the
vfs/mm decide based on the status of the PG_uptodate flag what to do
next. I'm assuming that a retry won't cause fscache to do another bio
attempt?

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