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

[David Wysochanski <dwysocha@redhat.com>]

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?
>

Because the context of the process calling nfs_readpage() is already
gone - this is called from keventd context coming up from fscache.
So if fscache fails, and then we retry here, and that fails, what is left
to be done?  This _is_ the retry code.  I guess I'm not following.  Do you
want the process to wait inside nfs_readpage() until fscache calls
nfs_readpage_from_fscache_complete?  If so, that would be new as
we have not done this before - this patch was just putting back the hunk I
mistakenly removed in commit 1e83b173b266


nfs_readpage() -> nfs_readpage_from_fscache -> fscache_read_or_alloc_page
* nfs_readpage_from_fscache_complete() will be called by fscache when
IO completes or there's an error
* fscache_read_or_alloc_page() returns 0 saying we've submitted the BIO

nfs_readpage()
...
    if (!IS_SYNC(inode)) {
        ret = nfs_readpage_from_fscache(desc.ctx, inode, page);
        if (ret == 0)
            goto out;
    }


int __nfs_readpage_from_fscache(struct nfs_open_context *ctx,
                struct inode *inode, struct page *page)
{
...
    ret = fscache_read_or_alloc_page(nfs_i_fscache(inode),
                     page,
                     nfs_readpage_from_fscache_complete,
                     ctx,
                     GFP_KERNEL);

   switch (ret) {
    case 0: /* read BIO submitted (page in fscache) */
        dfprintk(FSCACHE,
             "NFS:    readpage_from_fscache: BIO submitted\n");
        nfs_inc_fscache_stats(inode, NFSIOS_FSCACHE_PAGES_READ_OK);
        return ret;




> > +
> > +               nfs_pageio_complete_read(&desc.pgio);
> >         }
> >  }
> >
>
> --
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
>
>