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

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2021-06-29 at 11:29 -0400, David Wysochanski wrote:
> On Tue, Jun 29, 2021 at 10:54 AM Trond Myklebust
> <trondmy@hammerspace.com> wrote:
> > 
> > On Tue, 2021-06-29 at 09:20 -0400, David Wysochanski wrote:
> > > On Tue, Jun 29, 2021 at 8:46 AM Trond Myklebust
> > > <trondmy@hammerspace.com> wrote:
> > > > 
> > > > On Tue, 2021-06-29 at 05:17 -0400, David Wysochanski wrote:
> > > > > On Mon, Jun 28, 2021 at 8:39 PM Trond Myklebust
> > > > > <trondmy@hammerspace.com> wrote:
> > > > > > 
> > > > > > 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?
> > > > > > 
> > > > > 
> > > > > Yes I think you're right and I'm following - let me test it
> > > > > and
> > > > > I'll
> > > > > send a v2.
> > > > > Then we can drop patch #3 right?
> > > > > 
> > > > Sounds good. Thanks Dave!
> > > > 
> > > 
> > > This approach works but it differs from the original when an
> > > fscache
> > > error occurs.
> > > The original (see below) would call back into NFS to read from
> > > the
> > > server, but
> > > now we just let the VM handle it.  The VM will re-issue the read,
> > > but
> > > will go back into
> > > fscache again (because it's enabled), which may fail again.
> > 
> > How about marking the page on failure, then? I don't believe we
> > currently use PG_owner_priv_1 (a.k.a. PageOwnerPriv1, PageChecked,
> > PagePinned, PageForeign, PageSwapCache, PageXenRemapped) for
> > anything
> > and according to legend it is supposed to be usable by the fs for
> > page
> > cache pages.
> > 
> > So what say we use SetPageChecked() to mark the page as having
> > failed
> > retrieval from fscache?
> > 
> 
> So this?  I confirm this patch on top of the one I just sent works.
> Want me to merge them together and send a v3?
> 
> Author: Dave Wysochanski <dwysocha@redhat.com>
> Date:   Tue Jun 29 11:10:15 2021 -0400
> 
>     NFS: Mark page with PG_checked if fscache IO completes in error
> 
>     If fscache is enabled and we try to read from fscache, but the
>     IO fails, mark the page with PG_checked.  Then when the VM
>     re-issues the IO, skip over fscache and just read from the
> server.
> 
>     Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
> 
> diff --git a/fs/nfs/fscache.c b/fs/nfs/fscache.c
> index 0966e147e973..687e98b08994 100644
> --- a/fs/nfs/fscache.c
> +++ b/fs/nfs/fscache.c
> @@ -404,10 +404,12 @@ static void
> nfs_readpage_from_fscache_complete(struct page *page,
>                  "NFS: readpage_from_fscache_complete
> (0x%p/0x%p/%d)\n",
>                  page, context, error);
> 
> -       /* if the read completes with an error, unlock the page and
> let
> -        * the VM reissue the readpage */
> +       /* if the read completes with an error, mark the page with
> PG_checked,
> +        * unlock the page, and let the VM reissue the readpage */
>         if (!error)
>                 SetPageUptodate(page);
> +       else
> +               SetPageChecked(page);
>         unlock_page(page);
>  }
> 
> @@ -423,6 +425,11 @@ int __nfs_readpage_from_fscache(struct
> nfs_open_context *ctx,
>                  "NFS: readpage_from_fscache(fsc:%p/p:%p(i:%lx
> f:%lx)/0x%p)\n",
>                  nfs_i_fscache(inode), page, page->index, page-
> >flags, inode);
> 
> +       if (PageChecked(page)) {
> +               ClearPageChecked(page)
> +               return 1;
> +       }
> +
> 
>         ret = fscache_read_or_alloc_page(nfs_i_fscache(inode),
>                                          page,
>                                         
> nfs_readpage_from_fscache_complete,
> 

Yes, but how about just changing the above to:

	if (PageChecked(page))
		return 1;
	SetPageChecked(page);

Then you can short-circuit all further checks in
__nfs_readpage_from_fscache() if they've already failed once.

Note that I don't think it is useful to clear PageChecked() once it has
been set. Once a call to nfs_readpage() succeeds, the page will need to
be evicted from the page cache before we can call nfs_readpage() on it
again.


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