Re: [PATCH v2 05/16] NFS: Don't discard readdir results

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2020-11-03 at 10:55 -0500, Benjamin Coddington wrote:
> On 3 Nov 2020, at 10:33, trondmy@kernel.org wrote:
> 
> > From: Trond Myklebust <trond.myklebust@hammerspace.com>
> > 
> > If a readdir call returns more data than we can fit into one page
> > cache page, then allocate a new one for that data rather than
> > discarding the data.
> > 
> > Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> > ---
> >  fs/nfs/dir.c | 46 ++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 42 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > index b4861a33ad60..788c2a2eeaa3 100644
> > --- a/fs/nfs/dir.c
> > +++ b/fs/nfs/dir.c
> > @@ -321,6 +321,26 @@ static void nfs_readdir_page_set_eof(struct
> > page 
> > *page)
> >         kunmap_atomic(array);
> >  }
> > 
> > +static void nfs_readdir_page_unlock_and_put(struct page *page)
> > +{
> > +       unlock_page(page);
> > +       put_page(page);
> > +}
> > +
> > +static struct page *nfs_readdir_page_get_next(struct address_space
> > *mapping,
> > +                                             pgoff_t index, u64
> > cookie)
> > +{
> > +       struct page *page;
> > +
> > +       page = nfs_readdir_page_get_locked(mapping, index, cookie);
> > +       if (page) {
> > +               if (nfs_readdir_page_last_cookie(page) == cookie)
> > +                       return page;
> > +               nfs_readdir_page_unlock_and_put(page);
> > +       }
> > +       return NULL;
> > +}
> > +
> >  static inline
> >  int is_32bit_api(void)
> >  {
> > @@ -638,13 +658,15 @@ void nfs_prime_dcache(struct dentry *parent, 
> > struct nfs_entry *entry,
> >  }
> > 
> >  /* Perform conversion from xdr to cache array */
> > -static
> > -int nfs_readdir_page_filler(nfs_readdir_descriptor_t *desc, struct
> > nfs_entry *entry,
> > -                               struct page **xdr_pages, struct
> > page *page, unsigned int buflen)
> > +static int nfs_readdir_page_filler(struct nfs_readdir_descriptor 
> > *desc,
> > +                                  struct nfs_entry *entry,
> > +                                  struct page **xdr_pages,
> > +                                  struct page *fillme, unsigned
> > int buflen)
> >  {
> > +       struct address_space *mapping = desc->file->f_mapping;
> >         struct xdr_stream stream;
> >         struct xdr_buf buf;
> > -       struct page *scratch;
> > +       struct page *scratch, *new, *page = fillme;
> >         int status;
> > 
> >         scratch = alloc_page(GFP_KERNEL);
> > @@ -667,6 +689,19 @@ int 
> > nfs_readdir_page_filler(nfs_readdir_descriptor_t *desc, struct 
> > nfs_entry *en
> >                                         desc->dir_verifier);
> > 
> >                 status = nfs_readdir_add_to_array(entry, page);
> > +               if (status != -ENOSPC)
> > +                       continue;
> > +
> > +               if (page->mapping != mapping)
> > +                       break;
> 
> ^^ How can this happen?
> 

We call the same routines from uncached_readdir(), so the above is
really there in order to exit when we see those anonymous pages.

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