Re: [PATCH v4 21/21] NFS: Do uncached readdir when we're seeking a cookie in an empty page cache

Trond Myklebust
2020-11-09T21:46:47Z

On Mon, 2020-11-09 at 16:41 -0500, Benjamin Coddington wrote:
> On 7 Nov 2020, at 9:03, trondmy@kernel.org wrote:
>
> > From: Trond Myklebust <trond.myklebust@hammerspace.com>
> >
> > If the directory is changing, causing the page cache to get
> > invalidated
> > while we are listing the contents, then the NFS client is currently
> > forced
> > to read in the entire directory contents from scratch, because it
> > needs
> > to perform a linear search for the readdir cookie. While this is
> > not
> > an issue for small directories, it does not scale to directories
> > with
> > millions of entries.
> > In order to be able to deal with large directories that are
> > changing,
> > add a heuristic to ensure that if the page cache is empty, and we
> > are
> > searching for a cookie that is not the zero cookie, we just default
> > to
> > performing uncached readdir.
> >
> > Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> > ---
> >  fs/nfs/dir.c | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> >
> > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > index 238872d116f7..d7a9efd31ecd 100644
> > --- a/fs/nfs/dir.c
> > +++ b/fs/nfs/dir.c
> > @@ -917,11 +917,28 @@ static int find_and_lock_cache_page(struct
> > nfs_readdir_descriptor *desc)
> >         return res;
> >  }
> >
> > +static bool nfs_readdir_dont_search_cache(struct
> > nfs_readdir_descriptor *desc)
> > +{
> > +       struct address_space *mapping = desc->file->f_mapping;
> > +       struct inode *dir = file_inode(desc->file);
> > +       unsigned int dtsize = NFS_SERVER(dir)->dtsize;
> > +       loff_t size = i_size_read(dir);
> > +
> > +       /*
> > +        * Default to uncached readdir if the page cache is empty,
> > and
> > +        * we're looking for a non-zero cookie in a large
> > directory.
> > +        */
> > +       return desc->dir_cookie != 0 && mapping->nrpages == 0 &&
> > size >
> > dtsize;
>
> inode size > dtsize is a little hand-wavy.  We have a lot of
> customers
> trying to
> reverse-engineer nfs_readdir() behavior instead of reading the code,
> this
> is sure to drive them crazy.
>
> That said, in the absence of an easy way to make it tunable, I don't
> have
> anything better to suggest.
>
> Reviewed-by: Benjamin Coddington <bcodding@redhat.com>

Right. It is a heuristic, but I would expect that the directory size is
going to be somewhat proportional to the number of RPC calls we need to
perform to read it. That number again is somewhat proportional to the
dtsize.

IOW: The general idea is correct.

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