Re: [PATCH v7 04/21] NFS: Calculate page offsets algorithmically

[Trond Myklebust <trondmy@hammerspace.com>]

On Thu, 2022-02-24 at 09:15 -0500, Benjamin Coddington wrote:
> On 23 Feb 2022, at 16:12, trondmy@kernel.org wrote:
> 
> > From: Trond Myklebust <trond.myklebust@hammerspace.com>
> > 
> > Instead of relying on counting the page offsets as we walk through
> > the
> > page cache, switch to calculating them algorithmically.
> > 
> > Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> > ---
> >  fs/nfs/dir.c | 18 +++++++++++++-----
> >  1 file changed, 13 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > index 8f17aaebcd77..f2258e926df2 100644
> > --- a/fs/nfs/dir.c
> > +++ b/fs/nfs/dir.c
> > @@ -248,17 +248,20 @@ static const char
> > *nfs_readdir_copy_name(const 
> > char *name, unsigned int len)
> >         return ret;
> >  }
> > 
> > +static size_t nfs_readdir_array_maxentries(void)
> > +{
> > +       return (PAGE_SIZE - sizeof(struct nfs_cache_array)) /
> > +              sizeof(struct nfs_cache_array_entry);
> > +}
> > +
> 
> Why the choice to use a runtime function call rather than the
> compiler's
> calculation?  I suspect that the end result is the same, as the
> compiler
> will optimize it away, but I'm curious if there's a good reason for 
> this.
> 

The comparison is more efficient because no pointer arithmetic is
needed. As you said, the above function always evaluates to a constant,
and the array->size has been pre-calculated.

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