linux-kernel - Re: [PATCH 32/32] NFS: Convert readpage to readahead and use netfs

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Message-ID: <CALF+zOm++OzAebR4wu+Hdf8Aa8GpXZu8Am9eVajVUiDMBJE63w@mail.gmail.com>
Date:   Tue, 26 Jan 2021 14:10:26 -0500
From:   David Wysochanski <dwysocha@...hat.com>
To:     Chuck Lever <chuck.lever@...cle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        David Howells <dhowells@...hat.com>,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jeff Layton <jlayton@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-cachefs <linux-cachefs@...hat.com>,
        "linux-afs@...ts.infradead.org" <linux-afs@...ts.infradead.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>,
        "v9fs-developer@...ts.sourceforge.net" 
        <v9fs-developer@...ts.sourceforge.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 32/32] NFS: Convert readpage to readahead and use
 netfs_readahead for fscache

On Tue, Jan 26, 2021 at 10:25 AM Chuck Lever <chuck.lever@...cle.com> wrote:
>
>
>
> > On Jan 25, 2021, at 8:36 PM, Matthew Wilcox <willy@...radead.org> wrote:
> >
> >
> > For Subject: s/readpage/readpages/
> >
> > On Mon, Jan 25, 2021 at 09:37:29PM +0000, David Howells wrote:
> >> +int __nfs_readahead_from_fscache(struct nfs_readdesc *desc,
> >> +                             struct readahead_control *rac)
> >
> > I thought you wanted it called ractl instead of rac?  That's what I've
> > been using in new code.
> >
> >> -    dfprintk(FSCACHE, "NFS: nfs_getpages_from_fscache (0x%p/%u/0x%p)\n",
> >> -             nfs_i_fscache(inode), npages, inode);
> >> +    dfprintk(FSCACHE, "NFS: nfs_readahead_from_fscache (0x%p/0x%p)\n",
> >> +             nfs_i_fscache(inode), inode);
> >
> > We do have readahead_count() if this is useful information to be logging.
>
> As a sidebar, the Linux NFS community is transitioning to tracepoints.
> It would be helpful (but not completely necessary) to use tracepoints
> in new code instead of printk.
>

The netfs API has a lot of good tracepoints and to be honest I think we can
get rid of fscache's rpcdebug, but let me take a closer look to be
sure.  I didn't use rpcdebug much, if at all, in the latest rounds of debugging.



>
> >> +static inline int nfs_readahead_from_fscache(struct nfs_readdesc *desc,
> >> +                                         struct readahead_control *rac)
> >> {
> >> -    if (NFS_I(inode)->fscache)
> >> -            return __nfs_readpages_from_fscache(ctx, inode, mapping, pages,
> >> -                                                nr_pages);
> >> +    if (NFS_I(rac->mapping->host)->fscache)
> >> +            return __nfs_readahead_from_fscache(desc, rac);
> >>      return -ENOBUFS;
> >> }
> >
> > Not entirely sure that it's worth having the two functions separated any more.
> >
> >>      /* attempt to read as many of the pages as possible from the cache
> >>       * - this returns -ENOBUFS immediately if the cookie is negative
> >>       */
> >> -    ret = nfs_readpages_from_fscache(desc.ctx, inode, mapping,
> >> -                                     pages, &nr_pages);
> >> +    ret = nfs_readahead_from_fscache(&desc, rac);
> >>      if (ret == 0)
> >>              goto read_complete; /* all pages were read */
> >>
> >>      nfs_pageio_init_read(&desc.pgio, inode, false,
> >>                           &nfs_async_read_completion_ops);
> >>
> >> -    ret = read_cache_pages(mapping, pages, readpage_async_filler, &desc);
> >> +    while ((page = readahead_page(rac))) {
> >> +            ret = readpage_async_filler(&desc, page);
> >> +            put_page(page);
> >> +    }
> >
> > I thought with the new API we didn't need to do this kind of thing
> > any more?  ie no matter whether fscache is configured in or not, it'll
> > submit the I/Os.
>
> --
> Chuck Lever
>
>
>