linux-kernel - Re: [PATCH v4 28/68] fscache: Provide a function to note the release of a page

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Date:   Thu, 06 Jan 2022 16:26:57 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        JeffleXu <jefflexu@...ux.alibaba.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 28/68] fscache: Provide a function to note the release of a page

Jeff Layton <jlayton@...nel.org> wrote:

> > +/**
> > + * fscache_note_page_release - Note that a netfs page got released
> > + * @cookie: The cookie corresponding to the file
> > + *
> > + * Note that a page that has been copied to the cache has been released.  This
> > + * means that future reads will need to look in the cache to see if it's there.
> > + */
> > +static inline
> > +void fscache_note_page_release(struct fscache_cookie *cookie)
> > +{
> > +	if (cookie &&
> > +	    test_bit(FSCACHE_COOKIE_HAVE_DATA, &cookie->flags) &&
> > +	    test_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags))
> > +		clear_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags);
> > +}
> > +
> >  #endif /* _LINUX_FSCACHE_H */
> > 
> > 
> 
> Is this logic correct?
> 
> FSCACHE_COOKIE_HAVE_DATA gets set in cachefiles_write_complete, but will
> that ever be called on a cookie that has no data? Will we ever call
> cachefiles_write at all when there is no data to be written?

FSCACHE_COOKIE_NO_DATA_TO_READ is set if we have no data in the cache yet
(ie. the backing file lookup was negative, the file is 0 length or the cookie
got invalidated).  It means that we have no data in the cache, not that the
file is necessarily empty on the server.

FSCACHE_COOKIE_HAVE_DATA is set once we've stored data in the backing file.
From that point on, we have data we *could* read - however, it's covered by
pages in the netfs pagecache until at such time one of those covering pages is
released.

So if we've written data to the cache (HAVE_DATA) and there wasn't any data in
the cache when we started (NO_DATA_TO_READ), it may no longer be true that we
can skip reading from the cache.

Read skipping is done by cachefiles_prepare_read().

Note that I'm not doing tracking on a per-page basis, but only on a per-file
basis.

David