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 10:55:12 -0500
From:   Jeff Layton <jlayton@...nel.org>
To:     David Howells <dhowells@...hat.com>, linux-cachefs@...hat.com
Cc:     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

On Wed, 2021-12-22 at 23:20 +0000, David Howells wrote:
> Provide a function to be called from a network filesystem's releasepage
> method to indicate that a page has been released that might have been a
> reflection of data upon the server - and now that data must be reloaded
> from the server or the cache.
> 
> This is used to end an optimisation for empty files, in particular files
> that have just been created locally, whereby we know there cannot yet be
> any data that we would need to read from the server or the cache.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: linux-cachefs@...hat.com
> Link: https://lore.kernel.org/r/163819617128.215744.4725572296135656508.stgit@warthog.procyon.org.uk/ # v1
> Link: https://lore.kernel.org/r/163906920354.143852.7511819614661372008.stgit@warthog.procyon.org.uk/ # v2
> Link: https://lore.kernel.org/r/163967128061.1823006.611781655060034988.stgit@warthog.procyon.org.uk/ # v3
> ---
> 
>  include/linux/fscache.h |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/linux/fscache.h b/include/linux/fscache.h
> index 18e725671594..28ce258c1f87 100644
> --- a/include/linux/fscache.h
> +++ b/include/linux/fscache.h
> @@ -607,4 +607,20 @@ static inline void fscache_clear_inode_writeback(struct fscache_cookie *cookie,
>  	}
>  }
>  
> +/**
> + * 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?

--
Jeff Layton <jlayton@...nel.org>