linux-kernel - Re: [RFC PATCH 54/61] afs: Wait on PG_fscache before modifying/releasing a page

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Date:   Tue, 5 May 2020 04:59:46 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Jeff Layton <jlayton@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        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: [RFC PATCH 54/61] afs: Wait on PG_fscache before
 modifying/releasing a page

On Mon, May 04, 2020 at 06:15:39PM +0100, David Howells wrote:
> PG_fscache is going to be used to indicate that a page is being written to
> the cache, and that the page should not be modified or released until it's
> finished.
> 
> Make afs_invalidatepage() and afs_releasepage() wait for it.

Well, why?  Keeping a refcount on the page will prevent it from going
away while it's being written to storage.  And the fact that it's
being written to this cache is no reason to delay the truncate of a file
(is it?)  Similarly, I don't see why we need to wait for the page to make
it to the cache before we start to modify it.  Certainly we'll need to
re-write it to the cache since the cache is now stale, but why should
we wait for the now-stale write to complete?