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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <YMdZbsvBNYBtZDC2@casper.infradead.org>
References: <YMdZbsvBNYBtZDC2@casper.infradead.org> <162367681795.460125.11729955608839747375.stgit@warthog.procyon.org.uk> <162367682522.460125.5652091227576721609.stgit@warthog.procyon.org.uk>
To:     Matthew Wilcox <willy@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Cc:     dhowells@redhat.com, jlayton@kernel.org,
        linux-afs@lists.infradead.org, ceph-devel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] afs: Fix afs_write_end() to handle short writes
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Date:   Mon, 14 Jun 2021 14:37:12 +0100
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Matthew Wilcox <willy@infradead.org> wrote:

> >  (1) If the page is not up to date, then we should just return 0
> >      (ie. indicating a zero-length copy).  The loop in
> >      generic_perform_write() will go around again, possibly breaking u=
p the
> >      iterator into discrete chunks.
> =

> Does this actually work?  What about the situation where you're reading
> the last page of a file and thus (almost) always reading fewer bytes
> than a PAGE_SIZE?

Al Viro made such a change for Ceph - and we're writing, not reading.

I was thinking that it would break if reading from a pipe, but Jeff pointe=
d
out that the iov_iter_advance() in generic_perform_write() uses the return
value of ->write_end() to advance the iterator.  So it might loop endlessl=
y,
but it doesn't appear it will corrupt your data.

David