linux-kernel - Re: [PATCH 2/3] afs: Fix afs_write

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Message-ID: <466590.1623677832@warthog.procyon.org.uk>
Date:   Mon, 14 Jun 2021 14:37:12 +0100
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     dhowells@...hat.com, jlayton@...nel.org,
        linux-afs@...ts.infradead.org, ceph-devel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] afs: Fix afs_write_end() to handle short writes

Matthew Wilcox <willy@...radead.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 up 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 pointed
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 endlessly,
but it doesn't appear it will corrupt your data.

David