linux-ext4 - Re: [PATCH 03/12] filemap: update ki_pos in generic_perform

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Message-ID: <20230828140202.GX3390869@ZenIV>
Date:   Mon, 28 Aug 2023 15:02:02 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
        Xiubo Li <xiubli@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>,
        Christian Brauner <brauner@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Damien Le Moal <dlemoal@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-block@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH 03/12] filemap: update ki_pos in generic_perform_write

On Mon, Aug 28, 2023 at 02:30:23PM +0200, Christoph Hellwig wrote:
> On Sun, Aug 27, 2023 at 08:41:22PM +0100, Al Viro wrote:
> > That part is somewhat fishy - there's a case where you return a positive value
> > and advance ->ki_pos by more than that amount.  I really wonder if all callers
> > of ->write_iter() are OK with that.  Consider e.g. this:
> 
> This should not exist in the latest version merged by Jens.  Can you
> check if you still  see issues in the version in the block tree or
> linux-next.

Still does - the problem has migrated into direct_write_fallback(), but
that hadn't changed the situation.  We are still left with ->ki_pos bumped
by generic_perform_write() (evaluated as an argument of direct_write_fallback()
now) and *not* retraced in case when direct_write_fallback() decides to
discard the buffered write result.  Both in -next and in mainline (since
6.5-rc1).