linux-kernel - Re: [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion

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Message-ID: <CAHpGcMJTPfJ3M8SjMgv88xkbJLy8mhXTGEkWy8WjXM0NBcsZ1A@mail.gmail.com>
Date:   Fri, 20 Aug 2021 16:43:34 +0200
From:   Andreas Grünbacher <andreas.gruenbacher@...il.com>
To:     Steven Whitehouse <swhiteho@...hat.com>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        ocfs2-devel@....oracle.com
Subject: Re: [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock
 holder auto-demotion

Am Fr., 20. Aug. 2021 um 15:49 Uhr schrieb Steven Whitehouse
<swhiteho@...hat.com>:
> We always used to manage to avoid holding fs locks when copying to/from userspace
> to avoid these complications.

I realize the intent, but that goal has never actually been achieved.
Direct I/O has *always* been calling get_user_pages() while holding
the inode glock in deferred mode.

The situation is slightly different for buffered I/O, but we have the
same problem there at least since switching to iomap. (And it's a
fundamental property of iomap that we want to hold the inode glock
across multi-page mappings, not an implementation deficit.)

Here's a slightly outdated version of a test case that makes all
versions of gfs2 prior to this patch queue unhappy:
https://lore.kernel.org/fstests/20210531152604.240462-1-agruenba@redhat.com/

Thanks,
Andreas