linux-kernel - Re: [RFC][PATCH 00/25] Network fs helper library & fscache kiocb API

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Date:   Thu, 21 Jan 2021 18:55:13 +0000
From:   David Howells <dhowells@...hat.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     dhowells@...hat.com, Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Takashi Iwai <tiwai@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        linux-afs@...ts.infradead.org, Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-cachefs@...hat.com, 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 00/25] Network fs helper library & fscache kiocb API

J. Bruce Fields <bfields@...ldses.org> wrote:

> > Fixing this requires a much bigger overhaul of cachefiles than this patchset
> > performs.
> 
> That sounds like "sometimes you may get file corruption and there's
> nothing you can do about it".  But I know people actually use fscache,
> so it must be reliable at least for some use cases.

Yes.  That's true for the upstream code because that uses bmap.  I'm switching
to use SEEK_HOLE/SEEK_DATA to get rid of the bmap usage, but it doesn't change
the issue.

> Is it that those "bridging" blocks only show up in certain corner cases
> that users can arrange to avoid?  Or that it's OK as long as you use
> certain specific file systems whose behavior goes beyond what's
> technically required by the bamp or seek interfaces?

That's a question for the xfs, ext4 and btrfs maintainers, and may vary
between kernel versions and fsck or filesystem packing utility versions.

David