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

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Message-ID: <1856291.1611259704@warthog.procyon.org.uk>
Date:   Thu, 21 Jan 2021 20:08:24 +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:

> > 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.
> 
> Sorry, when you say "that's true", what part are you referring to?

Sometimes, theoretically, you may get file corruption due to this.

> > 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.
> 
> So, I'm still confused: there must be some case where we know fscache
> actually works reliably and doesn't corrupt your data, right?

Using ext2/3, for example.  I don't know under what circumstances xfs, ext4
and btrfs might insert/remove blocks of zeros, but I'm told it can happen.

David