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From:   David Howells <dhowells@redhat.com>
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To:     "J. Bruce Fields" <bfields@fieldses.org>
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Subject: Re: [RFC][PATCH 00/25] Network fs helper library & fscache kiocb API
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Date:   Thu, 21 Jan 2021 20:08:24 +0000
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J. Bruce Fields <bfields@fieldses.org> wrote:

> > J. Bruce Fields <bfields@fieldses.org> wrote:
> > =

> > > > Fixing this requires a much bigger overhaul of cachefiles than thi=
s 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 fscach=
e,
> > > 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 ca=
ses
> > > 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 var=
y
> > between kernel versions and fsck or filesystem packing utility version=
s.
> =

> 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, ext=
4
and btrfs might insert/remove blocks of zeros, but I'm told it can happen.

David