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

[J. Bruce Fields]

On Thu, Jan 21, 2021 at 06:55:13PM +0000, David Howells wrote:
> J. Bruce Fields <bfields@xxxxxxxxxxxx> 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?

> 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?

--b.