On Thu, Jan 21, 2021 at 06:55:13PM +0000, David Howells wrote:

J. Bruce Fields [off-list ref] wrote:

quoted

quoted

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.

quoted

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.