Re: fscache: Redesigning the on-disk cache

Dave Chinner
2021-03-08T21:56:52Z

On Mon, Mar 08, 2021 at 09:13:55AM +0000, David Howells wrote:
> Amir Goldstein <amir73il@gmail.com> wrote:
>
> > > (0a) As (0) but using SEEK_DATA/SEEK_HOLE instead of bmap and opening the
> > > file for every whole operation (which may combine reads and writes).
> >
> > I read that NFSv4 supports hole punching, so when using ->bmap() or SEEK_DATA
> > to keep track of present data, it's hard to distinguish between an
> > invalid cached range and a valid "cached hole".
>
> I wasn't exactly intending to permit caching over NFS. That leads to fun
> making sure that the superblock you're caching isn't the one that has the
> cache in it.
>
> However, we will need to handle hole-punching being done on a cached netfs,
> even if that's just to completely invalidate the cache for that file.
>
> > With ->fiemap() you can at least make the distinction between a non existing
> > and an UNWRITTEN extent.
>
> I can't use that for XFS, Ext4 or btrfs, I suspect. Christoph and Dave's
> assertion is that the cache can't rely on the backing filesystem's metadata
> because these can arbitrarily insert or remove blocks of zeros to bridge or
> split extents.

Well, that's not the big problem. The issue that makes FIEMAP
unusable for determining if there is user data present in a file is
that on-disk extent maps aren't exactly coherent with in-memory user
data state.

That is, we can have a hole on disk with delalloc user data in
memory. There's user data in the file, just not on disk. Same goes
for unwritten extents - there can be dirty data in memory over an
unwritten extent, and it won't get converted to written until the
data is written back and the filesystem runs a conversion
transaction.

So, yeah, if you use FIEMAP to determine where data lies in a file
that is being actively modified, you're going get corrupt data
sooner rather than later. SEEK_HOLE/DATA are coherent with in
memory user data, so don't have this problem.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com