Problems with determining data presence by examining extents?

David Howells
2020-01-14T16:48:43Z

Again with regard to my rewrite of fscache and cachefiles:

https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter

I've got rid of my use of bmap()! Hooray!

However, I'm informed that I can't trust the extent map of a backing file to
tell me accurately whether content exists in a file because:

(a) Not-quite-contiguous extents may be joined by insertion of blocks of
zeros by the filesystem optimising itself. This would give me a false
positive when trying to detect the presence of data.

(b) Blocks of zeros that I write into the file may get punched out by
filesystem optimisation since a read back would be expected to read zeros
there anyway, provided it's below the EOF. This would give me a false
negative.

Is there some setting I can use to prevent these scenarios on a file - or can
one be added?

Without being able to trust the filesystem to tell me accurately what I've
written into it, I have to use some other mechanism. Currently, I've switched
to storing a map in an xattr with 1 bit per 256k block, but that gets hard to
use if the file grows particularly large and also has integrity consequences -
though those are hopefully limited as I'm now using DIO to store data into the
cache.

If it helps, I'm downloading data in aligned 256k blocks and storing data in
those same aligned 256k blocks, so if that makes it easier...

David