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

[David Howells]

J. Bruce Fields <bfields at fieldses.org> wrote:

> On Wed, Jan 20, 2021 at 10:21:24PM +0000, David Howells wrote:
> >      Note that this uses SEEK_HOLE/SEEK_DATA to locate the data available
> >      to be read from the cache.  Whilst this is an improvement from the
> >      bmap interface, it still has a problem with regard to a modern
> >      extent-based filesystem inserting or removing bridging blocks of
> >      zeros.
> 
> What are the consequences from the point of view of a user?

The cache can get both false positive and false negative results on checks for
the presence of data because an extent-based filesystem can, at will, insert
or remove blocks of contiguous zeros to make the extents easier to encode
(ie. bridge them or split them).

A false-positive means that you get a block of zeros in the middle of your
file that very probably shouldn't be there (ie. file corruption); a
false-negative means that we go and reload the missing chunk from the server.

The problem exists in cachefiles whether we use bmap or we use
SEEK_HOLE/SEEK_DATA.  The only way round it is to keep track of what data is
present independently of backing filesystem's metadata.

To this end, it shouldn't (mis)behave differently than the code already there
- except that it handles better the case in which the backing filesystem
blocksize != PAGE_SIZE (which may not be relevant on an extent-based
filesystem anyway if it packs parts of different files together in a single
block) because the current implementation only bmaps the first block in a page
and doesn't probe for the rest.

Fixing this requires a much bigger overhaul of cachefiles than this patchset
performs.

Also, it works towards getting rid of this use of bmap, but that's not user
visible.

David