Re: PROBLEM: [REGRESSION 7.1-rc4 -> 7.1-rc5] 9p: silent NUL corruption of file data on cache modes with CACHE_WRITEBACK

[Michael Mulqueen <mike@method-b.uk>]

Thanks all for your replies.


On 11/09/2026 08:16, David Howells wrote:

> Are you actually using this with a cache?  Or just enabling the "use a
> cache" options?

This is my mistake, I hadn't set up cachefiles in my test VM. So yes, 
I'm seeing what you've seen with the reproducer. Apologies for that
misdirection! 

For posterity all the previously tested versions that were CORRUPT, except
v7.3-rc2 which I come to below, do this:

  none             read   0/20  write   0/20  [cache=0x0]
  readahead        read   0/20  write   0/20  [cache=0x1]
  mmap             read  20/20  write  20/20  [cache=0x5]
  loose            read  20/20  write  20/20  [cache=0xf]
  fscache/unbound  read  20/20  write  20/20  [cache=0x8f]
  fscache/bound    read   0/20  write  20/20  [cache=0x8f]

Where fscache/unbound is not having cachefiles properly configured and
fscache/bound is where it's actually active. It's the same reproducer 
as before, just run twice for the fscache case - the difference is 
cachefiles configuration outside the script.

On the 7.3 release candidates, I get a different result:

  none             read   0/20  write   0/20  [cache=0x0]
  readahead        read   0/20  write   0/20  [cache=0x1]
  mmap             read  20/20  write  20/20  [cache=0x5]
  loose            read  20/20  write  20/20  [cache=0xf]
  fscache/unbound  read  20/20  write  20/20  [cache=0x8f]
  fscache/bound    read  20/20  write  20/20  [cache=0x8f]

It appears that cachefiles isn't actually storing anything on 7.3. I've
used the same configuration for every version, cachefiles is active, but
seems to encounter an error when actually storing in 7.3. I haven't had 
time to investigate that. I only mention this for completeness, it's 
not material to fixing this bug, just affects how it presents.

On 11/09/2026 10:25, David Howells wrote:
> Does the attached work for you?

Yes, thanks, I get this on patched 7.2 and on patched 7.3-rc2:

  none             read   0/20  write   0/20  [cache=0x0]
  readahead        read   0/20  write   0/20  [cache=0x1]
  mmap             read   0/20  write   0/20  [cache=0x5]
  loose            read   0/20  write   0/20  [cache=0xf]
  fscache/unbound  read   0/20  write   0/20  [cache=0x8f]
  fscache/bound    read   0/20  write   0/20  [cache=0x8f]

So it looks like it's fixed. I haven't had a chance to try the patch 
on a real workflow, only the reproducer, but I will next week and report 
back.

Cheers,
Mike