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Subjectcleancache followup from LSF10/MM summit
Hi Christophe (and others interested in cleancache progress) --

Thanks for taking some time to talk with me about cleancache
at LSF summit! You had some interesting thoughts and suggestions
that I said I would investigate. They are:

1) use inode kva as key instead of i_ino
2) eliminate cleancache shim and call zcache directly
3) fs's requiring key > inode_t (e.g. 64-bit-fs on 32-bit-kernel)
4) eliminate fs-specific code entirely (e.g. "opt-in")
5) eliminate global variable

Here's my conclusions:

1) You suggested using the inode kva as a "key" for cleancache.
I think your goal was to make it more fs-independent and also
to eliminate the need for using a per-fs enabler and "pool id".
I looked at this but it will not work because cleancache
retains page cache data pages persistently even when the
inode has been pruned from the inode_unused_list and only
flushes the data pages if the file gets removed/truncated. If
cleancache used the inode kva, there would be coherency issues
when the inode kva is reused. Alternately, if cleancache
flushed the pages when the inode kva was freed, much of
the value of cleancache would be lost because the cache
of pages in cleancache is potentially much larger than
the page cache and is most useful if the pages survive
inode cache removal.

If I misunderstood your proposal or if you disagree, please
let me know.

2) You suggested eliminating the cleancache shim layer and just
directly calling zcache, effectively eliminating Xen as
a user. During and after LSF summit, I talked to developers
from Google who are interested in investigating the cleancache
interface for use with cgroups, an IBM developer who was
interested in cleancache for optimizing NUMA, and soon I
will be talking to HP Labs about using it as an interface
for "memory blades". I also think Rik van Riel and Mel Gorman
were intrigued about its use for collecting better memory
utilization statistics to drive guest/host memory "rightsizing".
While it is true that none of these are current users yet, even
if you prefer to ignore Xen tmem as a user, it seems silly to
throw away the cleanly-layered generic cleancache interface now,
only to add it back later when more users are added.

3) You re-emphasized the problem where cleancache's use of
the inode number as a key will cause problems on many 64-bit
filesystems especially running on a 32-bit kernel. With
help from Andreas Dilger, I'm trying to work out a generic
solution for this using s_export_op->encode_fh which would
be used for any fs that provides it to guarantee a unique
multi-word key for a file, while preserving the
shorter i_ino as a key for fs's for which i_ino is unique.

4) Though you were out of the room during the cleancache
lightning talk, other filesystem developers seemed OK
with the "opt-in" approach (as documented in lwn.net)...
one even asked "can't you just add a bit to the superblock?"
to which I answered "that's essentially what the one
line opt-in addition does". Not sure if you are still
objecting to that, but especially given that the 64-bit-fs-on
32-bit-kernel issue above only affects some filesystems,
I'm still thinking it is necessary.

5) You commented (before LSF) that the global variable should
be avoided which is certainly valid, and I will try Nitin's
suggestion to add a registration interface.

Did I miss anything?

I plan to submit a V4 for cleancache soon, and hope you will
be inclined to ack this time.

Thanks,
Dan


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