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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
