On Thu, Mar 10, 2011 at 04:43:31PM -0500, Chris Mason wrote: > Excerpts from Vivek Goyal's message of 2011-03-10 16:38:32 -0500: > > On Thu, Mar 10, 2011 at 02:24:07PM -0700, Andreas Dilger wrote: > > > On 2011-03-10, at 2:15 PM, Chris Mason wrote: > > > > Excerpts from Vivek Goyal's message of 2011-03-10 14:41:06 -0500: > > > >> On Thu, Mar 10, 2011 at 02:11:15PM -0500, Vivek Goyal wrote: > > > >>>>> I think the person who dirtied the page can store the information in > > > >>>>> page->private (assuming buffer heads were not generated) and if flusher > > > >>>>> thread later ends up generating buffer heads and ends up modifying > > > >>>>> page->private, this can be copied in buffer heads? > > > >>>> > > > >>>> This scares me a bit. > > > >>>> > > > >>>> As I understand it, fs/ code expects total ownership of page->private. > > > >>>> This adds a responsibility for every user to copy the data through and > > > >>>> store it in the buffer head (or anything else). btrfs seems to do > > > >>>> something entirely different in some cases and store a different kind > > > >>>> of value. > > > >>> > > > >>> If filesystems are using page->private for some other purpose also, then > > > >>> I guess we have issues. > > > >>> > > > >>> I am ccing linux-fsdevel to have some feedback on the idea of trying > > > >>> to store cgroup id of page dirtying thread in page->private and/or buffer > > > >>> head for tracking which group originally dirtied the page in IO controller > > > >>> during writeback. > > > >> > > > >> A quick "grep" showed that btrfs, ceph and logfs are using page->private > > > >> for other purposes also. > > > >> > > > >> I was under the impression that either page->private is null or it > > > >> points to buffer heads for the writeback case. So storing the info > > > >> directly in either buffer head directly or first in page->private and > > > >> then transferring it to buffer heads would have helped. > > > > > > > > Right, btrfs has its own uses for page->private, and we expect to own > > > > it. With a proper callback, the FS could store the extra information you > > > > need in out own structs. > > > > > > There is no requirement that page->private ever points to a > > > buffer_head, and Lustre clients use it for its own tracking > > > structure (never touching buffer_heads at all). Any > > > assumption about what a filesystem is storing in page->private > > > in other parts of the code is just broken. > > > > Andreas, > > > > As Chris mentioned, will providing callbacks so that filesystem > > can save/restore page->private be reasonable? > > Just to clarify, I think saving/restoring page->private is going > to be hard. I'd rather just have a call back that says here's a > page, storage this for the block io controller please, and another > one that returns any previously stored info. Agreed - there is absolutely no guarantee that some other thread doesn't grab the page while it is under writeback and dereference page->private expecting there to be buffer heads or some filesystem specific structure to be there. Hence swapping out the expected structure with something different is problematic. However, I think there's bigger issues. e.g. page->private might point to multiple bufferheads that map to non-contiguous disk blocks that were written by different threads - what happens if we get two concurrent IOs to the one page, perhaps with different cgroup IDs? Further, page->private might not even point to a per-page specific structure - it might point to a structure shared by multiple pages (e.g. an extent map). Adding a callback like this requires filesystems to be able to store per-page or per-block information for external users. Indeed, one of the areas of development in XFS right now is to move away from storing internal per-block/per-page information because of the memory overhead it causes. IMO, if you really need some per-page information, then just put it in the struct page - you can't hide the memory overhead just by having the filesystem to store it for you. That just adds unnecessary complexity... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
