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? Thanks Vivek -- 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
