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. -chris -- 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
