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. Cheers, Andreas -- 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
