Has anyone looked into whether it might make sense to have have a lock which which blocks vmtruncate() from racing against the writeback code? Arguments in favor of the status quo is that if the inode happens to be undergoing writeback, and we know we're about to truncate the sucker, it's best if we can stop the writeback ASAP and avoid the wasted work. However, modern file systems have to do multiple pass against the page cache; first to find out how many dirty pages are present so that the delayed allocation can be efficiently targetted against the free space on disk; then the file system has to allocate the necessary extent, and the finally the buffer heads attached to the pages need to be populated and the page sent out for writeback. However what if while this is happening, on another CPU, some of the pages are truncated and then (perhaps on a 3rd CPU) the pages are written to again once again subject to delayed allocation? Right now, ext4 doesn't protect against this; I'm working on it fixing it. But fixing it in the file system, while possible, is nasty, since it means that we have to constantly recheck against file systems get_blocks() filesystem against every single page, after locking each page, just in case the we happen to be racing against a truncate. I suppose I could put in a lock to prevent the fs-level truncate from completing until the inode's writeback is complete, but then it's still possible that we will have allocated too much space (since the truncate happened right after we finished counting out the delalloc pages), and we need to make sure extent is marked uninitalized lest we crash right after the allocate and before the truncate takes place. Ugh! I'm prety sure I can make it work, but it won't necessarily be the fastest thing in the world, and it will require taking a bunch of extra locks, some of them for every single page being written. It would be simpler to simply add a mutual exclusion between truncate and writeback; which has a downside, as I've acknowledged --- but which case is more common and thus worth optimizing for? The normal writeback case, or being able to avoid some extra disk writes in the case where the truncate is issued exactly while the write back code is processing the inode? Any thoughts or suggestions would be greatly appreciated. I've looked at the xfs and btrfs code for some ideas, but dealing with current writeback and truncate is nasty, especially if there's a subsequent delalloc write happening in parallel with the writeback and immediately after the truncate. After studying the code quite extensively over the weekend, I'm still not entirely sure that XFS and btrfs gets this case right (I know ext4 currently doesn't). Of course, it's not clear whether users will trip against this in practice, but it's nevertheless still a botch, and I'm wondering if it's simpler to avoid the concurrent vmtruncate/writeback case entirely. - Ted -- 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
