On Fri 04-06-10 12:02:43, Dave Chinner wrote: > On Thu, Jun 03, 2010 at 11:46:34AM -0400, Chris Mason wrote: > > On Wed, Jun 02, 2010 at 11:41:21PM +1000, Nick Piggin wrote: > > > Closing the while it is dirty, while it is being written back window > > > still leaves a pretty big window. Also, how do you handle mmap writes? > > > Write protect and checksum the destination page after every store? Or > > > leave some window between when the pagecache is dirtied and when it is > > > written back? So I don't know whether it's worth putting a lot of effort > > > into this case. > > > > So, changing gears to how do we protect filesystem page cache pages > > instead of the generic idea of dif/dix, btrfs crcs just before writing, > > which does leave a pretty big window for the page to get corrupted. > > The storage layer shouldn't care or know about that though, we hand it a > > crc and it makes sure data matching that crc goes to the media. > > I think the only way to get accurate CRCs is to stop modifications > from occurring while the page is under writeback. i.e. when a page > transitions from dirty to writeback we need to unmap any writable > mappings on the page, and then any new modifications (either by the > write() path or through ->fault) need to block waiting for > page writeback to complete before they can proceed... Actually, we already write-protect the page in clear_page_dirty_for_io so the first part already happens. Any filesystem can do wait_on_page_writeback() in its ->page_mkwrite function so even the second part shouldn't be hard. I'm just a bit worried about the performance implications / hidden deadlocks... Also we'd have to wait_on_page_writeback() in ->write_begin function to protect against ordinary writes but that's the easy part... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
