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    SubjectRe: Wrong DIF guard tag on ext2 write
    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@suse.cz>
    SUSE Labs, CR


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