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    SubjectRe: Dirtiable inode bdi default != sb bdi btrfs
    On Wed 29-09-10 10:19:36, Christoph Hellwig wrote:
    > ---
    > From: Christoph Hellwig <hch@lst.de>
    > Subject: [PATCH] writeback: always use sb->s_bdi for writeback purposes
    >
    ...
    > The one exception for now is the block device filesystem which really
    > wants different writeback contexts for it's different (internal) inodes
    > to handle the writeout more efficiently. For now we do this with
    > a hack in fs-writeback.c because we're so late in the cycle, but in
    > the future I plan to replace this with a superblock method that allows
    > for multiple writeback contexts per filesystem.
    Another exception I know about is mtd_inodefs filesystem
    (drivers/mtd/mtdchar.c).

    > Signed-off-by: Christoph Hellwig <hch@lst.de>
    >
    > Index: linux-2.6/fs/fs-writeback.c
    > ===================================================================
    > --- linux-2.6.orig/fs/fs-writeback.c 2010-09-29 16:58:41.750557721 +0900
    > +++ linux-2.6/fs/fs-writeback.c 2010-09-29 17:11:35.040557719 +0900
    > @@ -72,22 +72,10 @@ int writeback_in_progress(struct backing
    > static inline struct backing_dev_info *inode_to_bdi(struct inode *inode)
    > {
    > struct super_block *sb = inode->i_sb;
    > - struct backing_dev_info *bdi = inode->i_mapping->backing_dev_info;
    >
    > - /*
    > - * For inodes on standard filesystems, we use superblock's bdi. For
    > - * inodes on virtual filesystems, we want to use inode mapping's bdi
    > - * because they can possibly point to something useful (think about
    > - * block_dev filesystem).
    > - */
    > - if (sb->s_bdi && sb->s_bdi != &noop_backing_dev_info) {
    > - /* Some device inodes could play dirty tricks. Catch them... */
    > - WARN(bdi != sb->s_bdi && bdi_cap_writeback_dirty(bdi),
    > - "Dirtiable inode bdi %s != sb bdi %s\n",
    > - bdi->name, sb->s_bdi->name);
    > - return sb->s_bdi;
    > - }
    > - return bdi;
    > + if (strcmp(sb->s_type->name, "bdev") == 0)
    > + return inode->i_mapping->backing_dev_info;
    > + return sb->s_bdi;
    So at least here you'd need also add a similar exception for
    "mtd_inodefs". Because of these exeptions I've chosen the
    (sb->s_bdi && sb->s_bdi != &noop_backing_dev_info) check rather than your
    exception based check. All in all I don't care much what ends up in the
    kernel as it's just a temporary solution...
    Also I've added the warning to catch situations where inodes would get
    filed to a different backing device after the patch. So far the reported
    warnings were harmless but still I'm more comfortable when it's there
    because otherwise we can so easily miss some device-driver-invented
    filesystem like mtd_inodefs which would break silently after the change...

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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