On Tue, 31 Mar 2009 16:33:35 -0700 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Mon, 30 Mar 2009 12:40:08 -0400 > Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > The dirtied_when value on an inode is supposed to represent the first > > time that an inode has one of its pages dirtied. This value is in units > > of jiffies. It's used in several places in the writeback code to > > determine when to write out an inode. > > > > The problem is that these checks assume that dirtied_when is updated > > periodically. If an inode is continuously being used for I/O it can be > > persistently marked as dirty and will continue to age. Once the time > > difference between dirtied_when and the jiffies value it is being > > compared to is greater than or equal to half the maximum of the jiffies > > type, the logic of the time_*() macros inverts and the opposite of what > > is needed is returned. On 32-bit architectures that's just under 25 days > > (assuming HZ == 1000). > > > > As the least-recently dirtied inode, it'll end up being the first one > > that pdflush will try to write out. sync_sb_inodes does this check: > > > > /* Was this inode dirtied after sync_sb_inodes was called? */ > > if (time_after(inode->dirtied_when, start)) > > break; > > > > ...but now dirtied_when appears to be in the future. sync_sb_inodes > > bails out without attempting to write any dirty inodes. When this > > occurs, pdflush will stop writing out inodes for this superblock. > > Nothing can unwedge it until jiffies moves out of the problematic > > window. > > > > This patch fixes this problem by changing the checks against > > dirtied_when to also check whether it appears to be in the future. If it > > does, then we consider the value to be far in the past. > > > > This should shrink the problematic window of time to such a small period > > as not to matter. > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > Acked-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > > Acked-by: Ian Kent <raven@xxxxxxxxxx> > > --- > > fs/fs-writeback.c | 11 +++++++---- > > 1 files changed, 7 insertions(+), 4 deletions(-) > > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > > index e3fe991..dba69a5 100644 > > --- a/fs/fs-writeback.c > > +++ b/fs/fs-writeback.c > > @@ -196,8 +196,9 @@ static void redirty_tail(struct inode *inode) > > struct inode *tail_inode; > > > > tail_inode = list_entry(sb->s_dirty.next, struct inode, i_list); > > - if (!time_after_eq(inode->dirtied_when, > > - tail_inode->dirtied_when)) > > + if (time_before(inode->dirtied_when, > > + tail_inode->dirtied_when) || > > + time_after(inode->dirtied_when, jiffies)) > > inode->dirtied_when = jiffies; > > } > > list_move(&inode->i_list, &sb->s_dirty); > > @@ -231,7 +232,8 @@ static void move_expired_inodes(struct list_head *delaying_queue, > > struct inode *inode = list_entry(delaying_queue->prev, > > struct inode, i_list); > > if (older_than_this && > > - time_after(inode->dirtied_when, *older_than_this)) > > + time_after(inode->dirtied_when, *older_than_this) && > > + time_before_eq(inode->dirtied_when, jiffies)) > > break; > > list_move(&inode->i_list, dispatch_queue); > > } > > @@ -493,7 +495,8 @@ void generic_sync_sb_inodes(struct super_block *sb, > > } > > > > /* Was this inode dirtied after sync_sb_inodes was called? */ > > - if (time_after(inode->dirtied_when, start)) > > + if (time_after(inode->dirtied_when, start) && > > + time_before_eq(inode->dirtied_when, jiffies)) > > break; > > > > It'd be nice to add/update the comments to explain what's going on. > Otherwise it's a wee bit obscure, no? > Thanks for picking this up, Andrew... Good point. I had some comments in the patch that I backported for RHEL5. I'll add some and send a respin tomorrow. Cheers, -- Jeff Layton <jlayton@xxxxxxxxxx> -- 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
