On Fri, Mar 27, 2009 at 01:03:27AM +0800, Jeff Layton wrote: > On Wed, 25 Mar 2009 22:16:18 +0800 > Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote: > > > > > > > Actually, I think you were right. We still have this check in > > > generic_sync_sb_inodes() even with Wu's January 2008 patches: > > > > > > /* Was this inode dirtied after sync_sb_inodes was called? */ > > > if (time_after(inode->dirtied_when, start)) > > > break; > > > > Yeah, ugly code. Jens' per-bdi flush daemons should eliminate it... > > > > I had a look over Jens' patches and they seem to be more concerned with > how the queues and daemons are organized (per-bdi rather than per-sb). > The actual way that inodes flow between the queues and get written out > don't look like they really change with his set. OK, sorry that I've not carefully reviewed the per-bdi flushing patchset. > They also don't eliminate the problematic check above. Regardless of > whether your or Jens' patches make it in, I think we'll still need > something like the following (untested) patch. > > If this looks ok, I'll flesh out the comments some and "officially" post > it. Thoughts? It's good in itself. However with more_io_wait queue, the first two chunks will be eliminated. Mind I carry this patch with my patchset? Thanks, Fengguang > --------------[snip]----------------- > > >From d10adff2d5f9a15d19c438119dbb2c410bd26e3c Mon Sep 17 00:00:00 2001 > From: Jeff Layton <jlayton@xxxxxxxxxx> > Date: Thu, 26 Mar 2009 12:54:52 -0400 > Subject: [PATCH] writeback: guard against jiffies wraparound on inode->dirtied_when checks > > 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. This value is 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. But 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 > however: > > /* 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 and > nothing will unwedge it until jiffies moves out of the problematic > window. > > This patch fixes this problem by changing the time_after checks against > dirtied_when to also check whether dirtied_when appears to be in the > future. If it does, then we consider the value to be in the past. > > This should shrink the problematic window to such a small period as not > to matter. > > Signed-off-by: Jeff Layton <jlayton@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; > > /* Is another pdflush already flushing this queue? */ > -- > 1.5.5.6 -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
