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SubjectRe: [PATCH 02/17] writeback: update dirtied_when for synced inode to prevent livelock
On Fri 20-05-11 05:31:19, Wu Fengguang wrote:
> > > > @@ -419,6 +419,15 @@ writeback_single_inode(struct inode *ino
> > > > spin_lock(&inode->i_lock);
> > > > inode->i_state &= ~I_SYNC;
> > > > if (!(inode->i_state & I_FREEING)) {
> > > > + /*
> > > > + * Sync livelock prevention. Each inode is tagged and synced in
> > > > + * one shot, so we can unconditionally update its dirty time to
> > > > + * prevent syncing it again. Note that time ordering of b_dirty
> > > > + * list will be kept because the following code either removes
> > > > + * the inode from b_dirty or calls redirty_tail().
> > > > + */
> > > > + if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_sync)
> > > > + inode->dirtied_when = jiffies;
> > >
> > > Shouldn't this update only ocur if the inode is still dirty?
> >
> > Yeah, that would be better even though the current form won't lead to
> > errors.
> >
> > Let's add one more test (inode->i_state & I_DIRTY)?
> > (I was actually aware of the trade offs and didn't bother to add it..)
>
> Oops, the (inode->i_state & I_DIRTY) test is not enough because
> when the inode is actively dirtied, I_DIRTY_PAGES won't be set when
> the flusher is writing out the pages with I_SYNC set...
Well, rather on contrary I_DIRTY_PAGES won't be set when noone dirtied
any page after we cleared I_DIRTY_PAGES.

> Well it would look clumsy to add another mapping_tagged(mapping,
> PAGECACHE_TAG_DIRTY) test. So I tend to revert to the original scheme
> of updating ->dirtied_when only on newly dirtied pages. It's the
> simplest form that can avoid unnecessarily polluting ->dirtied_when.
Hmm, but won't now something like:
while true; do touch f; done
livelock sync? We always manage to write something - 1 inode - and the
inode will never be clean (before the IO completes, the other process
manages to dirty the inode again with very high probability).

Honza
>
> @@ -432,6 +432,15 @@ writeback_single_inode(struct inode *ino
> requeue_io(inode);
> } else {
> /*
> + * Sync livelock prevention. Each inode is
> + * tagged and synced in one shot, so we can
> + * unconditionally update its dirty time to
> + * prevent syncing it again.
> + */
> + if (wbc->sync_mode == WB_SYNC_ALL ||
> + wbc->tagged_writepages)
> + inode->dirtied_when = jiffies;
> + /*
> * Writeback blocked by something other than
> * congestion. Delay the inode for some time to
> * avoid spinning on the CPU (100% iowait)
>
> Thanks,
> Fengguang
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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