Re: Oops while rebalancing, now unmountable.

[Christoph Hellwig <hch@infradead.org>]

On Mon, Nov 15, 2010 at 08:29:14PM +0100, Andrea Arcangeli wrote:
> Scary stuff, so WB_SYNC_NONE wouldn't submit the dirty part of the
> page down for I/O, so that it's all clean after wait_on_page_writeback
> returns? (well of course unless the dirty bit was set again)

It might not if we have lock contention or other resource starvation.
That's the reason why WB_SYNC_NONE was added - to not block the flusher
threads.

> I didn't realize the stack overflow issue was specific to delalloc.

It's not.  It's specific to direct reclaim.  Only ext4 special cases
delalloc, but I'm not sure if that's intentional or just an accidental
side effect of the mess that the ext4 writeback code is.

> In short with THP it's khugepaged that is supposed to run the
> ->writepage in migrate.c and it will run it once every 10 sec even
> when it fails (and not in a 100% cpu wasting loop like kswapd), so if
> you did something magic for kswapd in XFS you should do for khugepaged
> too.

If you have a PF_ flag for it that's easy to add once it goes into
mainline.

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