Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 16 Jun 2010 18:33:51 +0200
Jan Kara <jack@suse.cz> wrote:

> We try to avoid livelocks of writeback when some steadily creates
> dirty pages in a mapping we are writing out. For memory-cleaning
> writeback, using nr_to_write works reasonably well but we cannot
> really use it for data integrity writeback. This patch tries to
> solve the problem.
> 
> The idea is simple: Tag all pages that should be written back
> with a special tag (TOWRITE) in the radix tree. This can be done
> rather quickly and thus livelocks should not happen in practice.
> Then we start doing the hard work of locking pages and sending
> them to disk only for those pages that have TOWRITE tag set.
> 
> Note: Adding new radix tree tag grows radix tree node from 288 to
> 296 bytes for 32-bit archs and from 552 to 560 bytes for 64-bit archs.
> However, the number of slab/slub items per page remains the same
> (13 and 7 respectively).
> 
>
> ...
>
> +void tag_pages_for_writeback(struct address_space *mapping,
> +			     pgoff_t start, pgoff_t end)
> +{
> +	unsigned long tagged;
> +
> +	do {
> +		spin_lock_irq(&mapping->tree_lock);
> +		tagged = radix_tree_range_tag_if_tagged(&mapping->page_tree,
> +				&start, end, WRITEBACK_TAG_BATCH,
> +				PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
> +		spin_unlock_irq(&mapping->tree_lock);
> +		cond_resched();
> +	} while (tagged >= WRITEBACK_TAG_BATCH);
> +}

grumble.  (tagged > WRITEBACK_TAG_BATCH) would be a bug, wouldn't it? 
So the ">=" is hiding a bug.

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