linux-kernel - Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

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Date:	Tue, 15 Jun 2010 06:53:41 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

> +	/*
> +	 * XXX: This is the Holy Hand Grenade of PotentiallyInvalidMapping. As
> +	 * the page lock has been dropped by ->writepage, that mapping could
> +	 * be anything
> +	 */

Why is this an XXX comment?

> +	/*
> +	 * Wait on writeback if requested to. This happens when
> +	 * direct reclaiming a large contiguous area and the
> +	 * first attempt to free a range of pages fails.
> +	 */
> +	if (PageWriteback(page) && sync_writeback == PAGEOUT_IO_SYNC)
> +		wait_on_page_writeback(page);
> +
> +	if (!PageWriteback(page)) {
> +		/* synchronous write or broken a_ops? */
> +		ClearPageReclaim(page);
> +	}

how about:

	if (PageWriteback(page) {
		if (sync_writeback == PAGEOUT_IO_SYNC)
			wait_on_page_writeback(page);
	} else {
		/* synchronous write or broken a_ops? */
		ClearPageReclaim(page);
	}

>  	if (!may_write_to_queue(mapping->backing_dev_info))
>  		return PAGE_KEEP;
>  
>  /*
> + * Clean a list of pages. It is expected that all the pages on page_list have been
> + * locked as part of isolation from the LRU.

A rather pointless line of 80 chars.  I see the point for long string
literals, but here's it's just a pain.

> + *
> + * XXX: Is there a problem with holding multiple page locks like this?

I think there is.  There's quite a few places that do hold multiple
pages locked, but they always lock pages in increasing page->inxex order.
Given that this locks basically in random order it could cause problems
for those places.

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