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

[Mel Gorman <mel@csn.ul.ie>]

On Tue, Jun 15, 2010 at 06:53:41AM -0400, Christoph Hellwig wrote:
> > +	/*
> > +	 * 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?
> 

With the page lock released, the mapping may be no longer valid. Nick
posted a patch in relation to it that I need to look at. The comment was
because Andrew highlight that this was buggy and I wanted to make sure I
didn't forget about it.

> > +	/*
> > +	 * 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);
> 	}
> 

Sure, that's tidier.

> >  	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.
> 

I'll trim it.

> > + *
> > + * 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.
> 

Hmm, ok. In that case, I'll have to release the locks on the list and
reacquire them. It was something I would have preferred to avoid. Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab