RE: [RFC 1/2] cma: remove __reclaim_pages

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Minchan,

On Friday, August 17, 2012 3:06 AM Minchan Kim wrote:

> On Thu, Aug 16, 2012 at 02:58:18PM +0100, Mel Gorman wrote:
> > On Tue, Aug 14, 2012 at 05:57:06PM +0900, Minchan Kim wrote:
> > > Now cma reclaims too many pages by __reclaim_pages which says
> > > following as
> > >
> > >         * Reclaim enough pages to make sure that contiguous allocation
> > >         * will not starve the system.
> > >
> > > Starve? What does it starve the system? The function which allocate
> > > free page for migration target would wake up kswapd and do direct reclaim
> > > if needed during migration so system doesn't starve.
> > >
> >
> > I thought this patch was overkill at the time it was introduced but
> > didn't have a concrete reason to reject it when I commented on it
> > https://lkml.org/lkml/2012/1/30/136 . Marek did want this and followed
> > up with "contiguous allocations should have higher priority than others"
> > which I took to mean that he was also ok with excessive reclaim.
> 
> I think OOM kill to background applications is more appropriate than
> big latency of foreground(ex, Camera app) application in your mobile phone.
> In other words, excessive reclaim is *really* bad which elapsed 8sec
> in my test as worst case. :(
> 
> >
> > > Let remove __reclaim_pages and related function and fields.
> > >
> >
> > That should be one patch and I don't object to it being removed as such
> > but it's Marek's call.
> 
> Marek. Any thought?

Well, I've introduced this function as a result of solving Mel's request:

http://www.spinics.net/lists/linux-mm/msg28485.html

Before that I solved it almost the same as in your current patch. I'm aware of 
the fact that __reclaim_pages() approach might be a little overkill, but I didn't
find anything better yet.

> > > I modified split_free_page slightly because I removed __reclaim_pages,
> > > isolate_freepages_range can fail by split_free_page's watermark check.
> > > It's very critical in CMA because it ends up failing alloc_contig_range.
> > >
> >
> > This is a big change and should have been in a patch on its
> > own. split_free_page checks watermarks because if the watermarks are
> > not obeyed a zone can become fully allocated. This can cause a system to
> > livelock under certain circumstances if a page cannot be allocated and a
> > free page is required before other pages can be freed.
> >
> > > I think we don't need the check in case of CMA because CMA allocates
> > > free pages by alloc_pages, not isolate_freepages_block in migrate_pages
> > > so watermark is already checked in alloc_pages.
> >
> > It uses alloc_pages when migrating pages out of the CMA area but note
> > that it uses isolate_freepages_block when allocating the CMA buffer when
> > alloc_contig_range calls isolate_freepages_range
> >
> > isolate_freepages_range(unsigned long start_pfn, unsigned long end_pfn)
> > {
> > 	for (pfn = start_pfn; pfn < end_pfn; pfn += isolated) {
> >                isolated = isolate_freepages_block(pfn, block_end_pfn,
> >                                                    &freelist, true);
> > 	}
> > 	map_pages(&freelist);
> > }
> >
> > so the actual CMA allocation itself is not using alloc_pages. By removing
> > the watermark check you allow the CMA to breach watermarks and puts the
> > system at risk of livelock.
> 
> Fair enough. I will look into that.

We will also looks into this issue. 

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center