Re: [RFC 0/3] reduce latency of direct async compaction

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Tue, Dec 08, 2015 at 01:14:39PM +0800, Aaron Lu wrote:
> On Tue, Dec 08, 2015 at 09:41:18AM +0900, Joonsoo Kim wrote:
> > On Mon, Dec 07, 2015 at 04:59:56PM +0800, Aaron Lu wrote:
> > > On Mon, Dec 07, 2015 at 04:35:24PM +0900, Joonsoo Kim wrote:
> > > > It looks like overhead still remain. I guess that migration scanner
> > > > would call pageblock_pfn_to_page() for more extended range so
> > > > overhead still remain.
> > > > 
> > > > I have an idea to solve his problem. Aaron, could you test following patch
> > > > on top of base? It tries to skip calling pageblock_pfn_to_page()
> > > 
> > > It doesn't apply on top of 25364a9e54fb8296837061bf684b76d20eec01fb
> > > cleanly, so I made some changes to make it apply and the result is:
> > > https://github.com/aaronlu/linux/commit/cb8d05829190b806ad3948ff9b9e08c8ba1daf63
> > 
> > Yes, that's okay. I made it on my working branch but it will not result in
> > any problem except applying.
> > 
> > > 
> > > There is a problem occured right after the test starts:
> > > [   58.080962] BUG: unable to handle kernel paging request at ffffea0082000018
> > > [   58.089124] IP: [<ffffffff81193f29>] compaction_alloc+0xf9/0x270
> > > [   58.096109] PGD 107ffd6067 PUD 207f7d5067 PMD 0
> > > [   58.101569] Oops: 0000 [#1] SMP 
> > 
> > I did some mistake. Please test following patch. It is also made
> > on my working branch so you need to resolve conflict but it would be
> > trivial.
> > 
> > I inserted some logs to check whether zone is contiguous or not.
> > Please check that normal zone is set to contiguous after testing.
> 
> Yes it is contiguous, but unfortunately, the problem remains:
> [   56.536930] check_zone_contiguous: Normal
> [   56.543467] check_zone_contiguous: Normal: contiguous
> [   56.549640] BUG: unable to handle kernel paging request at ffffea0082000018
> [   56.557717] IP: [<ffffffff81193f29>] compaction_alloc+0xf9/0x270
> [   56.564719] PGD 107ffd6067 PUD 207f7d5067 PMD 0
> 

Maybe, I find the reason. cc->free_pfn can be initialized to invalid pfn
that isn't checked so optimized pageblock_pfn_to_page() causes BUG().

I add work-around for this problem at isolate_freepages(). Please test
following one.

Thanks.

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