Re: Race in new page migration code?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Christoph Lameter wrote:
> On Sat, 14 Jan 2006, Nick Piggin wrote:
> 
> 
>>>We take that reference count on the page:
>>
>>Yes, after you have dropped all your claims to pin this page
>>(ie. pte lock). You really can't take a refcount on a page that
> 
> 
> Oh. Now I see. I screwed that up by a fix I added.... We cannot drop the 
> ptl here. So back to the way it was before. Remove the draining from 
> isolate_lru_page and do it before scanning for pages so that we do not
> have to drop the ptl. 
> 

OK (either way is fine), but you should still drop the __isolate_lru_page
nonsense and revert it like my patch does.

> Also remove the WARN_ON since its now even possible that other actions of 
> the VM move the pages into the LRU lists while we scan for pages to
> migrate.
> 

Well, it has always been possible since vmscan started batching scans a
long time ago. Actually seeing as you only take a read lock on the semaphore
it is probably also possible to have a concurrent migrate operation cause
this as well.

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.

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