Re: [PATCH 6/9] ksm: mem cgroup charge swapin copy

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Hugh Dickins <hugh.dickins@tiscali.co.uk> [2009-11-25 17:12:13]:

> On Wed, 25 Nov 2009, Balbir Singh wrote:
> > * Hugh Dickins <hugh.dickins@tiscali.co.uk> [2009-11-24 16:51:13]:
> > 
> > > But ksm swapping does require one small change in mem cgroup handling.
> > > When do_swap_page()'s call to ksm_might_need_to_copy() does indeed
> > > substitute a duplicate page to accommodate a different anon_vma (or a
> > > different index), that page escaped mem cgroup accounting, because of
> > > the !PageSwapCache check in mem_cgroup_try_charge_swapin().
> > > 
> > 
> > The duplicate page doesn't show up as PageSwapCache
> 
> That's right.
> 
> > or are we optimizing
> > for the race condition where the page is not in SwapCache?
> 
> No, optimization wasn't on my mind at all.  To be honest, it's slightly
> worsening the case of the race in which another thread has independently
> faulted it in, and then removed it from swap cache.  But I think we'll
> agree that that's rare enough a case that a few more cycles doing it
> won't matter.
>

Thanks for clarifying, yes I agree that the condition is rare and
nothing for us to worry about about at the moment.
 
> > I should probably look at the full series.
> 
> 2/9 is the one which brings the problem: it's ksm_might_need_to_copy()
> (an inline which tests for the condition) and ksm_does_need_to_copy()
> (which makes a duplicate page when the condition has been found so).
> 
> The problem arises because an Anon struct page contains a pointer to
> its anon_vma, used to locate its ptes when swapping.  Suddenly, with
> KSM swapping, an anon page may get read in from swap, faulted in and
> pointed to its anon_vma, everything fine; but then faulted in again
> somewhere else, and needs to be pointed to a different anon_vma...
> 
> Lose its anon_vma and it becomes unswappable, not a good choice when
> trying to extend swappability: so instead we allocate a duplicate page
> just to point to the different anon_vma; and if they last long enough,
> unchanged, KSM will come around again to find them the same and
> remerge them.  Not an efficient solution, but a simple solution,
> much in keeping with the way KSM already works.
> 
> The duplicate page is not PageSwapCache: certainly it crossed my mind
> to try making it PageSwapCache like the original, but I think that
> raises lots of other problems (how do we make the radix_tree slot
> for that offset hold two page pointers?).
>

Thanks for the detailed explanation, it does help me understand what
is going on. 

-- 
	Balbir

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