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Date: Tue, 20 Mar 2018 15:15:13 -0700 (PDT)
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To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Michal Hocko <mhocko@kernel.org>, "Li,Rongqing" <lirongqing@baidu.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>, "hannes@cmpxchg.org" <hannes@cmpxchg.org>

On Wed, 21 Mar 2018, Andrey Ryabinin wrote:

> >>> It would probably be best to limit the 
> >>> nr_pages to the amount that needs to be reclaimed, though, rather than 
> >>> over reclaiming.
> >>
> >> How do you achieve that? The charging path is not synchornized with the
> >> shrinking one at all.
> >>
> > 
> > The point is to get a better guess at how many pages, up to 
> > SWAP_CLUSTER_MAX, that need to be reclaimed instead of 1.
> > 
> >>> If you wanted to be invasive, you could change page_counter_limit() to 
> >>> return the count - limit, fix up the callers that look for -EBUSY, and 
> >>> then use max(val, SWAP_CLUSTER_MAX) as your nr_pages.
> >>
> >> I am not sure I understand
> >>
> > 
> > Have page_counter_limit() return the number of pages over limit, i.e. 
> > count - limit, since it compares the two anyway.  Fix up existing callers 
> > and then clamp that value to SWAP_CLUSTER_MAX in 
> > mem_cgroup_resize_limit().  It's a more accurate guess than either 1 or 
> > 1024.
> > 
> 
> JFYI, it's never 1, it's always SWAP_CLUSTER_MAX.
> See try_to_free_mem_cgroup_pages():
> ....	
> 	struct scan_control sc = {
> 		.nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> 

Is SWAP_CLUSTER_MAX the best answer if I'm lowering the limit by 1GB?