linux-kernel - Re: [PATCH -mm] vmscan: move reclaim_state handling to shrink

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Message-Id: <20150114153449.038bc61b1bd6fc262f9cea01@linux-foundation.org>
Date:	Wed, 14 Jan 2015 15:34:49 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm] vmscan: move reclaim_state handling to shrink_slab

On Wed, 14 Jan 2015 16:55:36 +0300 Vladimir Davydov <vdavydov@...allels.com> wrote:

> current->reclaim_state is only used to count the number of slab pages
> reclaimed by shrink_slab(). So instead of initializing it before we are
> going to call try_to_free_pages() or shrink_zone(), let's set in
> directly in shrink_slab().
> 
> This patch also makes shrink_slab() return the number of reclaimed slab
> pages (obtained from reclaim_state) instead of the number of reclaimed
> objects, because the latter is not of much use - it was only checked by
> drop_slab() to decide whether it should continue reclaim or abort. The
> number of reclaimed pages is more appropriate, because it also can be
> used by shrink_zone() to accumulate scan_control->nr_reclaimed.

Not sure that this is a good change.  If shrink_slab() managed to free
some objects but didn't free any pages then that's a good sign that
additional calls to shrink_slab() *will* free some pages.  With this
change, drop_slab_node() can give up too early.

The general philosophy throughout here is: "pass it nr_to_scan, it
returns nr_scanned/nr_freed".  Switching the return value to
nr_pages_freed kinda breaks that paradigm.

> Note that after this patch try_to_free_mem_cgroup_pages() will count not
> only reclaimed user pages, but also slab pages, which is expected,
> because it can reclaim kmem from kmem-active sub cgroups.
> 
>  mm/page_alloc.c |    4 ---
>  mm/vmscan.c     |   73 ++++++++++++++++++++-----------------------------------
>  2 files changed, 27 insertions(+), 50 deletions(-)

That's nice though.


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