linux-kernel - Re: [PATCH 1/2] mem-hotplug: use GFP_HIGHUSER_MOVABLE in, alloc_migrate

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Date:	Tue, 19 Jul 2016 15:50:42 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Xishi Qiu <qiuxishi@...wei.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mem-hotplug: use GFP_HIGHUSER_MOVABLE in,
 alloc_migrate_target()

On Mon, Jul 18, 2016 at 04:31:11PM +0800, Xishi Qiu wrote:
> On 2016/7/18 16:05, Vlastimil Babka wrote:
> 
> > On 07/18/2016 10:00 AM, Xishi Qiu wrote:
> >> On 2016/7/18 13:51, Joonsoo Kim wrote:
> >>
> >>> On Fri, Jul 15, 2016 at 10:47:06AM +0800, Xishi Qiu wrote:
> >>>> alloc_migrate_target() is called from migrate_pages(), and the page
> >>>> is always from user space, so we can add __GFP_HIGHMEM directly.
> >>>
> >>> No, all migratable pages are not from user space. For example,
> >>> blockdev file cache has __GFP_MOVABLE and migratable but it has no
> >>> __GFP_HIGHMEM and __GFP_USER.
> >>>
> >>
> >> Hi Joonsoo,
> >>
> >> So the original code "gfp_t gfp_mask = GFP_USER | __GFP_MOVABLE;"
> >> is not correct?
> > 
> > It's not incorrect. GFP_USER just specifies some reclaim flags, and may perhaps restrict allocation through __GFP_HARDWALL, where the original
> > page could have been allocated without the restriction. But it doesn't put the place in an unexpected address range, as placing a non-highmem page into highmem could. __GFP_MOVABLE then just controls a heuristic for placement within a zone.
> > 
> >>> And, zram's memory isn't GFP_HIGHUSER_MOVABLE but has __GFP_MOVABLE.
> >>>
> >>
> >> Can we distinguish __GFP_MOVABLE or GFP_HIGHUSER_MOVABLE when doing
> >> mem-hotplug?
> > 
> > I don't understand the question here, can you rephrase with more detail? Thanks.
> > 
> 
> Hi Joonsoo,

Above is answered by Vlastimil. :)

> When we do memory offline, and the zone is movable zone,
> can we use "alloc_pages_node(nid, GFP_HIGHUSER_MOVABLE, 0);" to alloc a
> new page? the nid is the next node.

I don't know much about memory offline, but, AFAIK, memory offline
could happen on non-movable zone like as ZONE_NORMAL. Perhaps, you can add
"if zone of the page is movable zone then alloc with GFP_HIGHUSER_MOVABLE".

Thanks.