LKML Archive on
help / color / mirror / Atom feed
From: Joonsoo Kim <>
To: Mel Gorman <>
Cc: Vlastimil Babka <>,
	Andrew Morton <>,
	Michal Hocko <>,
	Linux Memory Management List <>,
	LKML <>,
	Johannes Weiner <>,
	Minchan Kim <>,
	Ye Xiaolong <>,
	Joonsoo Kim <>
Subject: Re: [PATCH] mm/page_alloc: use ac->high_zoneidx for classzone_idx
Date: Tue, 8 May 2018 10:00:59 +0900	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hello, Mel.

Thanks for precious input!

2018-05-04 19:33 GMT+09:00 Mel Gorman <>:
> On Fri, May 04, 2018 at 09:03:02AM +0200, Vlastimil Babka wrote:
>> > min watermark for NORMAL zone on node 0
>> > allocation initiated on node 0: 750 + 4096 = 4846
>> > allocation initiated on node 1: 750 + 0 = 750
>> >
>> > This watermark difference could cause too many numa_miss allocation
>> > in some situation and then performance could be downgraded.
>> >
>> > Recently, there was a regression report about this problem on CMA patches
>> > since CMA memory are placed in ZONE_MOVABLE by those patches. I checked
>> > that problem is disappeared with this fix that uses high_zoneidx
>> > for classzone_idx.
>> >
>> >
>> >
>> > Using high_zoneidx for classzone_idx is more consistent way than previous
>> > approach because system's memory layout doesn't affect anything to it.
>> So to summarize;
>> - ac->high_zoneidx is computed via the arcane gfp_zone(gfp_mask) and
>> represents the highest zone the allocation can use
> It's arcane but it was simply a fast-path calculation. A much older
> definition would be easier to understand but it was slower.
>> - classzone_idx was supposed to be the highest zone that the allocation
>> can use, that is actually available in the system. Somehow that became
>> the highest zone that is available on the preferred node (in the default
>> node-order zonelist), which causes the watermark inconsistencies you
>> mention.
> I think it *always* was the index of the first preferred zone of a
> zonelist. The treatment of classzone has changed a lot over the years and
> I didn't do a historical check but the general intent was always "protect
> some pages in lower zones". This was particularly important for 32-bit
> and highmem albeit that is less of a concern today. When it transferred to
> NUMA, I don't think it ever was seriously considered if it should change
> as the critical node was likely to be node 0 with all the zones and the
> remote nodes all used the highest zone. CMA/MOVABLE changed that slightly
> by allowing the possibility of node0 having a "higher" zone than every

I think that this problem is related to not only protection of the
lowmem (that is
lower than normal) but also node balance.

In fact, problem reported by zeroday-bot is caused by node1 having a
"higher" zone. In this case, node0's lowmem is protected well but
node balance of the allocation is broken since node1's normal memory cannot
be protected from allocation that is initiated on remote node.

> other node. When MOVABLE was introduced, it wasn't much of a problem as
> the purpose of MOVABLE was for systems that dynamically needed to allocate
> hugetlbfs later in the runtime but for CMA, it was a lot more critical
> for ordinary usage so this is primarily a CMA thing.

I'm not sure that it's primarily a CMA thing. There is an another critical setup
for this problem, that is, memory hotplug. If someone plug-in a new memory to
the MOVABLE zone, "higher" zone will be created in a specific node and
this problem happens. I have checked this with QEMU.

>> I don't see a problem with your change. I would be worried about
>> inflated reserves when e.g. ZONE_MOVABLE doesn't exist, but that doesn't
>> seem to be the case. My laptop has empty ZONE_MOVABLE and the
>> ZONE_NORMAL protection for movable is 0.
>> But there had to be some reason for classzone_idx to be like this and
>> not simple high_zoneidx. Maybe Mel remembers? Maybe it was important
>> then, but is not anymore? Sigh, it seems to be pre-git.
> classzone predates my involvement with Linux but I would be less concerneed
> about what the original intent was and instead ensure that classzone index
> is consistent, sane and potentially renamed while preserving the intent of
> "reserve pages in lower zones when an allocation request can use higher
> zones". While historically the critical intent was to preserve Normal and
> to a lesser extent DMA on 32-bit systems, there still should be some care
> of DMA32 so we should not lose that.


> With the patch, the allocator looks like it would be fine as just
> reservations change. I think it's unlikely that CMA usage will result
> in lowmem starvation.  Compaction becomes a bit weird as classzone index
> has no special meaning versis highmem and I think it'll be very easy to
> forget. Similarly, vmscan can reclaim pages from remote nodes and zones
> that are higher than the original request. That is not likely to be a
> problem but it's a change in behaviour and easy to miss.
> Fundamentally, I find it extremely weird we now have two variables that are
> essentially the same thing. They should be collapsed into one variable,
> renamed and documented on what the index means for page allocator,
> compaction, vmscan and the special casing around CMA.

I will update this patch to reflect your comment. If someone have an idea
on renaming this variable, please let me know.


  reply	other threads:[~2018-05-08  1:01 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-04  4:30 js1304
2018-05-04  7:03 ` Vlastimil Babka
2018-05-04  7:31   ` Joonsoo Kim
2018-05-04 10:33   ` Mel Gorman
2018-05-08  1:00     ` Joonsoo Kim [this message]
2018-05-16  9:35       ` Vlastimil Babka
2018-05-16 10:28         ` Mel Gorman
2018-05-08 23:13     ` Andrew Morton

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='' \ \ \ \ \ \ \ \ \ \ \ \
    --subject='Re: [PATCH] mm/page_alloc: use ac->high_zoneidx for classzone_idx' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).