Re: [RFC 1/2] Protect larger order pages from breaking up

From: Michal Hocko <mhocko@kernel.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Christoph Lameter <cl@linux.com>,
	Matthew Wilcox <willy@infradead.org>,
	linux-mm@kvack.org, linux-rdma@vger.kernel.org,
	akpm@linux-foundation.org,
	Thomas Schoebel-Theuer <tst@schoebel-theuer.de>,
	andi@firstfloor.org, Rik van Riel <riel@redhat.com>,
	Guy Shattah <sguy@mellanox.com>,
	Anshuman Khandual <khandual@linux.vnet.ibm.com>,
	Michal Nazarewicz <mina86@mina86.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	David Nellans <dnellans@nvidia.com>,
	Laura Abbott <labbott@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Dave Hansen <dave.hansen@intel.com>,
	Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [RFC 1/2] Protect larger order pages from breaking up
Date: Mon, 19 Feb 2018 15:42:16 +0100	[thread overview]
Message-ID: <20180219144216.GP21134@dhcp22.suse.cz> (raw)
In-Reply-To: <20180219101935.cb3gnkbjimn5hbud@techsingularity.net>

On Mon 19-02-18 10:19:35, Mel Gorman wrote:
[...]
> Access to the pool is unprotected so you might create a reserve for jumbo
> frames only to have them consumed by something else entirely. It's not
> clear if that is even fixable as GFP flags are too coarse.
> 
> It is not covered in the changelog why MIGRATE_HIGHATOMIC was not
> sufficient for jumbo frames which are generally expected to be allocated
> from atomic context. If there is a problem there then maybe
> MIGRATE_HIGHATOMIC should be made more strict instead of a hack like
> this. It'll be very difficult, if not impossible, for this to be tuned
> properly.
> 
> Finally, while I accept that fragmentation over time is a problem for
> unmovable allocations (fragmentation protection was originally designed
> for THP/hugetlbfs), this is papering over the problem. If greater
> protections are needed then the right approach is to be more strict about
> fallbacks. Specifically, unmovable allocations should migrate all movable
> pages out of migrate_unmovable pageblocks before falling back and that
> can be controlled by policy due to the overhead of migration. For atomic
> allocations, allow fallback but use kcompact or a workqueue to migrate
> movable pages out of migrate_unmovable pageblocks to limit fallbacks in
> the future.

Completely agreed!

> I'm not a fan of this patch.

Yes, I think the approach is just wrong. It will just hit all sorts of
weird corner cases and won't work reliable for those who care.
-- 
Michal Hocko
SUSE Labs

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