Re: [PATCH] mm/slub: Reduce memory consumption in extreme scenarios

From: Vlastimil Babka <vbabka@suse.cz>
To: Mike Rapoport <rppt@kernel.org>
Cc: "chenjun (AM)" <chenjun102@huawei.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"cl@linux.com" <cl@linux.com>,
	"penberg@kernel.org" <penberg@kernel.org>,
	"rientjes@google.com" <rientjes@google.com>,
	"iamjoonsoo.kim@lge.com" <iamjoonsoo.kim@lge.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	Hyeonggon Yoo <42.hyeyoo@gmail.com>,
	"xuqiang (M)" <xuqiang36@huawei.com>,
	"Wangkefeng (OS Kernel Lab)" <wangkefeng.wang@huawei.com>,
	Michal Hocko <mhocko@kernel.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] mm/slub: Reduce memory consumption in extreme scenarios
Date: Tue, 21 Mar 2023 10:41:12 +0100	[thread overview]
Message-ID: <20b896db-9dd6-fcc3-a72a-ce0044d4ab75@suse.cz> (raw)
In-Reply-To: <ZBgjZn7WOqO5ruws@kernel.org>

On 3/20/23 10:12, Mike Rapoport wrote:
> On Mon, Mar 20, 2023 at 09:05:57AM +0100, Vlastimil Babka wrote:
>> On 3/19/23 08:22, chenjun (AM) wrote:
>> > 在 2023/3/17 20:06, Vlastimil Babka 写道:
>> > 
>> > If we ignore __GFP_ZERO passed by kzalloc， kzalloc will not work.
>> > Could we just unmask __GFP_RECLAIMABLE | __GFP_RECLAIM?
>> > 
>> > pc.flags &= ~(__GFP_RECLAIMABLE | __GFP_RECLAIM)
>> > pc.flags |= __GFP_THISNODE
>> 
>> __GFP_RECLAIMABLE would be wrong, but also ignored as new_slab() does:
>> 	flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK)
>> 
>> which would filter out __GFP_ZERO as well. That's not a problem as kzalloc()
>> will zero out the individual allocated objects, so it doesn't matter if we
>> don't zero out the whole slab page.
>> 
>> But I wonder, if we're not past due time for a helper e.g.
>> gfp_opportunistic(flags) that would turn any allocation flags to a
>> GFP_NOWAIT while keeping the rest of relevant flags intact, and thus there
>> would be one canonical way to do it - I'm sure there's a number of places
>> with their own variants now?
>> With such helper we'd just add __GFP_THISNODE to the result here as that's
>> specific to this particular opportunistic allocation.
> 
> I like the idea, but maybe gfp_no_reclaim() would be clearer?

Well, that name would say how it's implemented, but not exactly as we also
want to add __GFP_NOWARN. "gfp_opportunistic()" or a better name with
similar meaning was meant to convey the intention of what this allocation is
trying to do, and I think that's better from the API users POV?