Date:   Mon, 20 Mar 2023 11:12:06 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     Vlastimil Babka <vbabka@suse.cz>
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>
Subject: Re: [PATCH] mm/slub: Reduce memory consumption in extreme scenarios
Message-ID: <ZBgjZn7WOqO5ruws@kernel.org>
References: <20230314123403.100158-1-chenjun102@huawei.com>
 <0cad1ff3-8339-a3eb-fc36-c8bda1392451@suse.cz>
 <344c7521d72e4107b451c19b329e9864@huawei.com>
 <8c700468-245d-72e9-99e7-b99d4547e6d8@suse.cz>
 <aeb2bd3990004b9eb4f151aa833ddcf2@huawei.com>
 <015855b3-ced3-8d84-e21d-cc6ce112b556@suse.cz>
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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:
> >> On 3/17/23 12:32, chenjun (AM) wrote:
> >>> 在 2023/3/14 22:41, Vlastimil Babka 写道:
> >>>>>    	pc.flags = gfpflags;
> >>>>> +
> >>>>> +	/*
> >>>>> +	 * when (node != NUMA_NO_NODE) && (gfpflags & __GFP_THISNODE)
> >>>>> +	 * 1) try to get a partial slab from target node with __GFP_THISNODE.
> >>>>> +	 * 2) if 1) failed, try to allocate a new slab from target node with
> >>>>> +	 *    __GFP_THISNODE.
> >>>>> +	 * 3) if 2) failed, retry 1) and 2) without __GFP_THISNODE constraint.
> >>>>> +	 */
> >>>>> +	if (node != NUMA_NO_NODE && !(gfpflags & __GFP_THISNODE) && try_thisnode)
> >>>>> +			pc.flags |= __GFP_THISNODE;
> >>>>
> >>>> Hmm I'm thinking we should also perhaps remove direct reclaim possibilities
> >>>> from the attempt 2). In your qemu test it should make no difference, as it
> >>>> fills everything with kernel memory that is not reclaimable. But in practice
> >>>> the target node might be filled with user memory, and I think it's better to
> >>>> quickly allocate on a different node than spend time in direct reclaim. So
> >>>> the following should work I think?
> >>>>
> >>>> pc.flags = GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE
> >>>>
> >>>
> >>> Hmm, Should it be that:
> >>>
> >>> pc.flags |= GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE
> >> 
> >> No, we need to ignore the other reclaim-related flags that the caller
> >> passed, or it wouldn't work as intended.
> >> The danger is that we ignore some flag that would be necessary to pass, but
> >> I don't think there's any?
> >> 
> >> 
> > 
> > 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?

-- 
Sincerely yours,
Mike.