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

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Date:   Wed, 29 Mar 2023 10:41:52 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     "chenjun (AM)" <chenjun102@...wei.com>,
        Mike Rapoport <rppt@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "cl@...ux.com" <cl@...ux.com>,
        "penberg@...nel.org" <penberg@...nel.org>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        "xuqiang (M)" <xuqiang36@...wei.com>,
        "Wangkefeng (OS Kernel Lab)" <wangkefeng.wang@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH] mm/slub: Reduce memory consumption in extreme scenarios

On 3/21/23 10:30, chenjun (AM) wrote:
> 在 2023/3/20 17:12, Mike Rapoport 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?
>> 
> 
> #define gfp_no_reclaim(gfpflag) (gfpflag & ~__GFP_DIRECT_RECLAIM)

I hoped for more feedback on the idea, but it's probably best proposed
outside of this slub-specific thread, so we could go for an open-coded
solution in slub for now.

Also just masking out __GFP_DIRECT_RECLAIM wouldn't be sufficient in any
case for the general solution/

> And here,
> 
> pc.flags = gfp_no_reclaim(gfpflags) | __GFP_THISNODE.

I'd still suggest as earlier:

pc.flags = GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE;

> Do I get it right？