linux-kernel - Re: [PATCH v6 6/9] mm: multigenerational lru: aging

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Date:   Tue, 11 Jan 2022 16:16:57 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        page-reclaim@...gle.com, x86@...nel.org,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>
Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging

On Mon, Jan 10, 2022 at 04:01:13PM +0100, Michal Hocko wrote:
> On Thu 06-01-22 17:12:18, Michal Hocko wrote:
> > On Tue 04-01-22 13:22:25, Yu Zhao wrote:
> > > +static struct lru_gen_mm_walk *alloc_mm_walk(void)
> > > +{
> > > +	if (!current->reclaim_state || !current->reclaim_state->mm_walk)
> > > +		return kvzalloc(sizeof(struct lru_gen_mm_walk), GFP_KERNEL);
> 
> One thing I have overlooked completely.

I appreciate your attention to details but GFP_KERNEL is legit in the
reclaim path. It's been used many years in our production, e.g.,
  page reclaim
    swap_writepage()
      frontswap_store()
        zswap_frontswap_store()
          zswap_entry_cache_alloc(GFP_KERNEL)

(And I always test my changes with lockdep, kasan, DEBUG_VM, etc., no
 warnings ever seen from using GFP_KERNEL in the reclaim path.)

> You cannot really use GFP_KERNEL
> allocation here because the reclaim context can be constrained (e.g.
> GFP_NOFS). This allocation will not do any reclaim as it is PF_MEMALLOC
> but I suspect that the lockdep will complain anyway.
> 
> Also kvmalloc is not really great here. a) vmalloc path is never
> executed for small objects and b) we do not really want to make a
> dependency between vmalloc and the reclaim (by vmalloc -> reclaim ->
> vmalloc).
> 
> Even if we rule out vmalloc and look at kmalloc alone. Is this really
> safe? I do not see any recursion prevention in the SL.B code. Maybe this
> just happens to work but the dependency should be really documented so
> that future SL.B changes won't break the whole scheme. 

Affirmative, as Vlastimil has clarified.

Thanks!