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

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Date:   Mon, 10 Jan 2022 17:25:15 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        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>,
        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 10-01-22 17:01:07, Vlastimil Babka wrote:
> On 1/10/22 16:01, 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. 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. 
> 
> Slab implementations drop all locks before calling into page allocator (thus
> possibly reclaim) so slab itself should be fine and I don't expect it to
> change. But we could eventually reach the page allocator recursively again,
> that's true and not great.

Thanks for double checking. If recursion is really intended and
something SL.B allocators should support then this is definitely worth
documenting so that a subtle change won't break in the future. 

-- 
Michal Hocko
SUSE Labs