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

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Date:   Thu, 6 Jan 2022 14:41:12 -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 Thu, Jan 06, 2022 at 05:12:16PM +0100, 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);
> > +
> > +	return current->reclaim_state->mm_walk;
> > +}
> > +
> > +static void free_mm_walk(struct lru_gen_mm_walk *walk)
> > +{
> > +	if (!current->reclaim_state || !current->reclaim_state->mm_walk)
> > +		kvfree(walk);
> > +}
> 
> Do I get it right that you are allocating from the reclaim context? What
> prevents this to completely deplete the memory as the reclaim context is
> PF_MEMALLOC?

Yes, and in general the same reason zram/zswap/etc. allocate memory in
the reclaim context: to make more free memory.

In this case, lru_gen_mm_walk is small (160 bytes); it's per direct
reclaimer; and direct reclaimers rarely come here, i.e., only when
kswapd can't keep up in terms of the aging, which is similar to the
condition where the inactive list is empty for the active/inactive
lru.