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

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Message-ID: <Ydf/7DDu94fMs0CG@dhcp22.suse.cz>
Date:   Fri, 7 Jan 2022 09:55:08 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Yu Zhao <yuzhao@...gle.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 06-01-22 14:41:12, Yu Zhao wrote:
> 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.

I have to admit that I am not really familiar with zram/zswap but I find
the concept of requiring memory to do the reclaim really problematic.
 
> 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.

Well, this is not a strong argument to be honest. Kswapd being stuck
and the majority of the reclaim being done in the direct reclaim
context is a situation I have seen many many times. We used to have
problems with direct reclaimers throttling to prevent an over eager OOM
situations.

Have you considered using a pool of preallocated objects instead?
-- 
Michal Hocko
SUSE Labs