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

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Message-ID: <Yd6tafG3CS7BoRYn@dhcp22.suse.cz>
Date:   Wed, 12 Jan 2022 11:28:57 +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 Tue 11-01-22 16:16:57, Yu Zhao wrote:
> 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.)

OK, I can see it now. __need_reclaim will check for PF_MEMALLOC and skip
the fs_reclaim tracking.

I still maintain I am not really happy about (nor in the zswap example)
allocations from the direct reclaim context. I would really recommend
using a pre-allocated pool of objects.

If there are strong reasons for not doing so then at lease change that
to kzalloc.

Thanks!
-- 
Michal Hocko
SUSE Labs