Date:   Wed, 12 Jan 2022 11:28:57 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Jesse Barnes <jsbarnes@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Michael Larabel <Michael@michaellarabel.com>,
        Rik van Riel <riel@surriel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>,
        Ying Huang <ying.huang@intel.com>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        page-reclaim@google.com, x86@kernel.org,
        Konstantin Kharlamov <Hi-Angel@yandex.ru>
Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging
Message-ID: <Yd6tafG3CS7BoRYn@dhcp22.suse.cz>
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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