Date:   Mon, 10 Jan 2022 17:25:15 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Yu Zhao <yuzhao@google.com>,
        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>,
        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: <Ydxd6zHs2uR5mNnn@dhcp22.suse.cz>
References: <20220104202227.2903605-1-yuzhao@google.com>
 <20220104202227.2903605-7-yuzhao@google.com>
 <YdcU4P+XWkbDUUoO@dhcp22.suse.cz>
 <YdxKORU9OWCv114O@dhcp22.suse.cz>
 <8edfd643-888b-fbe6-97c0-21f900767c27@suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <8edfd643-888b-fbe6-97c0-21f900767c27@suse.cz>
Precedence: bulk

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