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Date:   Mon, 26 Sep 2022 16:05:45 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Florian Westphal <fw@strlen.de>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org, vbabka@suse.cz,
        akpm@linux-foundation.org, urezki@gmail.com,
        netdev@vger.kernel.org, netfilter-devel@vger.kernel.org,
        Martin Zaharinov <micron10@gmail.com>
Subject: Re: [PATCH mm] mm: fix BUG with kvzalloc+GFP_ATOMIC
Message-ID: <YzGxuZ8jQ2sO4ZML@dhcp22.suse.cz>
References: <20220923103858.26729-1-fw@strlen.de>
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 <20220923133512.GE22541@breakpoint.cc>
 <YzFZf0Onm6/UH7/I@dhcp22.suse.cz>
 <20220926075639.GA908@breakpoint.cc>
 <YzFplwSxwwsLpzzX@dhcp22.suse.cz>
 <YzFxHlYoncuDl2fM@dhcp22.suse.cz>
 <20220926100800.GB12777@breakpoint.cc>
 <YzGUyWlYd15uLu7G@dhcp22.suse.cz>
 <20220926130808.GD12777@breakpoint.cc>
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On Mon 26-09-22 15:08:08, Florian Westphal wrote:
[...]
> To the best of my knowledge there are users of this interface that
> invoke it with rcu read lock held, and since those always nest, the
> rcu_read_unlock() won't move us to GFP_KERNEL territory.

Fiar point. I can see a comment above rhashtable_insert_fast which is
supposed to be used by drivers and explicitly documented to be
compatible with an atomic context. So the above is clearly a no-go

In that case I would propose to go with the patch going
http://lkml.kernel.org/r/YzFplwSxwwsLpzzX@dhcp22.suse.cz direction.
For that we could use an alternative implementation for the vmalloc
fallback for non-sleeping request in the future. A single branch for an
unlikely event is not nice but acceptable I would say. Especially if we
want to go in line with Linus' original "prevent hack for gfp_mask in
callers".
-- 
Michal Hocko
SUSE Labs