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charset="us-ascii" ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1657849151; a=rsa-sha256; cv=none; b=2YEgiJ6xwUlLx44zlSZ+oFnvHap2SfGYnk5SySnX1OAzrwVHmHMGtu+Yh0XnH50fyrBewd pe5z25V16pYmv+p8foig2W5jxujfsdhggGFBax2wYcuCeAC9r2W/zEJoMY326N4zSVpoSO 94UQehIjEiFa1gfhYmYxyq8V+mzSJH0= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b="JfY/dZhi"; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf14.hostedemail.com: domain of 3PsXQYggKCCkXMFPJJQGLTTLQJ.HTRQNSZc-RRPaFHP.TWL@flex--shakeelb.bounces.google.com designates 209.85.128.202 as permitted sender) smtp.mailfrom=3PsXQYggKCCkXMFPJJQGLTTLQJ.HTRQNSZc-RRPaFHP.TWL@flex--shakeelb.bounces.google.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1657849151; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=mAeXqOZZBVPdbq4yhQiUP5VN0GJajqrt77b0GTm8ddQ=; b=ar/8eNo8EG+S97U6cWE4JkdY6726G6rWEHQRm6rLUNTzP0tY5WQvSJ+rCPXPdWF/HOSr6S GxKD0ATjCKAuBRsC0PUTwNMK6jdDsGQjBAXZo3qs+gd8TTrIntj39XvG2XGuYt7SqqHPSS cZxVkwHbmSxo34VGOY3bJ4qNJnSf3kI= X-Stat-Signature: 417c96pd51bdsp3x1ubmo7d81h15utcg X-Rspam-User: X-Rspamd-Queue-Id: 8938F1000B8 Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b="JfY/dZhi"; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf14.hostedemail.com: domain of 3PsXQYggKCCkXMFPJJQGLTTLQJ.HTRQNSZc-RRPaFHP.TWL@flex--shakeelb.bounces.google.com designates 209.85.128.202 as permitted sender) smtp.mailfrom=3PsXQYggKCCkXMFPJJQGLTTLQJ.HTRQNSZc-RRPaFHP.TWL@flex--shakeelb.bounces.google.com X-Rspamd-Server: rspam11 X-HE-Tag: 1657849151-540684 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Jul 15, 2022 at 07:24:55AM +0900, Tetsuo Handa wrote: > On 2022/07/14 23:18, Jan Kara wrote: > > Hello, > > > > so this lockdep report looks real but is more related to OOM handling than > > to ext4 as such. The immediate problem I can see is that > > mem_cgroup_print_oom_meminfo() which is called under oom_lock calls > > memory_stat_format() which does GFP_KERNEL allocations to allocate buffers > > for dumping of MM statistics. This creates oom_lock -> fs reclaim > > dependency and because OOM can be hit (and thus oom_lock acquired) in > > practically any allocation (regardless of GFP_NOFS) this has a potential of > > creating real deadlock cycles. > > > > So should mem_cgroup_print_oom_meminfo() be using > > memalloc_nofs_save/restore() to avoid such deadlocks? Or perhaps someone > > sees another solution? Generally allocating memory to report OOM looks a > > bit dangerous to me ;). mem_cgroup_print_oom_meminfo() is called only for memcg OOMs. So, the situaion would be dangerous only if the system is also OOM at that time. > > > > Honza > > I think mem_cgroup_print_oom_meminfo() should use GFP_ATOMIC, for it will fall into > infinite loop if kmalloc(GFP_NOFS) under oom_lock reached __alloc_pages_may_oom() path. I would prefer GFP_NOWAIT. This is printing info for memcg OOMs and if the system is low on memory then memcg OOMs has lower importance than the system state.