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Tue, 01 Sep 2026 07:25:56 -0700 (PDT) Date: Tue, 1 Sep 2026 10:25:52 -0400 From: Johannes Weiner To: David Stevens Cc: Shakeel Butt , Michal Hocko , Roman Gushchin , Muchun Song , Andrew Morton , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] memcg: Don't call schedule_work when no spinning is allowed Message-ID: <20260901142552.GG3004@cmpxchg.org> References: <20260831234339.280376-1-stevensd@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: gmxtxd8q4wkjo4qptmiix1tanyrtkegw X-Rspamd-Queue-Id: 711F440005 X-HE-Tag: 1788272758-362720 X-HE-Meta: U2FsdGVkX1/mW6bDuZw245176IL04mC5lucUDvGQ8rFFShCPcSiCBDi+CsvLkcf+9CUxIEQopAOnHumeOAGRi0aooPRXFmXF5z1nLhsRrOP0S/MW4h4upFH0XVVtEWb2YbIk3AteSctV1Nmi0e8H4aOIRH4ERVVkBzt7HxhOAJov146cV8yIU3g+rnkHFlsuhSGbuChtWON3UzzyNy8fzIo2NEBS/DPPaQaUng8TCRuYf5cFRFgOaAXuEb9tsl46O7RICq/dh76N9t5P77H8VCinyBblkal8BytoZGdEKP13khHCFpEyVu8KIBbr6SLeJaRY7UzwTlsQ74xHVDGsUj3jCx5wQR1tAFzpMTgGdHyZqNZxRrnfqo7Oo0lSzmVIaqJtsgu6EF8VfPgCQagBMCGUNzL+w4GSLbmkWZISLuRW4BitP0g3iVpEcLUyFg8VGr5z8oX3zncsJzl+isYngqNDm4o4IaurA6uH0bkQzr1/uFkmo5Un2Ly5+7O6yWTPlqjv0UvQk0uWVddu5AiUOX1LRCPGZmSbqhc7xWNn82cPc8sBLtfU7/6ITioSfP2jFI1P1Y5Ef7c0fTG1rhHO7nalJ1I1PGocv7JcUOjEdh7Yc90HzR/IaV5jOFjCaDq8S/uPeJpM5acTG+GJmfVBt61hpFhgO0GvDKWdL5RvmgFk5Yc0/QEviwTHy0LKDJ9s6rGXSE81wdYXbhJdF02oO1DqsroVv4Iy55zAaTiGPdnKQf6jrX5FPjhUIncj7mnw6hqKOMwuGQuzp4nr6UoOn8y9E7T6VdUxVOKhIvuDYI7vkXFluFJsymwFySc8MSKowC6djaYufaej9wWfqrxtMwyfIY61rTP7ueI0TH/dnmnSUEnfVG9aU5y/2r7ucZ6rR9IxChGVu4qzJqo3XaEayylBxQofoENfY5tS7o23CgN77qhl32gG1l2XZP2wRuElMZUavmUUtK623ny0rFf 1vRGM5v5 NjXYa0znkzVpNm9q1Etmq3SSHHhVM+WzOIWVPiIxI/i6rloJcGo63me3BU/fM2JQ/W0AmCyAueB4Dio8ifUo/reAF5/6Hs4xq+ruyVIPk/zrWg5A6Iuy+XOVuvVZhD7wSFb+a/9CpB+6m1AQfpokToDp1KwV+pMbgZ2sK5W1/8f+7WK0mOdk2l4ieOSXStJaiZrbfapjNxnKLsNlUnYT/SNikMJDrPFfk26CE1c3m2tmUwc7RBIBP7Q7fE62JUsIw1e9yBroSaUq6dcNnPT+G487MOISV8DstivweIBdnvq99s7YrPtbH45pDERBESIf/4qU+2HPvw3buRkqm6ba5IT0zHYLFbSAe0M6BrD4NCBrqelz6gSjgmNYaye+CYVE/Aq1l8FnTEzoMEOk664cxVgFhCyMMIcQZFrLCW4+wma+U/KCUGxvA7NFJdpojgSbKoliMYjo3rUz5ByRC4Tp4YyC2g911HAsenxSZlpQWZWURfyzBwIZmeZokrQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 31, 2026 at 06:04:57PM -0700, David Stevens wrote: > On Mon, Aug 31, 2026 at 5:10 PM Shakeel Butt wrote: > > > > On Mon, Aug 31, 2026 at 04:43:39PM -0700, David Stevens wrote: > > > Memcg charging can be done from any context, but calling schedule_work() > > > isn't safe from an NMI. If memory.high is breached from a context where > > > spinning isn't allowed, use irq_work to schedule the reclaim work. > > > > > > Fixes: 3ac4638a734a ("memcg: make memcg_rstat_updated nmi safe") > > > Signed-off-by: David Stevens > > > > Did you hit this issue or just code inspection? I assume this is the > > done_restock code path. > > I just found this via code inspection. I spent a little bit trying to > trigger it for real, but the only way I managed was by writing a hacky > driver absuing alloc_pages_nolock(). > > > > --- > > > include/linux/memcontrol.h | 1 + > > > mm/memcontrol.c | 12 +++++++++++- > > > 2 files changed, 12 insertions(+), 1 deletion(-) > > > > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > > > index 8170bb8066a2..036d973ceca6 100644 > > > --- a/include/linux/memcontrol.h > > > +++ b/include/linux/memcontrol.h > > > @@ -219,6 +219,7 @@ struct mem_cgroup { > > > spinlock_t peaks_lock; > > > > > > /* Range enforcement for interrupt charges */ > > > + struct irq_work high_irq_work; > > > > Instead of adding more complexity, let's just return if we can not spin on > > done_restock path. > > > > There would be no guarantee that memcg reclaim would ever be > triggered, which also would also stop MEMCG_HIGH events from being > generated. Overall that seems a more serious than just dropping > userspace notifications like is done for MEMCG_MAX. I'm leaning that way too. It's an indefinite error, and it's a freely programmable surface. IMO, a few lines of relatively straight-forward, self-explanatory code is better than code that needs a comment and leaves a problem that somebody in the future might run into.