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[217.140.101.70]) by mx.google.com with ESMTP id v27si3688232edb.444.2019.01.23.10.40.13 for ; Wed, 23 Jan 2019 10:40:14 -0800 (PST) Subject: Re: [PATCH v7 22/25] ACPI / APEI: Kick the memory_failure() queue for synchronous errors References: <20181203180613.228133-1-james.morse@arm.com> <20181203180613.228133-23-james.morse@arm.com> <20190121175850.GO29166@zn.tnic> From: James Morse Message-ID: <58053f17-5f03-8408-7252-a38ed3d448a9@arm.com> Date: Wed, 23 Jan 2019 18:40:08 +0000 MIME-Version: 1.0 In-Reply-To: <20190121175850.GO29166@zn.tnic> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Borislav Petkov Cc: linux-acpi@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, Marc Zyngier , Christoffer Dall , Will Deacon , Catalin Marinas , Naoya Horiguchi , Rafael Wysocki , Len Brown , Tony Luck , Dongjiu Geng , Xie XiuQi , Fan Wu Hi Boris, On 21/01/2019 17:58, Borislav Petkov wrote: > On Mon, Dec 03, 2018 at 06:06:10PM +0000, James Morse wrote: >> memory_failure() offlines or repairs pages of memory that have been >> discovered to be corrupt. These may be detected by an external >> component, (e.g. the memory controller), and notified via an IRQ. >> In this case the work is queued as not all of memory_failure()s work >> can happen in IRQ context. >> >> If the error was detected as a result of user-space accessing a >> corrupt memory location the CPU may take an abort instead. On arm64 >> this is a 'synchronous external abort', and on a firmware first >> system it is replayed using NOTIFY_SEA. >> >> This notification has NMI like properties, (it can interrupt >> IRQ-masked code), so the memory_failure() work is queued. If we >> return to user-space before the queued memory_failure() work is >> processed, we will take the fault again. This loop may cause platform >> firmware to exceed some threshold and reboot when Linux could have >> recovered from this error. >> >> If a ghes notification type indicates that it may be triggered again >> when we return to user-space, use the task-work and notify-resume >> hooks to kick the relevant memory_failure() queue before returning >> to user-space. >> --- >> I assume that if NOTIFY_NMI is coming from SMM it must suffer from >> this problem too. > > Good question. > > I'm guessing all those things should be queued on a normal struct > work_struct queue, no? ghes_notify_nmi() does this today with its: | irq_work_queue(&ghes_proc_irq_work); Once its in IRQ context, the irq_work pokes memory_failure_queue(), which schedule_work_on()s. Finally we schedule() in process context, and can unmap the affected memory. The problem is between each of these steps we might return to user-space and run the instruction that tripped all this to begin with. My SMM comment was because the CPU must jump from user-space->SMM, which injects an NMI into the kernel. The kernel's EIP must point into user-space, so returning from the NMI without doing the memory_failure() work puts us back the same position we started in. > Now, memory_failure_queue() does that and can run from IRQ context so > you need only an irq_work which can queue from NMI context. We do it > this way in the MCA code: > (was there something missing here?) > We queue in an irq_work in NMI context and work through the items in > process context. How are you getting from NMI to process context in one go? This patch causes the IRQ->process transition. The arch specific bit of this gives the irq work queue a kick if returning from the NMI would unmask IRQs. This makes it look like we moved from NMI to IRQ context without returning to user-space. Once ghes_handle_memory_failure() runs in IRQ context, it task_work_add()s the call to ghes_kick_memory_failure(). Finally on the way out of the kernel to user-space that task_work runs and the memory_failure() work happens in process context. During all this the user-space program counter can point at a poisoned location, but we don't return there until the memory_failure() work has been done. >> @@ -407,7 +447,22 @@ static void ghes_handle_memory_failure(struct acpi_hest_generic_data *gdata, int >> >> if (flags != -1) >> memory_failure_queue(pfn, flags); >> -#endif >> + >> + /* >> + * If the notification indicates that it was the interrupted >> + * instruction that caused the error, try to kick the >> + * memory_failure() queue before returning to user-space. >> + */ >> + if (ghes_is_synchronous(ghes) && current->mm != &init_mm) { >> + callback = kzalloc(sizeof(*callback), GFP_ATOMIC); > > Can we avoid that GFP_ATOMIC allocation and kfree() in > ghes_kick_memory_failure()? > > I mean, that struct ghes_memory_failure_work is small enough and we > already do lockless allocation: > > estatus_node = (void *)gen_pool_alloc(ghes_estatus_pool, node_len); > > so I guess we could add that ghes_memory_failure_work struct to that > estatus_node, hand it into ghes_do_proc() and then free it. I forget estatus_node is a linux thing, not an ACPI-spec thing! Hmmm, ghes_handle_memory_failure() runs for POLLED and irq error sources too, they don't have an estatus_node. We don't care about this ret_to_user() problem as they are all asynchronous, this is why we have ghes_is_synchronous()... It feels like there should be a way to do this, let me have a go... Thanks, James