linux-kernel - Re: [PATCH v2] mm, hwpoison: Try to recover from copy-on write faults

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Date:   Thu, 20 Oct 2022 21:08:24 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Shuai Xue <xueshuai@...ux.alibaba.com>
Cc:     Naoya Horiguchi <naoya.horiguchi@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] mm, hwpoison: Try to recover from copy-on write faults

On Fri, Oct 21, 2022 at 09:52:01AM +0800, Shuai Xue wrote:
> 
> 
> 在 2022/10/21 AM4:05, Tony Luck 写道:
> > On Thu, Oct 20, 2022 at 09:57:04AM +0800, Shuai Xue wrote:
> >>
> >>
> >> 在 2022/10/20 AM1:08, Tony Luck 写道:

> > I'm experimenting with using sched_work() to handle the call to
> > memory_failure() (echoing what the machine check handler does using
> > task_work)_add() to avoid the same problem of not being able to directly
> > call memory_failure()).
> 
> Work queues permit work to be deferred outside of the interrupt context
> into the kernel process context. If we return to user-space before the
> queued memory_failure() work is processed, we will take the fault again,
> as we discussed recently.
> 
>     commit 7f17b4a121d0d ACPI: APEI: Kick the memory_failure() queue for synchronous errors
>     commit 415fed694fe11 ACPI: APEI: do not add task_work to kernel thread to avoid memory leak
> 
> So, in my opinion, we should add memory failure as a task work, like
> do_machine_check does, e.g.
> 
>     queue_task_work(&m, msg, kill_me_maybe);

Maybe ... but this case isn't pending back to a user instruction
that is trying to READ the poison memory address. The task is just
trying to WRITE to any address within the page.

So this is much more like a patrol scrub error found asynchronously
by the memory controller (in this case found asynchronously by the
Linux page copy function).  So I don't feel that it's really the
responsibility of the current task.

When we do return to user mode the task is going to be busy servicing
a SIGBUS ... so shouldn't try to touch the poison page before the
memory_failure() called by the worker thread cleans things up.

> > +	INIT_WORK(&p->work, do_sched_memory_failure);
> > +	p->pfn = pfn;
> > +	schedule_work(&p->work);
> > +}
> 
> I think there is already a function to do such work in mm/memory-failure.c.
> 
> 	void memory_failure_queue(unsigned long pfn, int flags)

Also pointed out by Miaohe Lin <linmiaohe@...wei.com> ... this does
exacly what I want, and is working well in tests so far. So perhaps
a cleaner solution than making the kill_me_maybe() function globally
visible.

-Tony