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Date:   Tue, 22 Oct 2019 12:24:57 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Oscar Salvador <osalvador@suse.de>
Cc:     n-horiguchi@ah.jp.nec.com, mike.kravetz@oracle.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 10/16] mm,hwpoison: Rework soft offline for free
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On Tue 22-10-19 11:58:52, Oscar Salvador wrote:
> On Tue, Oct 22, 2019 at 11:22:56AM +0200, Michal Hocko wrote:
> > Hmm, that might be a misunderstanding on my end. I thought that it is
> > the MCE handler to say whether the failure is recoverable or not. If yes
> > then we can touch the content of the memory (that would imply the
> > migration). Other than that both paths should be essentially the same,
> > no? Well unrecoverable case would be essentially force migration failure
> > path.
> > 
> > MADV_HWPOISON is explicitly documented to test MCE handling IIUC:
> > : This feature is intended for testing of memory error-handling
> > : code; it is available only if the kernel was configured with
> > : CONFIG_MEMORY_FAILURE.
> > 
> > There is no explicit note about the type of the error that is injected
> > but I think it is reasonably safe to assume this is a recoverable one.
> 
> MADV_HWPOISON stands for hard-offline.
> MADV_SOFT_OFFLINE stands for soft-offline.
> 
> MADV_SOFT_OFFLINE (since Linux 2.6.33)
>               Soft offline the pages in the range specified by addr and
>               length.  The memory of each page in the specified range is
>               preserved (i.e., when next accessed, the same content will be
>               visible, but in a new physical page frame), and the original
>               page is offlined (i.e., no longer used, and taken out of
>               normal memory management).  The effect of the
>               MADV_SOFT_OFFLINE operation is invisible to (i.e., does not
>               change the semantics of) the calling process.
> 
>               This feature is intended for testing of memory error-handling
>               code; it is available only if the kernel was configured with
>               CONFIG_MEMORY_FAILURE.

I have missed that one somehow. Thanks for pointing out.

[...]

> AFAICS, for hard-offline case, a recovered event would be if:
> 
> - the page to shut down is already free
> - the page was unmapped
> 
> In some cases we need to kill the process if it holds dirty pages.

Yes, I would expect that the page table would be poisoned and the
process receive a SIGBUS when accessing that memory.

> But we never migrate contents in hard-offline path.
> I guess it is because we cannot really trust the contents anymore.

Yes, that makes a perfect sense. What I am saying that the migration
(aka trying to recover) is the main and only difference. The soft
offline should poison page tables when not able to migrate as well
IIUC.
-- 
Michal Hocko
SUSE Labs