linux-kernel - Re: [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages

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Date:   Thu, 22 Jun 2017 22:07:18 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     "Elliott, Robert (Persistent Memory)" <elliott@....com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Borislav Petkov <bp@...e.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "Kani, Toshimitsu" <toshi.kani@....com>,
        "Vaden, Tom (HPE Server OS Architecture)" <tom.vaden@....com>
Subject: Re: [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of
 poison pages

On Wed, Jun 21, 2017 at 1:30 PM, Luck, Tony <tony.luck@...el.com> wrote:
>> Persistent memory does have unpoisoning and would require this inverse
>> operation - see drivers/nvdimm/pmem.c pmem_clear_poison() and core.c
>> nvdimm_clear_poison().
>
> Nice.  Well this code will need to cooperate with that ... in particular if the page
> is in an area that can be unpoisoned ... then we should do that *instead* of marking
> the page not present (which breaks up huge/large pages and so affects performance).
>
> Instead of calling it "arch_unmap_pfn" it could be called something like arch_handle_poison()
> and do something like:
>
> void arch_handle_poison(unsigned long pfn)
> {
>         if this is a pmem page && pmem_clear_poison(pfn)
>                 return
>         if this is a nvdimm page && nvdimm_clear_poison(pfn)
>                 return
>         /* can't clear, map out from 1:1 region */
>         ... code from my patch ...
> }
>
> I'm just not sure how those first two "if" bits work ... particularly in terms of CONFIG dependencies and system
> capabilities.  Perhaps each of pmem and nvdimm could register their unpoison functions and this code could
> just call each in turn?

We don't unpoison pmem without new data to write in it's place. What
context is arch_handle_poison() called? Ideally we only "clear" poison
when we know we are trying to write zero over the poisoned range.