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Subject: Re: [PATCH RFCv2] mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to
 prefault/prealloc memory
From:   David Hildenbrand <david@redhat.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>, Michal Hocko <mhocko@suse.com>,
        Oscar Salvador <osalvador@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Minchan Kim <minchan@kernel.org>, Jann Horn <jannh@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dave Hansen <dave.hansen@intel.com>,
        Hugh Dickins <hughd@google.com>,
        Rik van Riel <riel@surriel.com>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
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        Max Filippov <jcmvbkbc@gmail.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Peter Xu <peterx@redhat.com>,
        Rolf Eike Beer <eike-kernel@sf-tec.de>,
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        linux-parisc@vger.kernel.org, linux-xtensa@linux-xtensa.org,
        linux-arch@vger.kernel.org, Linux API <linux-api@vger.kernel.org>
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Date:   Mon, 15 Mar 2021 17:28:40 +0100
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On 15.03.21 14:26, David Hildenbrand wrote:
> On 15.03.21 14:03, Kirill A. Shutemov wrote:
>> On Mon, Mar 15, 2021 at 01:25:40PM +0100, David Hildenbrand wrote:
>>> On 15.03.21 13:22, Kirill A. Shutemov wrote:
>>>> On Mon, Mar 08, 2021 at 05:45:20PM +0100, David Hildenbrand wrote:
>>>>> +			case -EHWPOISON: /* Skip over any poisoned pages. */
>>>>> +				start += PAGE_SIZE;
>>>>> +				continue;
>>>>
>>>> Why is it good approach? It's not abvious to me.
>>>
>>> My main motivation was to simplify return code handling. I don't want to
>>> return -EHWPOISON to user space
>>
>> Why? Hiding the problem under the rug doesn't help anybody. SIGBUS later
>> is not better than an error upfront.
> 
> Well, if you think about "prefaulting page tables", the first intuition
> is certainly not to check for poisoned pages, right? After all, you are
> not actually accessing memory, you are allocating memory if required and
> fill page tables. OTOH, mlock() will also choke on poisoned pages.
> 
> With the current semantics, you can start and run a VM just fine.
> Preallocation/prefaulting succeeded after all. On access you will get a
> SIGBUS, from which e.g., QEMU can recover by injecting an MCE into the
> guest - just like if you would hit a poisoned page later.
> 
> The problem we are talking about is most probably very rare, especially
> when using MADV_POPULATE_ for actual preallocation.
> 
> I don't have a strong opinion; not bailing out on poisoned pages felt
> like the right thing to do.

I'll switch to propagating -EHWPOISON, it matches how e.g., mlock() 
behaves -- not ignoring poisoned pages. Thanks!

-- 
Thanks,

David / dhildenb