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List:       linux-parisc
Subject:    Re: [PATCH RFCv2] mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault/prealloc memory
From:       David Hildenbrand <david () redhat ! com>
Date:       2021-03-15 16:28:40
Message-ID: df583cb8-ed13-92a1-811f-46d193ab4ae7 () redhat ! com
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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

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