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>,
	"James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
	Helge Deller <deller@gmx.de>, Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Peter Xu <peterx@redhat.com>,
	Rolf Eike Beer <eike-kernel@sf-tec.de>,
	linux-alpha@vger.kernel.org, linux-mips@vger.kernel.org,
	linux-parisc@vger.kernel.org, linux-xtensa@linux-xtensa.org,
	linux-arch@vger.kernel.org, Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH RFCv2] mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault/prealloc memory
Date: Mon, 15 Mar 2021 17:28:40 +0100	[thread overview]
Message-ID: <df583cb8-ed13-92a1-811f-46d193ab4ae7@redhat.com> (raw)
In-Reply-To: <e59d6301-6ba8-1d7f-5c15-60364eec3fe1@redhat.com>

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