linux-kernel - Re: [PATCH v2 2/5] mm: add PTE_MARKER

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Message-ID: <43dc4de2-1838-45dd-8d8f-3e5b95624c74@redhat.com>
Date: Tue, 22 Oct 2024 21:13:28 +0200
From: David Hildenbrand <david@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, "Paul E . McKenney"
 <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Muchun Song <muchun.song@...ux.dev>,
 Richard Henderson <richard.henderson@...aro.org>,
 Ivan Kokshaysky <ink@...assic.park.msu.ru>, Matt Turner
 <mattst88@...il.com>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
 "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
 Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
 Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...db.de>,
 linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
 linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
 Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
 linux-kselftest@...r.kernel.org, Sidhartha Kumar
 <sidhartha.kumar@...cle.com>, Jeff Xu <jeffxu@...omium.org>,
 Christoph Hellwig <hch@...radead.org>, linux-api@...r.kernel.org,
 John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v2 2/5] mm: add PTE_MARKER_GUARD PTE marker

On 21.10.24 19:26, Vlastimil Babka wrote:
> On 10/21/24 19:14, Lorenzo Stoakes wrote:
>> On Mon, Oct 21, 2024 at 07:00:53PM +0200, David Hildenbrand wrote:
>>>
>>>>
>>>>>
>>>>>>
>>>>>> Also the existing logic is that existing markers (HW poison, uffd-simulated HW
>>>>>> poison, uffd wp marker) are retained and no error raised on MADV_DONTNEED, and
>>>>>> no error on MADV_FREE either, so it'd be consistent with existing behaviour.
>>>>>
>>>>>
>>>>> HW poison / uffd-simulated HW poison are expected to be zapped: it's just
>>>>> like a mapped page with HWPOISON. So that is correct.
>>>>
>>>> Well, poison is _not_ zapped on MADV_DONTNEED but _is_ on MADV_FREE :) anyway, I
>>>
>>> Huh?
>>>
>>> madvise_dontneed_single_vma()->zap_page_range_single(details=NULL)->unmap_single_vma(details=NULL)
>>> ... zap_pte_range()
>>>
>>> } else if (is_hwpoison_entry(entry) ||
>>> 	   is_poisoned_swp_entry(entry)) {
>>> 	if (!should_zap_cows(details))
>>> 		continue;
>>> 	...
>>>
>>> Should just zap them.
>>>
>>> What am I missing?
>>
>> Yeah ok it's me who's missing something here, I hadn't noticed details == NULL
>> so should_zap_cows() is true, my mistake!
> 
> Well, good to know it's consistent then. As I've explained I see why zapping
> actual hwpoison makes sense for MADV_DONTNEED/MADV_FREE. That it's done also
> for uffd poison is not completely clear, but maybe it was just easier to
> implement. 

Note that in VM context "uffd poison" really just is "this was hwpoison 
on the source VM, so we mimic that on the destination VM, because the 
data *is* lost" -- so you want the exact same behavior.

For example, when a VM reboots you might just want to ZAP these hwpoison 
entries, and get fresh pages on next access.

So to me it makes sense that they are treated equally.

-- 
Cheers,

David / dhildenb