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Date: Thu, 14 Mar 2024 18:12:38 +0100
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Subject: Re: [PATCH v1] x86/mm/pat: fix VM_PAT handling in COW mappings
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From: David Hildenbrand <david@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
 Wupeng Ma <mawupeng1@huawei.com>, Dave Hansen <dave.hansen@linux.intel.com>,
 Andy Lutomirski <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
 Andrew Morton <akpm@linux-foundation.org>
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On 14.03.24 17:42, David Hildenbrand wrote:
> On 12.03.24 20:38, David Hildenbrand wrote:
>> On 12.03.24 20:22, Matthew Wilcox wrote:
>>> On Tue, Mar 12, 2024 at 07:11:18PM +0100, David Hildenbrand wrote:
>>>> PAT handling won't do the right thing in COW mappings: the first PTE
>>>> (or, in fact, all PTEs) can be replaced during write faults to point at
>>>> anon folios. Reliably recovering the correct PFN and cachemode using
>>>> follow_phys() from PTEs will not work in COW mappings.
>>>
>>> I guess the first question is: Why do we want to support COW mappings
>>> of VM_PAT areas?  What breaks if we just disallow it?
>>
>> Well, that was my first approach. Then I decided to be less radical (IOW
>> make my life easier by breaking less user space) and "fix it" with
>> minimal effort.
>>
>> Chances of breaking some weird user space is possible, although I
>> believe for most such mappings MAP_PRIVATE doesn't make too much sense
>> sense.
>>
>> Nasty COW support for VM_PFNMAP mappings dates back forever. So does PAT
>> support.
>>
>> I can try finding digging through some possible user space users tomorrow.
> 
> As discussed, MAP_PRIVATE doesn't make too much sense for most PFNMAP
> mappings.
> 
> However, /dev/mem and /proc/vmcore are still used with MAP_PRIVATE in
> some cases.
> 
> Side note: /proc/vmcore is a bit weird: mmap_vmcore() sets VM_MIXEDMAP,
> and then we might call remap_pfn_range(), which sets VM_PFNMAP. I'm not
> so sure if that's what we want to happen ...

Correction: at least mmap_vmcore() ends up clearing VM_MAYWRITE. So no 
COW mapping. We could do the same to at least keep PROT_READ|MAP_PRIVATE 
working. If user space specified PROT_WRITE for whatever reason, it's 
not that easy.

-- 
Cheers,

David / dhildenb