Re: [PATCH v1] x86/mm/pat: fix VM_PAT handling in COW mappings

[mawupeng <mawupeng1@huawei.com>]

On 2024/3/15 0:42, 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 ...
> 
> As far as I can see, makedumpfile always mmap's memory to be dumped (/dev/mem, /proc/vmcore) using PROT_READ+MAP_PRIVATE, resulting in a COW mapping.
> 
> 
> In my opinion, we should use this fairly simple fix to keep it working for now and look into disabling any MAP_PRIVATE of VM_PFNMAP separately, for all architectures.
> 
> But I'll leave the decision to x86 maintainers.

Hi, x86 maintainers:

kindle ping.

>