Re: [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings

[David Hildenbrand <david@redhat.com>]

On 09.08.22 00:08, Peter Xu wrote:
> On Mon, Aug 08, 2022 at 04:21:39PM -0400, Peter Xu wrote:
>> On Mon, Aug 08, 2022 at 06:25:21PM +0200, David Hildenbrand wrote:
>>>>> Relying on VM_SHARED to detect MAP_PRIVATE vs. MAP_SHARED is
>>>>> unfortunately wrong.
>>>>>
>>>>> If you're curious, take a look at f83a275dbc5c ("mm: account for
>>>>> MAP_SHARED mappings using VM_MAYSHARE and not VM_SHARED in hugetlbfs")
>>>>> and mmap() code.
>>>>>
>>>>> Long story short: if the file is read-only, we only have VM_MAYSHARE but
>>>>> not VM_SHARED (and consequently also not VM_MAYWRITE).
>>>>
>>>> To ask in another way: if file is RO but mapped RW (mmap() will have
>>>> VM_SHARED cleared but VM_MAYSHARE set), then if we check VM_MAYSHARE here
>>>> won't we grant write bit errornously while we shouldn't? As the user
>>>> doesn't really have write permission to the file.
>>>
>>> Thus the VM_WRITE check. :)
>>>
>>> I wonder if we should just do it cleanly and introduce the maybe_mkwrite
>>> semantics here as well. Then there is no need for additional VM_WRITE
>>> checks and hugetlb will work just like !hugetlb.
>>
>> Hmm yeah I think the VM_MAYSHARE check is correct, since we'll need to fail
>> the cases where MAYSHARE && !SHARE - we used to silently let it pass.
> 
> Sorry I think this is a wrong statement I made..  IIUC we'll fail correctly
> with/without the patch on any write to hugetlb RO regions.
> 
> Then I just don't see a difference on checking VM_SHARED or VM_MAYSHARE
> here, it's just that VM_MAYSHARE check should work too like VM_SHARED so I
> don't see a problem.
> 
> It also means I can't think of any valid case of having VM_WRITE when
> reaching here, then the WARN_ON_ONCE() is okay but maybe also redundant.
> Using maybe_mkwrite() seems misleading to me if FOLL_FORCE not ready for
> hugetlbfs after all.
> 

The main reason we'd have it would be to scream out lout and fail
gracefully if someone would -- for example -- use it for something like
FOLL_FORCE. I mean triggering a write fault without VM_WRITE on !hugetlb
works, so it would be easy to assume that it similarly simply works for
hugetlb as well. And the code most probably wouldn't even blow up
immediately :)

-- 
Thanks,

David / dhildenb