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

[David Hildenbrand]

On 10.08.22 21:29, Peter Xu wrote:
> On Wed, Aug 10, 2022 at 11:37:13AM +0200, David Hildenbrand wrote:
>> 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.
> 
> Having that WARN_ON_ONCE() is okay to me, but just to double check we're on
> the same page: why there's concern on using FOLL_FORCE? IIUC we're talking
> about shared mappings here, then no FOLL_FORCE possible at all?  IOW,
> "!is_cow_mapping()" should fail in check_vma_flags() already.

This code path also covers the anon case.
> 
> The other thing is I'm wondering whether patch 2 should be postponed anyway
> so that we can wait for a full resolution of the problem from Mike.

To make the code more robust and avoid any other such surprises I prefer
to have this in rather earlier than later.

As the commit says "Let's add a safety net ..."

-- 
Thanks,

David / dhildenb