linux-kernel - Re: mprotect and hugetlb mappings

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Message-ID: <ebaf79fe-25e4-9d42-4201-547ad7c8d480@redhat.com>
Date:   Mon, 17 Jul 2023 21:00:24 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Rik van Riel <riel@...riel.com>,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: mprotect and hugetlb mappings

On 17.07.23 20:52, Mike Kravetz wrote:
> On 07/17/23 18:19, Vlastimil Babka wrote:
>> On 7/6/23 01:53, Mike Kravetz wrote:
>>> On 07/06/23 00:22, Matthew Wilcox wrote:
>>>> On Wed, Jul 05, 2023 at 04:08:08PM -0700, Mike Kravetz wrote:
>>>>> I was recently asked about the behavior of mprotect on a hugetlb
>>>>> mapping where addr or addr+len is not hugetlb page size aligned.  As
>>>>> one might expect, EINVAL is returned in such cases.  However, the man
>>>>> page makes no mention of alignment requirements for hugetlb mappings.
>>>>>
>>>>> I am happy to submit man page updates if people agree this is the correct
>>>>> behavior.  We might even want to check alignment earlier in the code
>>>>> path as we fail when trying to split the vma today.
>>>>>
>>>>> An alternative behavior would be to operate on whole hugetlb pages within
>>>>> the range addr - addr+len.
>>>>
>>>> After a careful re-reading of the mprotect() man page, I suggest the
>>>> following behaviour ...
>>>>
>>>> addr must be a multiple of the hpage size.  Otherwise -EINVAL.
>>>> len should be rounded up to hpage size.
>>>>
>>>> I wonder how likely this change would be to break userspace code.
>>>> Maybe some test cases.
>>>
>>> My concern is that this is the approach I took with huegtlb MADV_DONTNEED,
>>> and this caused problems discussed and eventually modified here:
>>> https://lore.kernel.org/linux-mm/20221021154546.57df96db@imladris.surriel.com/
>>>
>>> In the MADV_DONTNEED case we were throwing away data.  With mprotect we are
>>> only modifying access to data.
>>
>> That can still confuse some userspace, no? I think realistically we can only
>> document the current implementation better, maybe improve it without
>> changing observed behavior as you suggested wrt the split vma fail. But
>> changing it would be dangerous.
> 
> Thanks for the comments Vlastimil.
> 
> That would be my thought/preferred path forward.  Simply document the
> current behavior, and MAYBE update code to be more explicit.
> 
> Any other thoughts?

I agree.

-- 
Cheers,

David / dhildenb