linux-kernel - Re: mprotect and hugetlb mappings

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Date:   Mon, 17 Jul 2023 18:19:18 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        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 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.