linux-kernel - Re: mprotect and hugetlb mappings

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Date:   Wed, 5 Jul 2023 16:53:22 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     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 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.
-- 
Mike Kravetz