linux-kernel - Re: mprotect and hugetlb mappings

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Message-ID: <ZKX7PerFpnoMT5Pc@casper.infradead.org>
Date:   Thu, 6 Jul 2023 00:22:37 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
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 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.