linux-kernel - Re: [PATCH] mm/gup: add missing gup_must_unshare() check to gup_huge

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Message-Id: <20230507173022.96c9d24cb1aa6f57ed50f5fa@linux-foundation.org>
Date:   Sun, 7 May 2023 17:30:22 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH] mm/gup: add missing gup_must_unshare() check to
 gup_huge_pgd()

On Sat,  6 May 2023 15:05:25 +0100 Lorenzo Stoakes <lstoakes@...il.com> wrote:

> All other instances of gup_huge_pXd() perform the unshare check, so update
> the PGD-specific function to do so as well.
> 
> While checking pgd_write() might seem unusual, this function already
> performs such a check via pgd_access_permitted() so this is in line with
> the existing implementation.

Rationale seems strange.  "Other sites do it so this should also".  Why
is this a desirable change?  Maybe the "other instances" shouldn't be
performing this check either?

IOW, what are the runtime effects of this change?

Thanks.