Re: [PATCH] mm/gup: add missing gup_must_unshare() check to gup_huge_pgd()

[Andrew Morton]

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