On Mon, 8 May 2023 02:45:12 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:
> On 08.05.23 02:30, Andrew Morton wrote:
> > 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?
>
> This change makes unshare handling across all GUP-fast variants consistent,
> which is desirable as GUP-fast is complicated enough already even when
> consistent :)
Thanks, I added the below to the changelog:
David said:
: This change makes unshare handling across all GUP-fast variants
: consistent, which is desirable as GUP-fast is complicated enough
: already even when consistent.
:
: This function was the only one I seemed to have missed (or left out and
: forgot why -- maybe because it's really dead code for now). The COW
: selftest would identify the problem, so far there was no report.
: Either the selftest wasn't run on corresponding architectures with that
: hugetlb size, or that code is still dead code and unused by
: architectures.
:
: the original commit(s) that added unsharing explain why we care about
: these checks:
:
: a7f226604170acd6 ("mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page")
: 84209e87c6963f92 ("mm/gup: reliable R/O long-term pinning in COW mappings")
