On 25.08.22 18:46, David Hildenbrand wrote:
> There seems to be no reason why FOLL_FORCE during GUP-fast would have to
> fallback to the slow path when stumbling over a PROT_NONE mapped page. We
> only have to trigger hinting faults in case FOLL_FORCE is not set, and any
> kind of fault handling naturally happens from the slow path -- where
> NUMA hinting accounting/handling would be performed.
>
> Note that the comment regarding THP migration is outdated:
> commit 2b4847e73004 ("mm: numa: serialise parallel get_user_page against
> THP migration") described that this was required for THP due to lack of PMD
> migration entries. Nowadays, we do have proper PMD migration entries in
> place -- see set_pmd_migration_entry(), which does a proper
> pmdp_invalidate() when placing the migration entry.
>
> So let's just reuse gup_can_follow_protnone() here to make it
> consistent and drop the somewhat outdated comments.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
> mm/gup.c | 14 +++-----------
> 1 file changed, 3 insertions(+), 11 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index a1355dbd848e..dfef23071dc8 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2350,11 +2350,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> struct page *page;
> struct folio *folio;
>
> - /*
> - * Similar to the PMD case below, NUMA hinting must take slow
> - * path using the pte_protnone check.
> - */
> - if (pte_protnone(pte))
> + if (pte_protnone(pte) && !gup_can_follow_protnone(flags))
> goto pte_unmap;
>
> if (!pte_access_permitted(pte, flags & FOLL_WRITE))
> @@ -2736,12 +2732,8 @@ static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned lo
>
> if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
> pmd_devmap(pmd))) {
> - /*
> - * NUMA hinting faults need to be handled in the GUP
> - * slowpath for accounting purposes and so that they
> - * can be serialised against THP migration.
> - */
> - if (pmd_protnone(pmd))
> + if (pmd_protnone(pmd) &&
> + !gup_can_follow_protnone(flags))
> return 0;
>
> if (!gup_huge_pmd(pmd, pmdp, addr, next, flags,
I just stumbled over something interesting. If we have a pte_protnone()
entry, ptep_clear_flush() might not flush, because the !pte_accessible()
does not hold.
Consequently, we could be in trouble when using ptep_clear_flush() on a
pte_protnone() PTE to make sure that GUP cannot run anymore.
Will give this a better thought, but most probably I'll replace this
patch by a proper documentation update here.
--
Thanks,
David / dhildenb
