Re: [PATCH v1 2/3] mm/gup: use gup_can_follow_protnone() also in GUP-fast

[David Hildenbrand <david@redhat.com>]

On 26.08.22 16:59, David Hildenbrand wrote:
> 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 Note that the comment regarding THP migration is outdated: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 consistent and drop the somewhat outdated comments.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@redhat.com>
>> ---
>>  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.

... and looking into the details of TLB flush and GUP-fast interaction
nowadays, that case is no longer relevant. A TLB flush is no longer
sufficient to stop concurrent GUP-fast ever since we introduced generic
RCU GUP-fast.

-- 
Thanks,

David / dhildenb