Re: [PATCH v1 15/39] mm/huge_memory: batch rmap operations in __split_huge_pmd_locked()

[David Hildenbrand <david@redhat.com>]

On 18.12.23 17:22, Ryan Roberts wrote:
> On 11/12/2023 15:56, David Hildenbrand wrote:
>> Let's use folio_add_anon_rmap_ptes(), batching the rmap operations.
>>
>> While at it, use more folio operations (but only in the code branch we're
>> touching), use VM_WARN_ON_FOLIO(), and pass RMAP_EXCLUSIVE instead of
>> manually setting PageAnonExclusive.
>>
>> We should never see non-anon pages on that branch: otherwise, the
>> existing page_add_anon_rmap() call would have been flawed already.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   mm/huge_memory.c | 23 +++++++++++++++--------
>>   1 file changed, 15 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 1f5634b2f374..82ad68fe0d12 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2398,6 +2398,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>   		unsigned long haddr, bool freeze)
>>   {
>>   	struct mm_struct *mm = vma->vm_mm;
>> +	struct folio *folio;
>>   	struct page *page;
>>   	pgtable_t pgtable;
>>   	pmd_t old_pmd, _pmd;
>> @@ -2493,16 +2494,18 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>   		uffd_wp = pmd_swp_uffd_wp(old_pmd);
>>   	} else {
>>   		page = pmd_page(old_pmd);
>> +		folio = page_folio(page);
>>   		if (pmd_dirty(old_pmd)) {
>>   			dirty = true;
>> -			SetPageDirty(page);
>> +			folio_set_dirty(folio);
>>   		}
>>   		write = pmd_write(old_pmd);
>>   		young = pmd_young(old_pmd);
>>   		soft_dirty = pmd_soft_dirty(old_pmd);
>>   		uffd_wp = pmd_uffd_wp(old_pmd);
>>   
>> -		VM_BUG_ON_PAGE(!page_count(page), page);
>> +		VM_WARN_ON_FOLIO(!folio_ref_count(folio), folio);
>> +		VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
> 
> Is this warning really correct? file-backed memory can be PMD-mapped with
> CONFIG_READ_ONLY_THP_FOR_FS, so presumably it can also have the need to be
> remapped as pte? Although I guess if we did have a file-backed folio, it
> definitely wouldn't be correct to call page_add_anon_rmap() /
> folio_add_anon_rmap_ptes()...

Yes, see the patch description where I spell that out.

PTE-remapping a file-back folio will simply zap the PMD and refault from 
the page cache after creating a page table.

So this is anon-only code.

-- 
Cheers,

David / dhildenb