Re: [PATCH v2 3/3] mm: rmap: Use flush_cache_range() to flush cache for hugetlb pages

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 5/4/2022 4:17 AM, Mike Kravetz wrote:
> On 4/27/22 03:52, Baolin Wang wrote:
>> Now we will use flush_cache_page() to flush cache for anonymous hugetlb
>> pages when unmapping or migrating a hugetlb page mapping, but the
>> flush_cache_page() only handles a PAGE_SIZE range on some architectures
>> (like arm32, arc and so on), which will cause potential cache issues.
>> Thus change to use flush_cache_range() to cover the whole size of a
>> hugetlb page.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/rmap.c | 90 ++++++++++++++++++++++++++++++++++-----------------------------
>>   1 file changed, 48 insertions(+), 42 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 4f0d115..6fdd198 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1528,13 +1528,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>   		anon_exclusive = folio_test_anon(folio) &&
>>   				 PageAnonExclusive(subpage);
>>   
>> -		if (folio_test_hugetlb(folio) && !folio_test_anon(folio)) {
>> -			/*
>> -			 * To call huge_pmd_unshare, i_mmap_rwsem must be
>> -			 * held in write mode.  Caller needs to explicitly
>> -			 * do this outside rmap routines.
>> -			 */
>> -			VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
>> +		if (folio_test_hugetlb(folio)) {
>>   			/*
>>   			 * huge_pmd_unshare may unmap an entire PMD page.
>>   			 * There is no way of knowing exactly which PMDs may
>> @@ -1544,22 +1538,31 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>   			 */
>>   			flush_cache_range(vma, range.start, range.end);
>>   
>> -			if (huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
>> -				flush_tlb_range(vma, range.start, range.end);
>> -				mmu_notifier_invalidate_range(mm, range.start,
>> -							      range.end);
>> -
>> +			if (!folio_test_anon(folio)) {
>>   				/*
>> -				 * The ref count of the PMD page was dropped
>> -				 * which is part of the way map counting
>> -				 * is done for shared PMDs.  Return 'true'
>> -				 * here.  When there is no other sharing,
>> -				 * huge_pmd_unshare returns false and we will
>> -				 * unmap the actual page and drop map count
>> -				 * to zero.
>> +				 * To call huge_pmd_unshare, i_mmap_rwsem must be
>> +				 * held in write mode.  Caller needs to explicitly
>> +				 * do this outside rmap routines.
>>   				 */
>> -				page_vma_mapped_walk_done(&pvmw);
>> -				break;
>> +				VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
>> +
>> +				if (huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
>> +					flush_tlb_range(vma, range.start, range.end);
>> +					mmu_notifier_invalidate_range(mm, range.start,
>> +								      range.end);
>> +
>> +					/*
>> +					 * The ref count of the PMD page was dropped
>> +					 * which is part of the way map counting
>> +					 * is done for shared PMDs.  Return 'true'
>> +					 * here.  When there is no other sharing,
>> +					 * huge_pmd_unshare returns false and we will
>> +					 * unmap the actual page and drop map count
>> +					 * to zero.
>> +					 */
>> +					page_vma_mapped_walk_done(&pvmw);
>> +					break;
>> +				}
>>   			}
>>   		} else {
>>   			flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
>> @@ -1885,13 +1888,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>>   		anon_exclusive = folio_test_anon(folio) &&
>>   				 PageAnonExclusive(subpage);
>>   
>> -		if (folio_test_hugetlb(folio) && !folio_test_anon(folio)) {
>> -			/*
>> -			 * To call huge_pmd_unshare, i_mmap_rwsem must be
>> -			 * held in write mode.  Caller needs to explicitly
>> -			 * do this outside rmap routines.
>> -			 */
>> -			VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
>> +		if (folio_test_hugetlb(folio)) {
>>   			/*
>>   			 * huge_pmd_unshare may unmap an entire PMD page.
>>   			 * There is no way of knowing exactly which PMDs may
>> @@ -1901,22 +1898,31 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>>   			 */
>>   			flush_cache_range(vma, range.start, range.end);
>>   
>> -			if (huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
>> -				flush_tlb_range(vma, range.start, range.end);
>> -				mmu_notifier_invalidate_range(mm, range.start,
>> -							      range.end);
>> -
>> +			if (!folio_test_anon(folio)) {
>>   				/*
>> -				 * The ref count of the PMD page was dropped
>> -				 * which is part of the way map counting
>> -				 * is done for shared PMDs.  Return 'true'
>> -				 * here.  When there is no other sharing,
>> -				 * huge_pmd_unshare returns false and we will
>> -				 * unmap the actual page and drop map count
>> -				 * to zero.
>> +				 * To call huge_pmd_unshare, i_mmap_rwsem must be
>> +				 * held in write mode.  Caller needs to explicitly
>> +				 * do this outside rmap routines.
>>   				 */
>> -				page_vma_mapped_walk_done(&pvmw);
>> -				break;
>> +				VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
>> +
>> +				if (huge_pmd_unshare(mm, vma, &address, pvmw.pte)) {
>> +					flush_tlb_range(vma, range.start, range.end);
>> +					mmu_notifier_invalidate_range(mm, range.start,
>> +								      range.end);
>> +
>> +					/*
>> +					 * The ref count of the PMD page was dropped
>> +					 * which is part of the way map counting
>> +					 * is done for shared PMDs.  Return 'true'
>> +					 * here.  When there is no other sharing,
>> +					 * huge_pmd_unshare returns false and we will
>> +					 * unmap the actual page and drop map count
>> +					 * to zero.
>> +					 */
>> +					page_vma_mapped_walk_done(&pvmw);
>> +					break;
>> +				}
>>   			}
>>   		} else {
>>   			flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
> 
> Thanks,
> The code looks fine.  It is unfortunate that we need so many levels of
> indenting and exceed 80 columns.  But, that is OK.

I'll do a cleanup to make it more readable with factoring the hugetlb 
case into a new function after the fix series[1].

[1] 
https://lore.kernel.org/linux-arm-kernel/20220503120343.6264e126@thinkpad/T/

> 
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
> I see you have a followup series to address the call to ptep_clear_flush()
> for hugetlb pages not unmapped via huge_pmd_share and will take a look at
> that soon.

Thanks.