Re: [PATCH 2/2] mm/migrate: move common code to numa_migrate_check (was numa_migrate_prep)

[Zi Yan]

On 8 Aug 2024, at 0:14, Huang, Ying wrote:

> Zi Yan <ziy@xxxxxxxxxx> writes:
>
>> do_numa_page() and do_huge_pmd_numa_page() share a lot of common code. To
>> reduce redundancy, move common code to numa_migrate_prep() and rename
>> the function to numa_migrate_check() to reflect its functionality.
>>
>> Now do_huge_pmd_numa_page() also checks shared folios to set TNF_SHARED
>> flag.
>>
>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> ---
>>  mm/huge_memory.c | 14 ++-------
>>  mm/internal.h    |  5 ++--
>>  mm/memory.c      | 76 ++++++++++++++++++++++++------------------------
>>  3 files changed, 44 insertions(+), 51 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index a3c018f2b554..9b312cae6775 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1699,18 +1699,10 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>  	if (!folio)
>>  		goto out_map;
>>
>> -	/* See similar comment in do_numa_page for explanation */
>> -	if (!writable)
>> -		flags |= TNF_NO_GROUP;
>> -
>>  	nid = folio_nid(folio);
>
> NITPICK: It appears that we can remove "nid" local variable.

I thought about it, but

1. if folio is NULL from vm_normal_folio_pmd(), nid remains NUMA_NO_NODE,
2. if migration succeeds, nid is changed to target_nid,
3. if migration fails, nid is the folio node id,

all three are consumed by if (nid != NUMA_NO_NODE) before task_numa_fault().

I will give it a try in next version and see if I can remove it.

>
>> -	/*
>> -	 * For memory tiering mode, cpupid of slow memory page is used
>> -	 * to record page access time.  So use default value.
>> -	 */
>> -	if (!folio_use_access_time(folio))
>> -		last_cpupid = folio_last_cpupid(folio);
>> -	target_nid = numa_migrate_prep(folio, vmf, haddr, nid, &flags);
>> +
>> +	target_nid = numa_migrate_check(folio, vmf, haddr, &flags, writable,
>> +					&last_cpupid);
>>  	if (target_nid == NUMA_NO_NODE)
>>  		goto out_map;
>>  	if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 52f7fc4e8ac3..fb16e18c9761 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -1191,8 +1191,9 @@ void vunmap_range_noflush(unsigned long start, unsigned long end);
>>
>>  void __vunmap_range_noflush(unsigned long start, unsigned long end);
>>
>> -int numa_migrate_prep(struct folio *folio, struct vm_fault *vmf,
>> -		      unsigned long addr, int page_nid, int *flags);
>> +int numa_migrate_check(struct folio *folio, struct vm_fault *vmf,
>> +		      unsigned long addr, int *flags, bool writable,
>> +		      int *last_cpupid);
>>
>>  void free_zone_device_folio(struct folio *folio);
>>  int migrate_device_coherent_page(struct page *page);
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 503d493263df..b093df652c11 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5368,16 +5368,43 @@ static vm_fault_t do_fault(struct vm_fault *vmf)
>>  	return ret;
>>  }
>>
>> -int numa_migrate_prep(struct folio *folio, struct vm_fault *vmf,
>> -		      unsigned long addr, int page_nid, int *flags)
>> +int numa_migrate_check(struct folio *folio, struct vm_fault *vmf,
>> +		      unsigned long addr, int *flags,
>> +		      bool writable, int *last_cpupid)
>>  {
>>  	struct vm_area_struct *vma = vmf->vma;
>>
>> +	/*
>> +	 * Avoid grouping on RO pages in general. RO pages shouldn't hurt as
>> +	 * much anyway since they can be in shared cache state. This misses
>> +	 * the case where a mapping is writable but the process never writes
>> +	 * to it but pte_write gets cleared during protection updates and
>> +	 * pte_dirty has unpredictable behaviour between PTE scan updates,
>> +	 * background writeback, dirty balancing and application behaviour.
>> +	 */
>> +	if (!writable)
>> +		*flags |= TNF_NO_GROUP;
>> +
>> +	/*
>> +	 * Flag if the folio is shared between multiple address spaces. This
>> +	 * is later used when determining whether to group tasks together
>> +	 */
>> +	if (folio_likely_mapped_shared(folio) && (vma->vm_flags & VM_SHARED))
>> +		*flags |= TNF_SHARED;
>> +	/*
>> +	 * For memory tiering mode, cpupid of slow memory page is used
>> +	 * to record page access time.  So use default value.
>> +	 */
>> +	if (folio_use_access_time(folio))
>> +		*last_cpupid = (-1 & LAST_CPUPID_MASK);
>> +	else
>> +		*last_cpupid = folio_last_cpupid(folio);
>> +
>>  	/* Record the current PID acceesing VMA */
>>  	vma_set_access_pid_bit(vma);
>>
>>  	count_vm_numa_event(NUMA_HINT_FAULTS);
>> -	if (page_nid == numa_node_id()) {
>> +	if (folio_nid(folio) == numa_node_id()) {
>>  		count_vm_numa_event(NUMA_HINT_FAULTS_LOCAL);
>>  		*flags |= TNF_FAULT_LOCAL;
>>  	}
>> @@ -5442,13 +5469,13 @@ static void numa_rebuild_large_mapping(struct vm_fault *vmf, struct vm_area_stru
>>  static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>  {
>>  	struct vm_area_struct *vma = vmf->vma;
>> +	pte_t old_pte = vmf->orig_pte;
>
> The usage of old_pte is different from other use cases.  Where,
>
> old_pte = *pte;
> check old_pte and orig_pte
> generate new_pte from old_pte
> set new_pte
>
> We have used this before in do_numa_page(), but not do that now.  But I
> still think that it's better to follow the convention partly if
> possible.  This makes code easier to be read.  I notices that we don't
> follow it in do_huge_pmd_numa_page(), we may change that?

Sure, since I am trying to make do_numa_page() and do_huge_pmd_numa_page()
use the same pattern.

>
>> +	pte_t pte;
>>  	struct folio *folio = NULL;
>>  	int nid = NUMA_NO_NODE;
>>  	bool writable = false, ignore_writable = false;
>>  	bool pte_write_upgrade = vma_wants_manual_pte_write_upgrade(vma);
>> -	int last_cpupid;
>> -	int target_nid;
>> -	pte_t pte, old_pte;
>> +	int target_nid, last_cpupid = (-1 & LAST_CPUPID_MASK);
>
> Because we will initialize last_cpupid in numa_migrate_check(), we don't
> need to initialize it here?

Will remove it.

Thanks for the review.

Best Regards,
Yan, Zi

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