Re: [RFC PATCH 3/3] mm/migrate: move common code to numa_migrate_check (was numa_migrate_prep)

[Zi Yan <ziy@nvidia.com>]

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On 18 Jul 2024, at 4:36, Huang, Ying wrote:

> Zi Yan <zi.yan@sent.com> writes:
>
>> From: Zi Yan <ziy@nvidia.com>
>>
>> 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.
>>
>> There is some code difference between do_numa_page() and
>> do_huge_pmd_numa_page() before the code move:
>>
>> 1. do_huge_pmd_numa_page() did not check shared folios to set TNF_SHARED.
>> 2. do_huge_pmd_numa_page() did not check and skip zone device folios.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>  mm/huge_memory.c | 28 ++++++-----------
>>  mm/internal.h    |  5 +--
>>  mm/memory.c      | 81 +++++++++++++++++++++++-------------------------
>>  3 files changed, 52 insertions(+), 62 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 8c11d6da4b36..66d67d13e0dc 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1670,10 +1670,10 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>  	pmd_t pmd;
>>  	struct folio *folio;
>>  	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>> -	int nid = NUMA_NO_NODE;
>> -	int target_nid, last_cpupid = (-1 & LAST_CPUPID_MASK);
>> +	int target_nid = NUMA_NO_NODE;
>> +	int last_cpupid = (-1 & LAST_CPUPID_MASK);
>>  	bool writable = false;
>> -	int flags = 0;
>> +	int flags = 0, nr_pages;
>>
>>  	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>>  	if (unlikely(!pmd_same(oldpmd, *vmf->pmd))) {
>> @@ -1693,21 +1693,13 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>  		writable = true;
>>
>>  	folio = vm_normal_folio_pmd(vma, haddr, pmd);
>> -	if (!folio)
>> +	if (!folio || folio_is_zone_device(folio))
>
> This change appears unrelated.  Can we put it in a separate patch?
>
> IIUC, this isn't necessary even in do_numa_page()?  Because in
> change_pte_range(), folio_is_zone_device() has been checked already.
> But It doesn't hurt too.
>
>>  		goto out_map;
>>
>> -	/* See similar comment in do_numa_page for explanation */
>> -	if (!writable)
>> -		flags |= TNF_NO_GROUP;
>> +	nr_pages = folio_nr_pages(folio);
>>
>> -	nid = folio_nid(folio);
>> -	/*
>> -	 * For memory tiering mode, cpupid of slow memory page is used
>> -	 * to record page access time.  So use default value.
>> -	 */
>> -	if (folio_has_cpupid(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, writable,
>> +			&flags, &last_cpupid);
>>  	if (target_nid == NUMA_NO_NODE)
>>  		goto out_map;
>>  	if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
>> @@ -1720,8 +1712,8 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>
>>  	if (!migrate_misplaced_folio(folio, vma, target_nid)) {
>>  		flags |= TNF_MIGRATED;
>> -		nid = target_nid;
>>  	} else {
>> +		target_nid = NUMA_NO_NODE;
>>  		flags |= TNF_MIGRATE_FAIL;
>>  		vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>>  		if (unlikely(!pmd_same(oldpmd, *vmf->pmd))) {
>> @@ -1732,8 +1724,8 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>  	}
>>
>>  out:
>> -	if (nid != NUMA_NO_NODE)
>> -		task_numa_fault(last_cpupid, nid, HPAGE_PMD_NR, flags);
>> +	if (target_nid != NUMA_NO_NODE)
>> +		task_numa_fault(last_cpupid, target_nid, nr_pages, flags);
>
> This appears a behavior change.  IIUC, there are 2 possible issues.
>
> 1) if migrate_misplaced_folio() fails, folio_nid() should be used as
> nid.  "target_nid" as variable name here is confusing, because
> folio_nid() is needed in fact.
>
> 2) if !pmd_same(), task_numa_fault() should be skipped.  The original
> code is buggy.
>
> Similar issues for do_numa_page().
>
> If my understanding were correct, we should implement a separate patch
> to fix 2) above.  And that may need to be backported.

Hmm, the original code seems OK after I checked the implementation.
There are two possible !pte_same()/!pmd_same() locations:
1) at the beginning of do_numa_page() and do_huge_pmd_numa_page() and the faulted
PTE/PMD changed before the folio can be checked, task_numa_fault() should not be
called.

2) when migrate_misplaced_folio() failed and the PTE/PMD changed, but the folio
has been determined and checked. task_numa_fault() should be called even if
!pte_same()/!pmd_same(),

Let me know if I get this wrong. Thanks.


Best Regards,
Yan, Zi

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