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

[Huang, Ying]

Zi Yan <ziy@xxxxxxxxxx> writes:

> On 18 Jul 2024, at 4:36, Huang, Ying wrote:
>
>> Zi Yan <zi.yan@xxxxxxxx> writes:
>>
>>> From: Zi Yan <ziy@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxx>
>>> ---
>>>  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.

Yes.

> 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(),

IIUC, if !pte_same()/!pmd_same(), the fault has been processed on
another CPU.  For example, do_numa_page()/do_huge_pmd_numa_page() has
been called on another CPU and task_numa_fault() has been called for the
PTE/PMD already.

> Let me know if I get this wrong. Thanks.
>

--
Best Regards,
Huang, Ying