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 21 Jul 2024, at 21:47, Huang, Ying wrote:

> Zi Yan <ziy@nvidia.com> writes:
>
>> 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.
>
> 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.

Hmm, this behavior at least dates back to 2015 at
commit 074c238177a7 ("mm: numa: slow PTE scan rate if migration failures occur”).
So cc Mel.

The code is https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/memory.c?id=074c238177a75f5e79af3b2cb6a84e54823ef950#n3102. I have not checked older
commits.

I wonder how far we should trace back.


Best Regards,
Yan, Zi

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