Message-ID: <DDB73347-5EBA-442A-A631-E533EE50EFC7@nvidia.com>
Date: Thu, 18 Jul 2024 10:40:37 -0400
From: Zi Yan <ziy@...dia.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/3] mm/migrate: move common code to
 numa_migrate_check (was numa_migrate_prep)

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

> Zi Yan <zi.yan@...t.com> writes:
>
>> From: Zi Yan <ziy@...dia.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@...dia.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.

OK, will make this change in a separate patch.

>
>>  		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.

Right. Will fix it in my code.

>
> 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.

Got it. Here is my plan:

1. Send the first two patches in this series separately, since they
are separate fixes. They can be picked up right now.
2. Send a separate patch to fix 2) above with cc: stable.
3. Clean up this patch and send it separately.

Thank you for the review. :)

>
>>
>>  	return 0;
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index b4d86436565b..7782b7bb3383 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -1217,8 +1217,9 @@ 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, bool writable,
>> +		      int *flags, 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 96c2f5b3d796..a252c0f13755 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5209,16 +5209,42 @@ 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, bool writable,
>> +		      int *flags, 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.
>> +	 */
>> +	if (folio_has_cpupid(folio))
>> +		*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;
>>  	}
>> @@ -5284,12 +5310,11 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>  {
>>  	struct vm_area_struct *vma = vmf->vma;
>>  	struct folio *folio = NULL;
>> -	int nid = NUMA_NO_NODE;
>> +	int target_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 last_cpupid = (-1 & LAST_CPUPID_MASK);
>> +	pte_t pte, old_pte = vmf->orig_pte;
>>  	int flags = 0, nr_pages;
>>
>>  	/*
>> @@ -5297,10 +5322,7 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>  	 * table lock, that its contents have not changed during fault handling.
>>  	 */
>>  	spin_lock(vmf->ptl);
>> -	/* Read the live PTE from the page tables: */
>> -	old_pte = ptep_get(vmf->pte);
>> -
>> -	if (unlikely(!pte_same(old_pte, vmf->orig_pte))) {
>> +	if (unlikely(!pte_same(old_pte, *vmf->pte))) {
>>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
>>  		goto out;
>>  	}
>> @@ -5320,35 +5342,10 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>  	if (!folio || folio_is_zone_device(folio))
>>  		goto out_map;
>>
>> -	/*
>> -	 * 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;
>> -
>> -	nid = folio_nid(folio);
>>  	nr_pages = folio_nr_pages(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 = (-1 & LAST_CPUPID_MASK);
>> -	else
>> -		last_cpupid = folio_last_cpupid(folio);
>> -	target_nid = numa_migrate_prep(folio, vmf, vmf->address, nid, &flags);
>> +
>> +	target_nid = numa_migrate_check(folio, vmf, vmf->address, writable,
>> +			&flags, &last_cpupid);
>>  	if (target_nid == NUMA_NO_NODE)
>>  		goto out_map;
>>  	if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
>> @@ -5362,9 +5359,9 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>
>>  	/* Migrate to the requested node */
>>  	if (!migrate_misplaced_folio(folio, vma, target_nid)) {
>> -		nid = target_nid;
>>  		flags |= TNF_MIGRATED;
>>  	} else {
>> +		target_nid = NUMA_NO_NODE;
>>  		flags |= TNF_MIGRATE_FAIL;
>>  		vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>>  					       vmf->address, &vmf->ptl);
>> @@ -5378,8 +5375,8 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>  	}
>>
>>  out:
>> -	if (nid != NUMA_NO_NODE)
>> -		task_numa_fault(last_cpupid, nid, nr_pages, flags);
>> +	if (target_nid != NUMA_NO_NODE)
>> +		task_numa_fault(last_cpupid, target_nid, nr_pages, flags);
>>  	return 0;
>>  out_map:
>>  	/*
>
> --
> Best Regards,
> Huang, Ying


Best Regards,
Yan, Zi

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