linux-kernel - Re: [PATCH v1 1/2] mm/migrate: make migrate_misplaced

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Date: Mon, 1 Jul 2024 09:44:56 +0200
From: David Hildenbrand <david@...hat.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1 1/2] mm/migrate: make migrate_misplaced_folio() return
 0 on success

On 01.07.24 09:36, Huang, Ying wrote:
> David Hildenbrand <david@...hat.com> writes:
> 
>> Let's just return 0 on success, which is less confusing.
>>
>> ... especially because we got it wrong in the migrate.h stub where we
>> have "return -EAGAIN; /* can't migrate now */" instead of "return 0;".
>> Likely this wrong return value doesn't currently matter, but it
>> certainly adds confusion.
>>
>> We'll add migrate_misplaced_folio_prepare() next, where we want to use
>> the same "return 0 on success" approach, so let's just clean this up.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>   mm/huge_memory.c | 5 ++---
>>   mm/memory.c      | 2 +-
>>   mm/migrate.c     | 4 ++--
>>   3 files changed, 5 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 0fffaa58a47a..fc27dabcd8e3 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1652,7 +1652,7 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>   	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>   	int nid = NUMA_NO_NODE;
>>   	int target_nid, last_cpupid = (-1 & LAST_CPUPID_MASK);
>> -	bool migrated = false, writable = false;
>> +	bool writable = false;
>>   	int flags = 0;
>>   
>>   	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>> @@ -1696,8 +1696,7 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>>   	spin_unlock(vmf->ptl);
>>   	writable = false;
>>   
>> -	migrated = migrate_misplaced_folio(folio, vma, target_nid);
>> -	if (migrated) {
>> +	if (!migrate_misplaced_folio(folio, vma, target_nid)) {
>>   		flags |= TNF_MIGRATED;
>>   		nid = target_nid;
>>   	} else {
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 00728ea95583..118660de5bcc 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5354,7 +5354,7 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>   	ignore_writable = true;
>>   
>>   	/* Migrate to the requested node */
>> -	if (migrate_misplaced_folio(folio, vma, target_nid)) {
>> +	if (!migrate_misplaced_folio(folio, vma, target_nid)) {
>>   		nid = target_nid;
>>   		flags |= TNF_MIGRATED;
>>   	} else {
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 781979567f64..0307b54879a0 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -2629,11 +2629,11 @@ int migrate_misplaced_folio(struct folio *folio, struct vm_area_struct *vma,
>>   					    nr_succeeded);
>>   	}
>>   	BUG_ON(!list_empty(&migratepages));
>> -	return isolated;
>> +	return isolated ? 0 : -EAGAIN;
> 
> Is it good to use -EAGAIN as error code always?  At least if
> nr_remaining < 0, we can use that?

Thanks for the review. I kept it simple for now, because the 
migrate_misplaced_folio_prepare() stub has:

	return -EAGAIN; /* can't migrate now */

and it doesn't really make a difference right now which exact return 
value we use.

(maybe some cases, like the stub, should return -EINVAL or -EOPNOTSUPP 
once we start caring about the exact value)

-- 
Cheers,

David / dhildenb