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

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Message-ID: <87plrxo96w.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Mon, 01 Jul 2024 15:36:23 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: David Hildenbrand <david@...hat.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

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?

Otherwise, this LGTM.  Thanks!

>  
>  out:
>  	folio_put(folio);
> -	return 0;
> +	return -EAGAIN;
>  }
>  #endif /* CONFIG_NUMA_BALANCING */
>  #endif /* CONFIG_NUMA */

--
Best Regards,
Huang, Ying