Re: [PATCH v1 2/2] mm/migrate: move NUMA hinting fault folio isolation + checks under PTL

[Zi Yan <ziy@nvidia.com>]

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On 20 Jun 2024, at 17:29, David Hildenbrand wrote:

> Currently we always take a folio reference even if migration will not
> even be tried or isolation failed, requiring us to grab+drop an additional
> reference.
>
> Further, we end up calling folio_likely_mapped_shared() while the folio
> might have already been unmapped, because after we dropped the PTL, that
> can easily happen. We want to stop touching mapcounts and friends from
> such context, and only call folio_likely_mapped_shared() while the folio
> is still mapped: mapcount information is pretty much stale and unreliable
> otherwise.
>
> So let's move checks into numamigrate_isolate_folio(), rename that
> function to migrate_misplaced_folio_prepare(), and call that function
> from callsites where we call migrate_misplaced_folio(), but still with
> the PTL held.
>
> We can now stop taking temporary folio references, and really only take
> a reference if folio isolation succeeded. Doing the
> folio_likely_mapped_shared() + golio isolation under PT lock is now similar
> to how we handle MADV_PAGEOUT.
>
> While at it, combine the folio_is_file_lru() checks.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/linux/migrate.h |  7 ++++
>  mm/huge_memory.c        |  8 ++--
>  mm/memory.c             |  9 +++--
>  mm/migrate.c            | 81 +++++++++++++++++++----------------------
>  4 files changed, 55 insertions(+), 50 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index f9d92482d117..644be30b69c8 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -139,9 +139,16 @@ const struct movable_operations *page_movable_ops(struct page *page)
>  }
>
>  #ifdef CONFIG_NUMA_BALANCING
> +int migrate_misplaced_folio_prepare(struct folio *folio,
> +		struct vm_area_struct *vma, int node);
>  int migrate_misplaced_folio(struct folio *folio, struct vm_area_struct *vma,
>  			   int node);
>  #else
> +static inline int migrate_misplaced_folio_prepare(struct folio *folio,
> +		struct vm_area_struct *vma, int node)
> +{
> +	return -EAGAIN; /* can't migrate now */
> +}
>  static inline int migrate_misplaced_folio(struct folio *folio,
>  					 struct vm_area_struct *vma, int node)
>  {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index fc27dabcd8e3..4b2817bb2c7d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1688,11 +1688,13 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>  	if (node_is_toptier(nid))
>  		last_cpupid = folio_last_cpupid(folio);
>  	target_nid = numa_migrate_prep(folio, vmf, haddr, nid, &flags);
> -	if (target_nid == NUMA_NO_NODE) {
> -		folio_put(folio);
> +	if (target_nid == NUMA_NO_NODE)
> +		goto out_map;
> +	if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
> +		flags |= TNF_MIGRATE_FAIL;
>  		goto out_map;
>  	}
> -
> +	/* The folio is isolated and isolation code holds a folio reference. */
>  	spin_unlock(vmf->ptl);
>  	writable = false;
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 118660de5bcc..4fd1ecfced4d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5207,8 +5207,6 @@ int numa_migrate_prep(struct folio *folio, struct vm_fault *vmf,
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>
> -	folio_get(folio);
> -
>  	/* Record the current PID acceesing VMA */
>  	vma_set_access_pid_bit(vma);

Without taking a reference here, is it safe to call mpol_misplaced() on the folio
at the return? Can this folio be freed?

--
Best Regards,
Yan, Zi

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