linux-kernel - Re: [v7 11/16] mm/migrate_device: add THP splitting during migration

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Message-ID: <20251019081954.luz3mp5ghdhii3vr@master>
Date: Sun, 19 Oct 2025 08:19:54 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Balbir Singh <balbirs@...dia.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linux-mm@...ck.org, akpm@...ux-foundation.org,
	David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
	Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
	Byungchul Park <byungchul@...com>,
	Gregory Price <gourry@...rry.net>,
	Ying Huang <ying.huang@...ux.alibaba.com>,
	Alistair Popple <apopple@...dia.com>,
	Oscar Salvador <osalvador@...e.de>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
	Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Ralph Campbell <rcampbell@...dia.com>,
	Mika Penttilä <mpenttil@...hat.com>,
	Matthew Brost <matthew.brost@...el.com>,
	Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v7 11/16] mm/migrate_device: add THP splitting during migration

On Wed, Oct 01, 2025 at 04:57:02PM +1000, Balbir Singh wrote:
[...]
> static int __folio_split(struct folio *folio, unsigned int new_order,
> 		struct page *split_at, struct page *lock_at,
>-		struct list_head *list, bool uniform_split)
>+		struct list_head *list, bool uniform_split, bool unmapped)
> {
> 	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
> 	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>@@ -3765,13 +3757,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> 		 * is taken to serialise against parallel split or collapse
> 		 * operations.
> 		 */
>-		anon_vma = folio_get_anon_vma(folio);
>-		if (!anon_vma) {
>-			ret = -EBUSY;
>-			goto out;
>+		if (!unmapped) {
>+			anon_vma = folio_get_anon_vma(folio);
>+			if (!anon_vma) {
>+				ret = -EBUSY;
>+				goto out;
>+			}
>+			anon_vma_lock_write(anon_vma);
> 		}
> 		mapping = NULL;
>-		anon_vma_lock_write(anon_vma);
> 	} else {
> 		unsigned int min_order;
> 		gfp_t gfp;
>@@ -3838,7 +3832,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> 		goto out_unlock;
> 	}
> 
>-	unmap_folio(folio);
>+	if (!unmapped)
>+		unmap_folio(folio);
> 
> 	/* block interrupt reentry in xa_lock and spinlock */
> 	local_irq_disable();
>@@ -3925,10 +3920,13 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> 
> 			next = folio_next(new_folio);
> 
>+			zone_device_private_split_cb(folio, new_folio);
>+
> 			expected_refs = folio_expected_ref_count(new_folio) + 1;
> 			folio_ref_unfreeze(new_folio, expected_refs);
> 
>-			lru_add_split_folio(folio, new_folio, lruvec, list);
>+			if (!unmapped)
>+				lru_add_split_folio(folio, new_folio, lruvec, list);
> 
> 			/*
> 			 * Anonymous folio with swap cache.
>@@ -3959,6 +3957,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> 			__filemap_remove_folio(new_folio, NULL);
> 			folio_put_refs(new_folio, nr_pages);
> 		}
>+
>+		zone_device_private_split_cb(folio, NULL);
> 		/*
> 		 * Unfreeze @folio only after all page cache entries, which
> 		 * used to point to it, have been updated with new folios.
>@@ -3982,6 +3982,9 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> 
> 	local_irq_enable();
> 
>+	if (unmapped)
>+		return ret;

As the comment of __folio_split() and __split_huge_page_to_list_to_order()
mentioned:

  * The large folio must be locked
  * After splitting, the after-split folio containing @lock_at remains locked

But here we seems to change the prerequisites.

Hmm.. I am not sure this is correct.

>+
> 	if (nr_shmem_dropped)
> 		shmem_uncharge(mapping->host, nr_shmem_dropped);
> 

-- 
Wei Yang
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