btrfs: allocate eb-attached btree pages as movable

Extent buffer pages allocated by alloc_extent_buffer() are attached to
btree_inode->i_mapping (the buffer_tree path), reach the LRU, and are
served by the btree_migrate_folio aops in fs/btrfs/disk-io.c. They are
migratable in practice once their owning extent buffer hits refs == 1,
which happens naturally. The buddy allocator classifies them by GFP,
however, and bare GFP_NOFS lands them in MIGRATE_UNMOVABLE pageblocks.

The result: every btree_inode page we read in pins an unmovable pageblock
from the page-superblock allocator's perspective, even though the page
itself can be moved.

Have each caller of btrfs_alloc_page_array, btrfs_alloc_folio_array,
and alloc_eb_folio_array pass in the full GFP mask directly, instead
of having the functions calculate it from boolean flags.

The alloc_extent_buffer call site passes GFP_NOFS | __GFP_NOFAIL |
__GFP_MOVABLE. All other call sites pass plain GFP_NOFS.

Three categories of caller stay on bare GFP_NOFS, deliberately:

  - alloc_dummy_extent_buffer / btrfs_clone_extent_buffer: the
    resulting eb is EXTENT_BUFFER_UNMAPPED, folio->mapping stays NULL,
    the folios never enter LRU, never get migrate_folio aops. Tagging
    them __GFP_MOVABLE would violate the page allocator's migrability
    contract and they would defeat compaction in MOVABLE pageblocks
    where isolate_migratepages_block skips non-LRU non-movable_ops
    pages outright.

  - btrfs_alloc_page_array callers in fs/btrfs/raid56.c (stripe
    pages), fs/btrfs/inode.c (encoded reads), fs/btrfs/ioctl.c (io_uring
    encoded reads), fs/btrfs/relocation.c (relocation buffers): same
    contract violation. raid56 stripe_pages additionally persist in
    the stripe cache (RBIO_CACHE_SIZE=1024) well beyond a single I/O,
    so they are not transient enough to hand-wave the contract.

  - btrfs_alloc_folio_array caller in fs/btrfs/scrub.c (stripe
    folios): same -- stripe->folios[] are private buffers freed via
    folio_put in release_scrub_stripe.

This change targets the dominant fragmentation source observed on the
page-superblock series: ~28 GB of btree_inode pages parked across
many tainted superpageblocks on a 250 GB test system with btrfs root,
preventing 1 GiB hugepage allocation from those regions. With the
movable hint, those pages now land in MOVABLE pageblocks where the
existing background defragger drains them through the standard
PB_has_movable gate, no LRU-sample fallback needed.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -617,24 +617,24 @@ static void end_bbio_data_read(struct btrfs_bio *bbio)
 }
 
 /*
- * Populate every free slot in a provided array with folios using GFP_NOFS.
+ * Populate every free slot in a provided array with folios.
  *
- * @nr_folios:   number of folios to allocate
- * @order:	 the order of the folios to be allocated
- * @folio_array: the array to fill with folios; any existing non-NULL entries in
- *		 the array will be skipped
+ * @nr_folios:    number of folios to allocate
+ * @order:	  folio order
+ * @folio_array:  array to fill with folios; non-NULL entries are skipped
+ * @gfp:          GFP flags for the allocation
  *
  * Return: 0        if all folios were able to be allocated;
  *         -ENOMEM  otherwise, the partially allocated folios would be freed and
  *                  the array slots zeroed
  */
 int btrfs_alloc_folio_array(unsigned int nr_folios, unsigned int order,
-			    struct folio **folio_array)
+			    struct folio **folio_array, gfp_t gfp)
 {
 	for (int i = 0; i < nr_folios; i++) {
 		if (folio_array[i])
 			continue;
-		folio_array[i] = folio_alloc(GFP_NOFS, order);
+		folio_array[i] = folio_alloc(gfp, order);
 		if (!folio_array[i])
 			goto error;
 	}
@@ -649,21 +649,18 @@ error:
 }
 
 /*
- * Populate every free slot in a provided array with pages, using GFP_NOFS.
+ * Populate every free slot in a provided array with pages.
  *
- * @nr_pages:   number of pages to allocate
- * @page_array: the array to fill with pages; any existing non-null entries in
- *		the array will be skipped
- * @nofail:	whether using __GFP_NOFAIL flag
+ * @nr_pages:    number of pages to allocate
+ * @page_array:  array to fill; non-NULL entries are skipped
+ * @gfp:         GFP flags for the allocation
  *
  * Return: 0        if all pages were able to be allocated;
  *         -ENOMEM  otherwise, the partially allocated pages would be freed and
  *                  the array slots zeroed
  */
-int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array,
-			   bool nofail)
+int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array, gfp_t gfp)
 {
-	const gfp_t gfp = nofail ? (GFP_NOFS | __GFP_NOFAIL) : GFP_NOFS;
 	unsigned int allocated;
 
 	for (allocated = 0; allocated < nr_pages;) {
@@ -687,13 +684,13 @@ int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array,
  *
  * For now, the folios populated are always in order 0 (aka, single page).
  */
-static int alloc_eb_folio_array(struct extent_buffer *eb, bool nofail)
+static int alloc_eb_folio_array(struct extent_buffer *eb, gfp_t gfp)
 {
 	struct page *page_array[INLINE_EXTENT_BUFFER_PAGES] = { 0 };
 	int num_pages = num_extent_pages(eb);
 	int ret;
 
-	ret = btrfs_alloc_page_array(num_pages, page_array, nofail);
+	ret = btrfs_alloc_page_array(num_pages, page_array, gfp);
 	if (ret < 0)
 		return ret;
 
@@ -3103,7 +3100,7 @@ struct extent_buffer *btrfs_clone_extent_buffer(const struct extent_buffer *src)
 	 */
 	set_bit(EXTENT_BUFFER_UNMAPPED, &new->bflags);
 
-	ret = alloc_eb_folio_array(new, false);
+	ret = alloc_eb_folio_array(new, GFP_NOFS);
 	if (ret)
 		goto release_eb;
 
@@ -3144,7 +3141,7 @@ struct extent_buffer *alloc_dummy_extent_buffer(struct btrfs_fs_info *fs_info,
 	if (!eb)
 		return NULL;
 
-	ret = alloc_eb_folio_array(eb, false);
+	ret = alloc_eb_folio_array(eb, GFP_NOFS);
 	if (ret)
 		goto release_eb;
 
@@ -3497,8 +3494,12 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 	}
 
 reallocate:
-	/* Allocate all pages first. */
-	ret = alloc_eb_folio_array(eb, true);
+	/*
+	 * Allocate all pages first. These will be attached to btree_inode->i_mapping
+	 * below (added to LRU, served by btree_migrate_folio), so request
+	 * __GFP_MOVABLE so the page allocator places them in MOVABLE pageblocks.
+	 */
+	ret = alloc_eb_folio_array(eb, GFP_NOFS | __GFP_NOFAIL | __GFP_MOVABLE);
 	if (ret < 0) {
 		btrfs_free_folio_state(prealloc);
 		goto out;

fs/btrfs/extent_io.h

@@ -400,10 +400,9 @@ static inline void btrfs_clear_folio_dirty_tag(struct folio *folio)
 	xa_unlock_irq(&folio->mapping->i_pages);
 }
 
-int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array,
-			   bool nofail);
+int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array, gfp_t gfp);
 int btrfs_alloc_folio_array(unsigned int nr_folios, unsigned int order,
-			    struct folio **folio_array);
+			    struct folio **folio_array, gfp_t gfp);
 
 #ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
 bool find_lock_delalloc_range(struct inode *inode,

fs/btrfs/inode.c

@@ -9496,7 +9496,7 @@ ssize_t btrfs_encoded_read_regular(struct kiocb *iocb, struct iov_iter *iter,
 	pages = kzalloc_objs(struct page *, nr_pages, GFP_NOFS);
 	if (!pages)
 		return -ENOMEM;
-	ret = btrfs_alloc_page_array(nr_pages, pages, false);
+	ret = btrfs_alloc_page_array(nr_pages, pages, GFP_NOFS);
 	if (ret) {
 		ret = -ENOMEM;
 		goto out;

fs/btrfs/ioctl.c

@@ -4617,7 +4617,7 @@ static int btrfs_uring_read_extent(struct kiocb *iocb, struct iov_iter *iter,
 	pages = kzalloc_objs(struct page *, nr_pages, GFP_NOFS);
 	if (!pages)
 		return -ENOMEM;
-	ret = btrfs_alloc_page_array(nr_pages, pages, 0);
+	ret = btrfs_alloc_page_array(nr_pages, pages, GFP_NOFS);
 	if (ret) {
 		ret = -ENOMEM;
 		goto out_fail;

fs/btrfs/raid56.c

@@ -1123,7 +1123,7 @@ static int alloc_rbio_pages(struct btrfs_raid_bio *rbio)
 {
 	int ret;
 
-	ret = btrfs_alloc_page_array(rbio->nr_pages, rbio->stripe_pages, false);
+	ret = btrfs_alloc_page_array(rbio->nr_pages, rbio->stripe_pages, GFP_NOFS);
 	if (ret < 0)
 		return ret;
 	/* Mapping all sectors */
@@ -1138,7 +1138,7 @@ static int alloc_rbio_parity_pages(struct btrfs_raid_bio *rbio)
 	int ret;
 
 	ret = btrfs_alloc_page_array(rbio->nr_pages - data_pages,
-				     rbio->stripe_pages + data_pages, false);
+				     rbio->stripe_pages + data_pages, GFP_NOFS);
 	if (ret < 0)
 		return ret;
 
@@ -1732,7 +1732,7 @@ static int alloc_rbio_data_pages(struct btrfs_raid_bio *rbio)
 	const int data_pages = rbio->nr_data * rbio->stripe_npages;
 	int ret;
 
-	ret = btrfs_alloc_page_array(data_pages, rbio->stripe_pages, false);
+	ret = btrfs_alloc_page_array(data_pages, rbio->stripe_pages, GFP_NOFS);
 	if (ret < 0)
 		return ret;
 

fs/btrfs/relocation.c

@@ -4051,7 +4051,7 @@ static int copy_remapped_data(struct btrfs_fs_info *fs_info, u64 old_addr,
 	if (!pages)
 		return -ENOMEM;
 
-	ret = btrfs_alloc_page_array(nr_pages, pages, 0);
+	ret = btrfs_alloc_page_array(nr_pages, pages, GFP_NOFS);
 	if (ret) {
 		ret = -ENOMEM;
 		goto end;

fs/btrfs/scrub.c

@@ -369,7 +369,8 @@ static int init_scrub_stripe(struct btrfs_fs_info *fs_info,
 
 	ASSERT(BTRFS_STRIPE_LEN >> min_folio_shift <= SCRUB_STRIPE_MAX_FOLIOS);
 	ret = btrfs_alloc_folio_array(BTRFS_STRIPE_LEN >> min_folio_shift,
-				      fs_info->block_min_order, stripe->folios);
+				      fs_info->block_min_order, stripe->folios,
+				      GFP_NOFS);
 	if (ret < 0)
 		goto error;