[PATCH v5 03/11] mm, swap: prepare the swap IO path for vswap

COLD16d

From: Nhat Pham <nphamcs@gmail.com>
Date: 2026-09-18 18:02:49
Also in: cgroups, linux-mm, lkml
Subsystem: memory management, memory management - swap, the rest, zswap compressed swap caching · Maintainers: Andrew Morton, Chris Li, Kairui Song, Linus Torvalds, Johannes Weiner, Yosry Ahmed, Nhat Pham

Revision v5 of 3 in this series.

Revisions (3)

2026-08-06 v3 [diff vs current]
2026-08-25 v4 [diff vs current]
2026-09-18 v5 current

In preparation for adding a physical swap backend for vswap, make the
swap IO path able to submit IO for a swap entry other than folio->swap.

The swap IO path derives the target device and sector from folio->swap.
For a vswap folio backed by a physical slot that entry is virtual, so it
identifies neither the backing device nor the sector to submit IO
against.

Compute the sector from an explicit entry (swap_folio_sector becomes
swap_entry_sector), thread that entry through swap_add_folio,
__swap_writeout and ops->can_merge, and stash it in swap_iocb so the
submit and completion paths address the IO from it rather than from
folio->swap.

This lets the batching path serve both vswap entries (backed by a
physical slot) and physical entries mapped directly into PTEs.

ops->can_merge changes anchor with it: swap_bdev_can_merge() compares
sectors and swap_fs_can_merge() byte offsets, and both now measure from the
batch head plus the accumulated length instead of from the previous folio
plus its size. The two agree at every step, since a batch only grows
through merges that passed the same test. The blkg comparison is unchanged
and still anchors on the last folio in the batch.

All callers pass folio->swap, so there is no functional change.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 include/linux/swap.h     |  2 +-
 include/linux/swap_ops.h |  9 ++++---
 mm/page_io.c             | 54 +++++++++++++++++++---------------------
 mm/swap.h                |  3 ++-
 mm/swapfile.c            |  6 ++---
 mm/zswap.c               |  2 +-
 6 files changed, 38 insertions(+), 38 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index abf658f7861f..5ab050b2457c 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h

@@ -409,7 +409,7 @@ extern int __swap_count(swp_entry_t entry);
 extern bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry);
 extern int swp_swapcount(swp_entry_t entry);
 extern struct swap_info_struct *get_swap_device(swp_entry_t entry);
-sector_t swap_folio_sector(struct folio *folio);
+sector_t swap_entry_sector(swp_entry_t entry);
 
 /*
  * If there is an existing swap slot reference (swap entry) and the caller

diff --git a/include/linux/swap_ops.h b/include/linux/swap_ops.h
index 57ac6c703f68..223c84548bde 100644
--- a/include/linux/swap_ops.h
+++ b/include/linux/swap_ops.h

@@ -12,6 +12,7 @@ struct swap_iocb {
 	struct bio_vec		bvecs[SWAP_CLUSTER_MAX];
 	int			nr_bvecs;
 	int			len;
+	swp_entry_t		entry;	/* first slot in the batch; addresses the IO */
 };
 
 struct swap_io_ctx {

@@ -30,15 +31,15 @@ struct swap_io_ctx {
 struct swap_ops {
 	unsigned int		flags;
 
-	bool (*can_merge)(struct folio *folio, struct folio *prev_folio,
-			size_t prev_folio_size, int rw);
+	bool (*can_merge)(struct folio *folio, swp_entry_t phys,
+			struct swap_iocb *sio, int rw);
 	void (*submit_write)(struct swap_io_ctx *ctx);
 	void (*submit_read)(struct swap_io_ctx *ctx);
 };
 
 void swap_fs_prepare_rw(struct swap_io_ctx *ctx, int rw, struct iov_iter *iter);
-bool swap_fs_can_merge(struct folio *folio, struct folio *prev_folio,
-		size_t prev_folio_size, int rw);
+bool swap_fs_can_merge(struct folio *folio, swp_entry_t phys,
+		struct swap_iocb *sio, int rw);
 int swap_fs_activate(struct swap_info_struct *sis, const struct swap_ops *ops);
 
 #endif /* _MM_SWAP_OPS_H */

diff --git a/mm/page_io.c b/mm/page_io.c
index 2e7fb335eb01..632683232622 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c

@@ -265,7 +265,7 @@ int swap_writeout(struct swap_io_ctx *ctx, struct folio *folio)
 		return AOP_WRITEPAGE_ACTIVATE;
 	}
 
-	__swap_writeout(ctx, folio);
+	__swap_writeout(ctx, folio, folio->swap);
 	return 0;
 out_unlock:
 	folio_unlock(folio);

@@ -336,24 +336,22 @@ int sio_pool_init(void)
 }
 
 static bool swap_can_merge(struct swap_io_ctx *ctx, struct folio *folio,
-		int rw)
+		swp_entry_t phys, int rw)
 {
-	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
-	struct bio_vec *last_bv = &ctx->sio->bvecs[ctx->sio->nr_bvecs - 1];
-	struct folio *prev_folio = bvec_folio(last_bv);
-	size_t prev_folio_size = folio_size(prev_folio);
+	struct swap_info_struct *sis = __swap_entry_to_info(phys);
 
 	if (ctx->sis != sis)
 		return false;
-	return sis->ops->can_merge(folio, prev_folio, prev_folio_size, rw);
+	return sis->ops->can_merge(folio, phys, ctx->sio, rw);
 }
 
-static void swap_add_folio(struct swap_io_ctx *ctx, struct folio *folio, int rw)
+static void swap_add_folio(struct swap_io_ctx *ctx, struct folio *folio,
+		swp_entry_t phys, int rw)
 {
-	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
+	struct swap_info_struct *sis = __swap_entry_to_info(phys);
 	struct swap_iocb *sio = ctx->sio;
 
-	if (sio && !swap_can_merge(ctx, folio, rw)) {
+	if (sio && !swap_can_merge(ctx, folio, phys, rw)) {
 		if (rw == WRITE)
 			swap_write_submit(ctx);
 		else

@@ -366,6 +364,7 @@ static void swap_add_folio(struct swap_io_ctx *ctx, struct folio *folio, int rw)
 		ctx->sio = sio = mempool_alloc(sio_pool, GFP_NOIO);
 		sio->nr_bvecs = 0;
 		sio->len = 0;
+		sio->entry = phys;
 	}
 	bvec_set_folio(&sio->bvecs[sio->nr_bvecs], folio, folio_size(folio), 0);
 	sio->len += folio_size(folio);

@@ -386,7 +385,8 @@ static void swap_add_folio(struct swap_io_ctx *ctx, struct folio *folio, int rw)
 	}
 }
 
-void __swap_writeout(struct swap_io_ctx *ctx, struct folio *folio)
+void __swap_writeout(struct swap_io_ctx *ctx, struct folio *folio,
+		swp_entry_t phys)
 {
 	VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio);

@@ -402,7 +402,7 @@ void __swap_writeout(struct swap_io_ctx *ctx, struct folio *folio)
 
 	folio_start_writeback(folio);
 	folio_unlock(folio);
-	swap_add_folio(ctx, folio, WRITE);
+	swap_add_folio(ctx, folio, phys, WRITE);
 }
 
 /*

@@ -506,7 +506,7 @@ void swap_read_folio(struct swap_io_ctx *ctx, struct folio *folio)
 
 	/* We have to read from slower devices. Increase zswap protection. */
 	zswap_folio_swapin(folio);
-	swap_add_folio(ctx, folio, READ);
+	swap_add_folio(ctx, folio, folio->swap, READ);
 
 finish:
 	if (workingset) {

@@ -538,8 +538,6 @@ static void swap_fs_write_complete(struct kiocb *iocb, long ret)
 	bool failed = ret != sio->len;
 
 	if (failed) {
-		struct folio *folio = bvec_folio(&sio->bvecs[0]);
-
 		/*
 		 * In the case of swap-over-nfs, this can be a temporary failure
 		 * if the system has limited memory for allocating transmit

@@ -547,7 +545,7 @@ static void swap_fs_write_complete(struct kiocb *iocb, long ret)
 		 * folio_rotate_reclaimable but rate-limit the messages.
 		 */
 		pr_err_ratelimited("Write error %ld on dio swapfile (%llu)\n",
-				   ret, swap_dev_pos(folio->swap));
+				   ret, swap_dev_pos(sio->entry));
 	}
 
 	swap_write_end(sio, failed);

@@ -619,7 +617,7 @@ static void swap_bdev_submit_write(struct swap_io_ctx *ctx)
 	bio_init(bio, ctx->sis->bdev, sio->bvecs, ARRAY_SIZE(sio->bvecs),
 			REQ_OP_WRITE | REQ_SWAP);
 	bio->bi_iter.bi_size = sio->len;
-	bio->bi_iter.bi_sector = swap_folio_sector(bio_first_folio_all(bio));
+	bio->bi_iter.bi_sector = swap_entry_sector(sio->entry);
 	bio_associate_blkg_from_folio(bio, bio_first_folio_all(bio));
 
 	if (ctx->sis->flags & SWP_SYNCHRONOUS_IO) {

@@ -639,7 +637,7 @@ static void swap_bdev_submit_read(struct swap_io_ctx *ctx)
 	bio_init(bio, ctx->sis->bdev, sio->bvecs, ARRAY_SIZE(sio->bvecs),
 			REQ_OP_READ);
 	bio->bi_iter.bi_size = sio->len;
-	bio->bi_iter.bi_sector = swap_folio_sector(bio_first_folio_all(bio));
+	bio->bi_iter.bi_sector = swap_entry_sector(sio->entry);
 
 	if (ctx->sis->flags & SWP_SYNCHRONOUS_IO) {
 		/*

@@ -657,13 +655,14 @@ static void swap_bdev_submit_read(struct swap_io_ctx *ctx)
 	}
 }
 
-static bool swap_bdev_can_merge(struct folio *folio, struct folio *prev_folio,
-		size_t prev_folio_size, int rw)
+static bool swap_bdev_can_merge(struct folio *folio, swp_entry_t phys,
+		struct swap_iocb *sio, int rw)
 {
-	if (swap_folio_sector(folio) !=
-	    swap_folio_sector(prev_folio) + (prev_folio_size >> SECTOR_SHIFT))
+	if (swap_entry_sector(phys) !=
+	    swap_entry_sector(sio->entry) + (sio->len >> SECTOR_SHIFT))
 		return false;
-	if (rw == WRITE && !folio_blkg_can_merge(folio, prev_folio))
+	if (rw == WRITE && !folio_blkg_can_merge(folio,
+			bvec_folio(&sio->bvecs[sio->nr_bvecs - 1])))
 		return false;
 	return true;
 }

@@ -679,7 +678,7 @@ void swap_fs_prepare_rw(struct swap_io_ctx *ctx, int rw, struct iov_iter *iter)
 	struct swap_iocb *sio = ctx->sio;
 
 	init_sync_kiocb(&sio->iocb, ctx->sis->swap_file);
-	sio->iocb.ki_pos = swap_dev_pos(bvec_folio(&sio->bvecs[0])->swap);
+	sio->iocb.ki_pos = swap_dev_pos(sio->entry);
 	if (rw == WRITE)
 		sio->iocb.ki_complete = swap_fs_write_complete;
 	else

@@ -690,11 +689,10 @@ void swap_fs_prepare_rw(struct swap_io_ctx *ctx, int rw, struct iov_iter *iter)
 }
 EXPORT_SYMBOL_GPL(swap_fs_prepare_rw);
 
-bool swap_fs_can_merge(struct folio *folio, struct folio *prev_folio,
-		size_t prev_folio_size, int rw)
+bool swap_fs_can_merge(struct folio *folio, swp_entry_t phys,
+		struct swap_iocb *sio, int rw)
 {
-	return swap_dev_pos(folio->swap) ==
-		swap_dev_pos(prev_folio->swap) + prev_folio_size;
+	return swap_dev_pos(phys) == swap_dev_pos(sio->entry) + sio->len;
 }
 EXPORT_SYMBOL_GPL(swap_fs_can_merge);

diff --git a/mm/swap.h b/mm/swap.h
index 81e47dc36a02..df323d5e8da8 100644
--- a/mm/swap.h
+++ b/mm/swap.h

@@ -320,7 +320,8 @@ void swap_read_folio(struct swap_io_ctx *ctx, struct folio *folio);
 void swap_read_submit(struct swap_io_ctx *ctx);
 void swap_write_submit(struct swap_io_ctx *ctx);
 int swap_writeout(struct swap_io_ctx *ctx, struct folio *folio);
-void __swap_writeout(struct swap_io_ctx *ctx, struct folio *folio);
+void __swap_writeout(struct swap_io_ctx *ctx, struct folio *folio,
+		swp_entry_t phys);
 
 /* linux/mm/swap_state.c */
 extern struct address_space swap_space __read_mostly;

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 67a2399cf1ae..af6a162c4450 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c

@@ -342,14 +342,14 @@ offset_to_swap_extent(struct swap_info_struct *sis, unsigned long offset)
 	BUG();
 }
 
-sector_t swap_folio_sector(struct folio *folio)
+sector_t swap_entry_sector(swp_entry_t entry)
 {
-	struct swap_info_struct *sis = __swap_entry_to_info(folio->swap);
+	struct swap_info_struct *sis = __swap_entry_to_info(entry);
 	struct swap_extent *se;
 	sector_t sector;
 	pgoff_t offset;
 
-	offset = swp_offset(folio->swap);
+	offset = swp_offset(entry);
 	se = offset_to_swap_extent(sis, offset);
 	sector = se->start_block + (offset - se->start_page);
 	return sector << (PAGE_SHIFT - 9);

diff --git a/mm/zswap.c b/mm/zswap.c
index 3466c80ac188..c2430dbfc653 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c

@@ -1094,7 +1094,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	folio_set_reclaim(folio);
 
 	/* start writeback */
-	__swap_writeout(&ctx, folio);
+	__swap_writeout(&ctx, folio, folio->swap);
 	swap_write_submit(&ctx);
 
 out:

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