+ mm-shmem-swap-never-use-swap-cache-and-readahead-for-swp_synchronous_io.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/shmem, swap: never use swap cache and readahead for SWP_SYNCHRONOUS_IO
has been added to the -mm mm-unstable branch.  Its filename is
     mm-shmem-swap-never-use-swap-cache-and-readahead-for-swp_synchronous_io.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-shmem-swap-never-use-swap-cache-and-readahead-for-swp_synchronous_io.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Kairui Song <kasong@tencent.com>
Subject: mm/shmem, swap: never use swap cache and readahead for SWP_SYNCHRONOUS_IO
Date: Sat, 5 Jul 2025 02:17:45 +0800

Currently if a THP swapin failed due to reasons like partially conflicting
swap cache or ZSWAP enabled, it will fallback to cached swapin.

Right now the swap cache still has a non-trivial overhead, and readahead
is not helpful for SWP_SYNCHRONOUS_IO devices, so we should always skip
the readahead and swap cache even if the swapin falls back to order 0.

So handle the fallback logic without falling back to the cached read.

Link: https://lkml.kernel.org/r/20250704181748.63181-7-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/shmem.c |   55 +++++++++++++++++++++++++++++++++------------------
 1 file changed, 36 insertions(+), 19 deletions(-)

--- a/mm/shmem.c~mm-shmem-swap-never-use-swap-cache-and-readahead-for-swp_synchronous_io
+++ a/mm/shmem.c
@@ -1975,13 +1975,16 @@ unlock:
 	return ERR_PTR(error);
 }
 
-static struct folio *shmem_swap_alloc_folio(struct inode *inode,
+static struct folio *shmem_swapin_direct(struct inode *inode,
 		struct vm_area_struct *vma, pgoff_t index,
-		swp_entry_t entry, int order, gfp_t gfp)
+		swp_entry_t swap, swp_entry_t index_entry,
+		int order, gfp_t gfp)
 {
 	struct shmem_inode_info *info = SHMEM_I(inode);
+	swp_entry_t entry = index_entry;
 	int nr_pages = 1 << order;
 	struct folio *new;
+	gfp_t alloc_gfp;
 	void *shadow;
 
 	/*
@@ -1989,6 +1992,7 @@ static struct folio *shmem_swap_alloc_fo
 	 * limit chance of success with further cpuset and node constraints.
 	 */
 	gfp &= ~GFP_CONSTRAINT_MASK;
+	alloc_gfp = gfp;
 	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
 		if (WARN_ON_ONCE(order))
 			return ERR_PTR(-EINVAL);
@@ -2003,19 +2007,22 @@ static struct folio *shmem_swap_alloc_fo
 		if ((vma && unlikely(userfaultfd_armed(vma))) ||
 		     !zswap_never_enabled() ||
 		     non_swapcache_batch(entry, nr_pages) != nr_pages)
-			return ERR_PTR(-EINVAL);
+			goto fallback;
 
-		gfp = limit_gfp_mask(vma_thp_gfp_mask(vma), gfp);
+		alloc_gfp = limit_gfp_mask(vma_thp_gfp_mask(vma), gfp);
+	}
+retry:
+	new = shmem_alloc_folio(alloc_gfp, order, info, index);
+	if (!new) {
+		new = ERR_PTR(-ENOMEM);
+		goto fallback;
 	}
-
-	new = shmem_alloc_folio(gfp, order, info, index);
-	if (!new)
-		return ERR_PTR(-ENOMEM);
 
 	if (mem_cgroup_swapin_charge_folio(new, vma ? vma->vm_mm : NULL,
-					   gfp, entry)) {
+					   alloc_gfp, entry)) {
 		folio_put(new);
-		return ERR_PTR(-ENOMEM);
+		new = ERR_PTR(-ENOMEM);
+		goto fallback;
 	}
 
 	/*
@@ -2030,7 +2037,9 @@ static struct folio *shmem_swap_alloc_fo
 	 */
 	if (swapcache_prepare(entry, nr_pages)) {
 		folio_put(new);
-		return ERR_PTR(-EEXIST);
+		new = ERR_PTR(-EEXIST);
+		/* Try smaller folio to avoid cache conflict */
+		goto fallback;
 	}
 
 	__folio_set_locked(new);
@@ -2044,6 +2053,15 @@ static struct folio *shmem_swap_alloc_fo
 	folio_add_lru(new);
 	swap_read_folio(new, NULL);
 	return new;
+fallback:
+	/* Order 0 swapin failed, nothing to fallback to, abort */
+	if (!order)
+		return new;
+	order = 0;
+	nr_pages = 1;
+	alloc_gfp = gfp;
+	entry = swap;
+	goto retry;
 }
 
 /*
@@ -2309,25 +2327,24 @@ static int shmem_swapin_folio(struct ino
 			count_vm_event(PGMAJFAULT);
 			count_memcg_event_mm(fault_mm, PGMAJFAULT);
 		}
-
 		/* Skip swapcache for synchronous device. */
 		if (data_race(si->flags & SWP_SYNCHRONOUS_IO)) {
-			folio = shmem_swap_alloc_folio(inode, vma, index,
-						       index_entry, order, gfp);
+			folio = shmem_swapin_direct(inode, vma, index, swap,
+						    index_entry, order, gfp);
 			if (!IS_ERR(folio)) {
-				swap = index_entry;
+				if (folio_test_large(folio))
+					swap = index_entry;
 				skip_swapcache = true;
 				goto alloced;
 			}
 
 			/*
-			 * Fallback to swapin order-0 folio unless the swap entry
-			 * already exists.
+			 * Direct swapin handled order 0 fallback already,
+			 * if it failed, abort.
 			 */
 			error = PTR_ERR(folio);
 			folio = NULL;
-			if (error == -EEXIST)
-				goto failed;
+			goto failed;
 		}
 		/* Cached swapin with readahead, only supports order 0 */
 		folio = shmem_swapin_cluster(swap, gfp, info, index);
_

Patches currently in -mm which might be from kasong@tencent.com are

mm-list_lru-refactor-the-locking-code.patch
mm-shmem-swap-improve-cached-mthp-handling-and-fix-potential-hung.patch
mm-shmem-swap-avoid-redundant-xarray-lookup-during-swapin.patch
mm-shmem-swap-tidy-up-thp-swapin-checks.patch
mm-shmem-swap-tidy-up-swap-entry-splitting.patch
mm-shmem-swap-avoid-false-positive-swap-cache-lookup.patch
mm-shmem-swap-never-use-swap-cache-and-readahead-for-swp_synchronous_io.patch
mm-shmem-swap-simplify-swapin-path-and-result-handling.patch
mm-shmem-swap-simplify-swap-entry-and-index-calculation-of-large-swapin.patch
mm-shmem-swap-fix-major-fault-counting.patch