+ swapfile-get-rid-of-volatile-and-avoid-redundant-read.patch added to mm-unstable branch

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

The patch titled
     Subject: swapfile: get rid of volatile and avoid redundant read
has been added to the -mm mm-unstable branch.  Its filename is
     swapfile-get-rid-of-volatile-and-avoid-redundant-read.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/swapfile-get-rid-of-volatile-and-avoid-redundant-read.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: swapfile: get rid of volatile and avoid redundant read
Date: Tue, 20 Dec 2022 02:58:37 +0800

Patch series "Clean up and fixes for swap", v2.

This series cleans up some code paths, saves a few cycles and reduces the
object size by a bit.  It also fixes some rare race issue with statistics.


This patch (of 4):

Convert a volatile variable to more readable READ_ONCE.  And this actually
avoids the code from reading the variable twice redundantly when it races.

Link: https://lkml.kernel.org/r/20221219185840.25441-1-ryncsn@gmail.com
Link: https://lkml.kernel.org/r/20221219185840.25441-2-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/swapfile.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/mm/swapfile.c~swapfile-get-rid-of-volatile-and-avoid-redundant-read
+++ a/mm/swapfile.c
@@ -1843,13 +1843,13 @@ static int unuse_pte_range(struct vm_are
 	pte_t *pte;
 	struct swap_info_struct *si;
 	int ret = 0;
-	volatile unsigned char *swap_map;
 
 	si = swap_info[type];
 	pte = pte_offset_map(pmd, addr);
 	do {
 		struct folio *folio;
 		unsigned long offset;
+		unsigned char swp_count;
 
 		if (!is_swap_pte(*pte))
 			continue;
@@ -1860,7 +1860,6 @@ static int unuse_pte_range(struct vm_are
 
 		offset = swp_offset(entry);
 		pte_unmap(pte);
-		swap_map = &si->swap_map[offset];
 		folio = swap_cache_get_folio(entry, vma, addr);
 		if (!folio) {
 			struct page *page;
@@ -1877,8 +1876,10 @@ static int unuse_pte_range(struct vm_are
 				folio = page_folio(page);
 		}
 		if (!folio) {
-			if (*swap_map == 0 || *swap_map == SWAP_MAP_BAD)
+			swp_count = READ_ONCE(si->swap_map[offset]);
+			if (swp_count == 0 || swp_count == SWAP_MAP_BAD)
 				goto try_next;
+
 			return -ENOMEM;
 		}
 
_

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

swapfile-get-rid-of-volatile-and-avoid-redundant-read.patch
swap-avoid-a-redundant-pte-map-if-ra-window-is-1.patch
swap-fold-swap_ra_clamp_pfn-into-swap_ra_info.patch
swap-avoid-holding-swap-reference-in-swap_cache_get_folio.patch