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@xxxxxxxxxxx>
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@xxxxxxxxx
Link: https://lkml.kernel.org/r/20221219185840.25441-2-ryncsn@xxxxxxxxx
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxxxx 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
