The patch titled
Subject: percpu_counter: extend _limited_add() to negative amounts
has been added to the -mm mm-unstable branch. Its filename is
percpu_counter-extend-_limited_add-to-negative-amounts.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/percpu_counter-extend-_limited_add-to-negative-amounts.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: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: percpu_counter: extend _limited_add() to negative amounts
Date: Wed, 11 Oct 2023 21:40:09 -0700 (PDT)
Though tmpfs does not need it, percpu_counter_limited_add() can be twice
as useful if it works sensibly with negative amounts (subs) - typically
decrements towards a limit of 0 or nearby: as suggested by Dave Chinner.
And in the course of that reworking, skip the percpu counter sum if it is
already obvious that the limit would be passed: as suggested by Tim Chen.
Extend the comment above __percpu_counter_limited_add(), defining the
behaviour with positive and negative amounts, allowing negative limits,
but not bothering about overflow beyond S64_MAX.
Link: https://lkml.kernel.org/r/8f86083b-c452-95d4-365b-f16a2e4ebcd4@xxxxxxxxxx
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Carlos Maiolino <cem@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Chuck Lever <chuck.lever@xxxxxxxxxx>
Cc: Darrick J. Wong <djwong@xxxxxxxxxx>
Cc: Dave Chinner <dchinner@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/percpu_counter.h | 11 +++++-
lib/percpu_counter.c | 54 ++++++++++++++++++++++---------
2 files changed, 49 insertions(+), 16 deletions(-)
--- a/include/linux/percpu_counter.h~percpu_counter-extend-_limited_add-to-negative-amounts
+++ a/include/linux/percpu_counter.h
@@ -198,14 +198,21 @@ static inline bool
percpu_counter_limited_add(struct percpu_counter *fbc, s64 limit, s64 amount)
{
unsigned long flags;
+ bool good = false;
s64 count;
+ if (amount == 0)
+ return true;
+
local_irq_save(flags);
count = fbc->count + amount;
- if (count <= limit)
+ if ((amount > 0 && count <= limit) ||
+ (amount < 0 && count >= limit)) {
fbc->count = count;
+ good = true;
+ }
local_irq_restore(flags);
- return count <= limit;
+ return good;
}
/* non-SMP percpu_counter_add_local is the same with percpu_counter_add */
--- a/lib/percpu_counter.c~percpu_counter-extend-_limited_add-to-negative-amounts
+++ a/lib/percpu_counter.c
@@ -279,8 +279,16 @@ int __percpu_counter_compare(struct perc
EXPORT_SYMBOL(__percpu_counter_compare);
/*
- * Compare counter, and add amount if the total is within limit.
- * Return true if amount was added, false if it would exceed limit.
+ * Compare counter, and add amount if total is: less than or equal to limit if
+ * amount is positive, or greater than or equal to limit if amount is negative.
+ * Return true if amount is added, or false if total would be beyond the limit.
+ *
+ * Negative limit is allowed, but unusual.
+ * When negative amounts (subs) are given to percpu_counter_limited_add(),
+ * the limit would most naturally be 0 - but other limits are also allowed.
+ *
+ * Overflow beyond S64_MAX is not allowed for: counter, limit and amount
+ * are all assumed to be sane (far from S64_MIN and S64_MAX).
*/
bool __percpu_counter_limited_add(struct percpu_counter *fbc,
s64 limit, s64 amount, s32 batch)
@@ -288,10 +296,10 @@ bool __percpu_counter_limited_add(struct
s64 count;
s64 unknown;
unsigned long flags;
- bool good;
+ bool good = false;
- if (amount > limit)
- return false;
+ if (amount == 0)
+ return true;
local_irq_save(flags);
unknown = batch * num_online_cpus();
@@ -299,7 +307,8 @@ bool __percpu_counter_limited_add(struct
/* Skip taking the lock when safe */
if (abs(count + amount) <= batch &&
- fbc->count + unknown <= limit) {
+ ((amount > 0 && fbc->count + unknown <= limit) ||
+ (amount < 0 && fbc->count - unknown >= limit))) {
this_cpu_add(*fbc->counters, amount);
local_irq_restore(flags);
return true;
@@ -309,7 +318,19 @@ bool __percpu_counter_limited_add(struct
count = fbc->count + amount;
/* Skip percpu_counter_sum() when safe */
- if (count + unknown > limit) {
+ if (amount > 0) {
+ if (count - unknown > limit)
+ goto out;
+ if (count + unknown <= limit)
+ good = true;
+ } else {
+ if (count + unknown < limit)
+ goto out;
+ if (count - unknown >= limit)
+ good = true;
+ }
+
+ if (!good) {
s32 *pcount;
int cpu;
@@ -317,15 +338,20 @@ bool __percpu_counter_limited_add(struct
pcount = per_cpu_ptr(fbc->counters, cpu);
count += *pcount;
}
+ if (amount > 0) {
+ if (count > limit)
+ goto out;
+ } else {
+ if (count < limit)
+ goto out;
+ }
+ good = true;
}
- good = count <= limit;
- if (good) {
- count = __this_cpu_read(*fbc->counters);
- fbc->count += count + amount;
- __this_cpu_sub(*fbc->counters, count);
- }
-
+ count = __this_cpu_read(*fbc->counters);
+ fbc->count += count + amount;
+ __this_cpu_sub(*fbc->counters, count);
+out:
raw_spin_unlock(&fbc->lock);
local_irq_restore(flags);
return good;
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
shmem-shrink-shmem_inode_info-dir_offsets-in-a-union.patch
shmem-remove-vma-arg-from-shmem_get_folio_gfp.patch
shmem-factor-shmem_falloc_wait-out-of-shmem_fault.patch
shmem-trivial-tidyups-removing-extra-blank-lines-etc.patch
shmem-shmem_acct_blocks-and-shmem_inode_acct_blocks.patch
shmem-move-memcg-charge-out-of-shmem_add_to_page_cache.patch
shmem-_add_to_page_cache-before-shmem_inode_acct_blocks.patch
shmempercpu_counter-add-_limited_addfbc-limit-amount.patch
percpu_counter-extend-_limited_add-to-negative-amounts.patch
hugetlbfs-drop-shared-numa-mempolicy-pretence.patch
kernfs-drop-shared-numa-mempolicy-hooks.patch
mempolicy-fix-migrate_pages2-syscall-return-nr_failed.patch
mempolicy-trivia-delete-those-ancient-pr_debugs.patch
mempolicy-trivia-slightly-more-consistent-naming.patch
mempolicy-trivia-use-pgoff_t-in-shared-mempolicy-tree.patch
mempolicy-mpol_shared_policy_init-without-pseudo-vma.patch
mempolicy-remove-confusing-mpol_mf_lazy-dead-code.patch
mm-add-page_rmappable_folio-wrapper.patch
mempolicy-alloc_pages_mpol-for-numa-policy-without-vma.patch
mempolicy-mmap_lock-is-not-needed-while-migrating-folios.patch
mempolicy-migration-attempt-to-match-interleave-nodes.patch
