The patch titled
Subject: zram: permit preemption with active compression stream
has been added to the -mm mm-unstable branch. Its filename is
zram-permit-preemption-with-active-compression-stream.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zram-permit-preemption-with-active-compression-stream.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: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Subject: zram: permit preemption with active compression stream
Date: Fri, 14 Feb 2025 13:50:14 +0900
Currently, per-CPU stream access is done from a non-preemptible (atomic)
section, which imposes the same atomicity requirements on compression
backends as entry spin-lock, and makes it impossible to use algorithms
that can schedule/wait/sleep during compression and decompression.
Switch to preemptible per-CPU model, similar to the one used in zswap.
Instead of a per-CPU local lock, each stream carries a mutex which is
locked throughout entire time zram uses it for compression or
decompression, so that cpu-dead event waits for zram to stop using a
particular per-CPU stream and release it.
Link: https://lkml.kernel.org/r/20250214045208.1388854-3-senozhatsky@xxxxxxxxxxxx
Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Suggested-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: Kairui Song <ryncsn@xxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
drivers/block/zram/zcomp.c | 41 ++++++++++++++++++++++----------
drivers/block/zram/zcomp.h | 6 ++--
drivers/block/zram/zram_drv.c | 20 +++++++--------
3 files changed, 42 insertions(+), 25 deletions(-)
--- a/drivers/block/zram/zcomp.c~zram-permit-preemption-with-active-compression-stream
+++ a/drivers/block/zram/zcomp.c
@@ -6,7 +6,7 @@
#include <linux/slab.h>
#include <linux/wait.h>
#include <linux/sched.h>
-#include <linux/cpu.h>
+#include <linux/cpuhotplug.h>
#include <linux/crypto.h>
#include <linux/vmalloc.h>
@@ -109,13 +109,29 @@ ssize_t zcomp_available_show(const char
struct zcomp_strm *zcomp_stream_get(struct zcomp *comp)
{
- local_lock(&comp->stream->lock);
- return this_cpu_ptr(comp->stream);
+ for (;;) {
+ struct zcomp_strm *zstrm = raw_cpu_ptr(comp->stream);
+
+ /*
+ * Inspired by zswap
+ *
+ * stream is returned with ->mutex locked which prevents
+ * cpu_dead() from releasing this stream under us, however
+ * there is still a race window between raw_cpu_ptr() and
+ * mutex_lock(), during which we could have been migrated
+ * from a CPU that has already destroyed its stream. If
+ * so then unlock and re-try on the current CPU.
+ */
+ mutex_lock(&zstrm->lock);
+ if (likely(zstrm->buffer))
+ return zstrm;
+ mutex_unlock(&zstrm->lock);
+ }
}
-void zcomp_stream_put(struct zcomp *comp)
+void zcomp_stream_put(struct zcomp_strm *zstrm)
{
- local_unlock(&comp->stream->lock);
+ mutex_unlock(&zstrm->lock);
}
int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
@@ -151,12 +167,9 @@ int zcomp_decompress(struct zcomp *comp,
int zcomp_cpu_up_prepare(unsigned int cpu, struct hlist_node *node)
{
struct zcomp *comp = hlist_entry(node, struct zcomp, node);
- struct zcomp_strm *zstrm;
+ struct zcomp_strm *zstrm = per_cpu_ptr(comp->stream, cpu);
int ret;
- zstrm = per_cpu_ptr(comp->stream, cpu);
- local_lock_init(&zstrm->lock);
-
ret = zcomp_strm_init(comp, zstrm);
if (ret)
pr_err("Can't allocate a compression stream\n");
@@ -166,16 +179,17 @@ int zcomp_cpu_up_prepare(unsigned int cp
int zcomp_cpu_dead(unsigned int cpu, struct hlist_node *node)
{
struct zcomp *comp = hlist_entry(node, struct zcomp, node);
- struct zcomp_strm *zstrm;
+ struct zcomp_strm *zstrm = per_cpu_ptr(comp->stream, cpu);
- zstrm = per_cpu_ptr(comp->stream, cpu);
+ mutex_lock(&zstrm->lock);
zcomp_strm_free(comp, zstrm);
+ mutex_unlock(&zstrm->lock);
return 0;
}
static int zcomp_init(struct zcomp *comp, struct zcomp_params *params)
{
- int ret;
+ int ret, cpu;
comp->stream = alloc_percpu(struct zcomp_strm);
if (!comp->stream)
@@ -186,6 +200,9 @@ static int zcomp_init(struct zcomp *comp
if (ret)
goto cleanup;
+ for_each_possible_cpu(cpu)
+ mutex_init(&per_cpu_ptr(comp->stream, cpu)->lock);
+
ret = cpuhp_state_add_instance(CPUHP_ZCOMP_PREPARE, &comp->node);
if (ret < 0)
goto cleanup;
--- a/drivers/block/zram/zcomp.h~zram-permit-preemption-with-active-compression-stream
+++ a/drivers/block/zram/zcomp.h
@@ -3,7 +3,7 @@
#ifndef _ZCOMP_H_
#define _ZCOMP_H_
-#include <linux/local_lock.h>
+#include <linux/mutex.h>
#define ZCOMP_PARAM_NO_LEVEL INT_MIN
@@ -31,7 +31,7 @@ struct zcomp_ctx {
};
struct zcomp_strm {
- local_lock_t lock;
+ struct mutex lock;
/* compression buffer */
void *buffer;
struct zcomp_ctx ctx;
@@ -77,7 +77,7 @@ struct zcomp *zcomp_create(const char *a
void zcomp_destroy(struct zcomp *comp);
struct zcomp_strm *zcomp_stream_get(struct zcomp *comp);
-void zcomp_stream_put(struct zcomp *comp);
+void zcomp_stream_put(struct zcomp_strm *zstrm);
int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
const void *src, unsigned int *dst_len);
--- a/drivers/block/zram/zram_drv.c~zram-permit-preemption-with-active-compression-stream
+++ a/drivers/block/zram/zram_drv.c
@@ -1650,7 +1650,7 @@ static int read_compressed_page(struct z
ret = zcomp_decompress(zram->comps[prio], zstrm, src, size, dst);
kunmap_local(dst);
zs_unmap_object(zram->mem_pool, handle);
- zcomp_stream_put(zram->comps[prio]);
+ zcomp_stream_put(zstrm);
return ret;
}
@@ -1811,14 +1811,14 @@ compress_again:
kunmap_local(mem);
if (unlikely(ret)) {
- zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+ zcomp_stream_put(zstrm);
pr_err("Compression failed! err=%d\n", ret);
zs_free(zram->mem_pool, handle);
return ret;
}
if (comp_len >= huge_class_size) {
- zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+ zcomp_stream_put(zstrm);
return write_incompressible_page(zram, page, index);
}
@@ -1842,7 +1842,7 @@ compress_again:
__GFP_HIGHMEM |
__GFP_MOVABLE);
if (IS_ERR_VALUE(handle)) {
- zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+ zcomp_stream_put(zstrm);
atomic64_inc(&zram->stats.writestall);
handle = zs_malloc(zram->mem_pool, comp_len,
GFP_NOIO | __GFP_HIGHMEM |
@@ -1854,7 +1854,7 @@ compress_again:
}
if (!zram_can_store_page(zram)) {
- zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+ zcomp_stream_put(zstrm);
zs_free(zram->mem_pool, handle);
return -ENOMEM;
}
@@ -1862,7 +1862,7 @@ compress_again:
dst = zs_map_object(zram->mem_pool, handle, ZS_MM_WO);
memcpy(dst, zstrm->buffer, comp_len);
- zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
+ zcomp_stream_put(zstrm);
zs_unmap_object(zram->mem_pool, handle);
zram_slot_lock(zram, index);
@@ -2021,7 +2021,7 @@ static int recompress_slot(struct zram *
kunmap_local(src);
if (ret) {
- zcomp_stream_put(zram->comps[prio]);
+ zcomp_stream_put(zstrm);
return ret;
}
@@ -2031,7 +2031,7 @@ static int recompress_slot(struct zram *
/* Continue until we make progress */
if (class_index_new >= class_index_old ||
(threshold && comp_len_new >= threshold)) {
- zcomp_stream_put(zram->comps[prio]);
+ zcomp_stream_put(zstrm);
continue;
}
@@ -2089,13 +2089,13 @@ static int recompress_slot(struct zram *
__GFP_HIGHMEM |
__GFP_MOVABLE);
if (IS_ERR_VALUE(handle_new)) {
- zcomp_stream_put(zram->comps[prio]);
+ zcomp_stream_put(zstrm);
return PTR_ERR((void *)handle_new);
}
dst = zs_map_object(zram->mem_pool, handle_new, ZS_MM_WO);
memcpy(dst, zstrm->buffer, comp_len_new);
- zcomp_stream_put(zram->comps[prio]);
+ zcomp_stream_put(zstrm);
zs_unmap_object(zram->mem_pool, handle_new);
_
Patches currently in -mm which might be from senozhatsky@xxxxxxxxxxxx are
zram-sleepable-entry-locking.patch
zram-permit-preemption-with-active-compression-stream.patch
zram-remove-unused-crypto-include.patch
zram-remove-max_comp_streams-device-attr.patch
zram-remove-two-staged-handle-allocation.patch
zram-remove-writestall-zram_stats-member.patch
zram-limit-max-recompress-prio-to-num_active_comps.patch
zram-filter-out-recomp-targets-based-on-priority.patch
zram-rework-recompression-loop.patch
zsmalloc-rename-pool-lock.patch
zsmalloc-make-zspage-lock-preemptible.patch
zsmalloc-introduce-new-object-mapping-api.patch
zram-switch-to-new-zsmalloc-object-mapping-api.patch
zram-permit-reclaim-in-zstd-custom-allocator.patch
zram-do-not-leak-page-on-recompress_store-error-path.patch
zram-do-not-leak-page-on-writeback_store-error-path.patch
zram-add-might_sleep-to-zcomp-api.patch
