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Message-ID: <Z6zF5QvTQwVoAhMP@google.com>
Date: Wed, 12 Feb 2025 16:01:41 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Kairui Song <ryncsn@...il.com>, Minchan Kim <minchan@...nel.org>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 02/18] zram: permit preemption with active compression
stream
On Wed, Feb 12, 2025 at 03:27:00PM +0900, Sergey Senozhatsky wrote:
> 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.
>
> Suggested-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> ---
> drivers/block/zram/zcomp.c | 36 +++++++++++++++++++++++++----------
> drivers/block/zram/zcomp.h | 6 +++---
> drivers/block/zram/zram_drv.c | 20 +++++++++----------
> 3 files changed, 39 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
> index bb514403e305..e83dd9a80a81 100644
> --- a/drivers/block/zram/zcomp.c
> +++ b/drivers/block/zram/zcomp.c
> @@ -7,6 +7,7 @@
> #include <linux/wait.h>
> #include <linux/sched.h>
> #include <linux/cpu.h>
> +#include <linux/cpuhotplug.h>
What code changes prompt this?
> #include <linux/crypto.h>
> #include <linux/vmalloc.h>
>
> @@ -54,6 +55,7 @@ static int zcomp_strm_init(struct zcomp *comp, struct zcomp_strm *zstrm)
> {
> int ret;
>
> + mutex_init(&zstrm->lock);
I don't think we can initialize the mutex in the hotplug callback. I
think the following scenario is possible:
CPU #1 CPU #2
zcomp_stream_get()
zstrm = raw_cpu_ptr()
/* task migrated to CPU 2 */
CPU goes offline
zcomp_cpu_dead()
mutex_lock()
..
mutex_unlock()
/* migrated task continues */
zcomp_stream_get()
mutex_lock()
CPU goes online
mutex_init()
mutex_unlock() /* problem */
In this case we'll end up initializing the mutex on CPU #1 while CPU #2
has it locked. When we unlocked it on CPU #2 we will corrupt it AFAICT.
This is why I moved the mutex initialization out of the hotplug callback
in zswap. I suspect to do something similar for zram we'd need to do it
in zcomp_init()?
> ret = comp->ops->create_ctx(comp->params, &zstrm->ctx);
> if (ret)
> return ret;
> @@ -109,13 +111,29 @@ ssize_t zcomp_available_show(const char *comp, char *buf)
>
> 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
> + * to a CPU that has already destroyed its stream. If so
"we could have been migrated from** a CPU that has already destroyed its
stream"? Right?
> + * 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 +169,9 @@ int zcomp_decompress(struct zcomp *comp, struct zcomp_strm *zstrm,
> 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");
> @@ -166,10 +181,11 @@ int zcomp_cpu_up_prepare(unsigned int cpu, struct hlist_node *node)
> 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;
> }
>
> diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
> index ad5762813842..23b8236b9090 100644
> --- a/drivers/block/zram/zcomp.h
> +++ b/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 *alg, struct zcomp_params *params);
> 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);
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 3708436f1d1f..43f460a45e3e 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -1608,7 +1608,7 @@ static int read_compressed_page(struct zram *zram, struct page *page, u32 index)
> 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;
> }
> @@ -1769,14 +1769,14 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
> 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);
> }
>
> @@ -1800,7 +1800,7 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
> __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 |
> @@ -1812,7 +1812,7 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
> }
>
> 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;
> }
> @@ -1820,7 +1820,7 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
> 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);
> @@ -1979,7 +1979,7 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
> kunmap_local(src);
>
> if (ret) {
> - zcomp_stream_put(zram->comps[prio]);
> + zcomp_stream_put(zstrm);
> return ret;
> }
>
> @@ -1989,7 +1989,7 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
> /* 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;
> }
>
> @@ -2047,13 +2047,13 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
> __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);
>
> --
> 2.48.1.502.g6dc24dfdaf-goog
>
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