The patch titled
Subject: zram: permit only one post-processing operation at a time
has been added to the -mm mm-unstable branch. Its filename is
zram-permit-only-one-post-processing-operation-at-a-time.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zram-permit-only-one-post-processing-operation-at-a-time.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 only one post-processing operation at a time
Date: Tue, 17 Sep 2024 11:09:07 +0900
Both recompress and writeback soon will unlock slots during processing,
which makes things too complex wrt possible race-conditions. We still
want to clear PP_SLOT in slot_free, because this is how we figure out that
slot that was selected for post-processing has been released under us and
when we start post-processing we check if slot still has PP_SLOT set. At
the same time, theoretically, we can have something like this:
CPU0 CPU1
recompress
scan slots
set PP_SLOT
unlock slot
slot_free
clear PP_SLOT
allocate PP_SLOT
writeback
scan slots
set PP_SLOT
unlock slot
select PP-slot
test PP_SLOT
So recompress will not detect that slot has been re-used and re-selected
for concurrent writeback post-processing.
Make sure that we only permit on post-processing operation at a time. So
now recompress and writeback post-processing don't race against each
other, we only need to handle slot re-use (slot_free and write), which is
handled individually by each pp operation.
Having recompress and writeback competing for the same slots is not
exactly good anyway (can't imagine anyone doing that).
Link: https://lkml.kernel.org/r/20240917021020.883356-3-senozhatsky@xxxxxxxxxxxx
Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/blockdev/zram.rst | 2 ++
drivers/block/zram/zram_drv.c | 16 ++++++++++++++++
drivers/block/zram/zram_drv.h | 1 +
3 files changed, 19 insertions(+)
--- a/Documentation/admin-guide/blockdev/zram.rst~zram-permit-only-one-post-processing-operation-at-a-time
+++ a/Documentation/admin-guide/blockdev/zram.rst
@@ -47,6 +47,8 @@ The list of possible return codes:
-ENOMEM zram was not able to allocate enough memory to fulfil your
needs.
-EINVAL invalid input has been provided.
+-EAGAIN re-try operation later (e.g. when attempting to run recompress
+ and writeback simultaneously).
======== =============================================================
If you use 'echo', the returned value is set by the 'echo' utility,
--- a/drivers/block/zram/zram_drv.c~zram-permit-only-one-post-processing-operation-at-a-time
+++ a/drivers/block/zram/zram_drv.c
@@ -627,6 +627,12 @@ static ssize_t writeback_store(struct de
goto release_init_lock;
}
+ /* Do not permit concurrent post-processing actions. */
+ if (atomic_xchg(&zram->pp_in_progress, 1)) {
+ up_read(&zram->init_lock);
+ return -EAGAIN;
+ }
+
if (!zram->backing_dev) {
ret = -ENODEV;
goto release_init_lock;
@@ -753,6 +759,7 @@ next:
free_block_bdev(zram, blk_idx);
__free_page(page);
release_init_lock:
+ atomic_set(&zram->pp_in_progress, 0);
up_read(&zram->init_lock);
return ret;
@@ -1883,6 +1890,12 @@ static ssize_t recompress_store(struct d
goto release_init_lock;
}
+ /* Do not permit concurrent post-processing actions. */
+ if (atomic_xchg(&zram->pp_in_progress, 1)) {
+ up_read(&zram->init_lock);
+ return -EAGAIN;
+ }
+
if (algo) {
bool found = false;
@@ -1950,6 +1963,7 @@ next:
__free_page(page);
release_init_lock:
+ atomic_set(&zram->pp_in_progress, 0);
up_read(&zram->init_lock);
return ret;
}
@@ -2146,6 +2160,7 @@ static void zram_reset_device(struct zra
zram->disksize = 0;
zram_destroy_comps(zram);
memset(&zram->stats, 0, sizeof(zram->stats));
+ atomic_set(&zram->pp_in_progress, 0);
reset_bdev(zram);
comp_algorithm_set(zram, ZRAM_PRIMARY_COMP, default_compressor);
@@ -2383,6 +2398,7 @@ static int zram_add(void)
zram->disk->fops = &zram_devops;
zram->disk->private_data = zram;
snprintf(zram->disk->disk_name, 16, "zram%d", device_id);
+ atomic_set(&zram->pp_in_progress, 0);
/* Actual capacity set using sysfs (/sys/block/zram<id>/disksize */
set_capacity(zram->disk, 0);
--- a/drivers/block/zram/zram_drv.h~zram-permit-only-one-post-processing-operation-at-a-time
+++ a/drivers/block/zram/zram_drv.h
@@ -140,5 +140,6 @@ struct zram {
#ifdef CONFIG_ZRAM_MEMORY_TRACKING
struct dentry *debugfs_dir;
#endif
+ atomic_t pp_in_progress;
};
#endif
_
Patches currently in -mm which might be from senozhatsky@xxxxxxxxxxxx are
zram-introduce-zram_pp_slot-flag.patch
zram-permit-only-one-post-processing-operation-at-a-time.patch
zram-rework-recompress-target-selection-strategy.patch
zram-rework-writeback-target-selection-strategy.patch
zram-do-not-mark-idle-slots-that-cannot-be-idle.patch
zram-reshuffle-zram_free_page-flags-operations.patch
zram-remove-under_wb-and-simplify-writeback.patch
