The patch titled
Subject: zram: remove UNDER_WB and simplify writeback
has been added to the -mm mm-unstable branch. Its filename is
zram-remove-under_wb-and-simplify-writeback.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zram-remove-under_wb-and-simplify-writeback.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: remove UNDER_WB and simplify writeback
Date: Tue, 17 Sep 2024 11:09:12 +0900
We now have only one active post-processing at any time, so we don't have
same race conditions that we had before. If slot selected for
post-processing gets freed or freed and reallocated it loses its PP_SLOT
flag and there is no way for such a slot to gain PP_SLOT flag again until
current post-processing terminates.
Link: https://lkml.kernel.org/r/20240917021020.883356-8-senozhatsky@xxxxxxxxxxxx
Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
drivers/block/zram/zram_drv.c | 53 +++++++++-----------------------
drivers/block/zram/zram_drv.h | 1
2 files changed, 16 insertions(+), 38 deletions(-)
--- a/drivers/block/zram/zram_drv.c~zram-remove-under_wb-and-simplify-writeback
+++ a/drivers/block/zram/zram_drv.c
@@ -390,10 +390,7 @@ static void mark_idle(struct zram *zram,
for (index = 0; index < nr_pages; index++) {
/*
- * Do not mark ZRAM_UNDER_WB slot as ZRAM_IDLE to close race.
- * See the comment in writeback_store.
- *
- * Also do not mark ZRAM_SAME slots as ZRAM_IDLE, because no
+ * Do not mark ZRAM_SAME slots as ZRAM_IDLE, because no
* post-processing (recompress, writeback) happens to the
* ZRAM_SAME slot.
*
@@ -402,7 +399,6 @@ static void mark_idle(struct zram *zram,
zram_slot_lock(zram, index);
if (!zram_allocated(zram, index) ||
zram_test_flag(zram, index, ZRAM_WB) ||
- zram_test_flag(zram, index, ZRAM_UNDER_WB) ||
zram_test_flag(zram, index, ZRAM_SAME)) {
zram_slot_unlock(zram, index);
continue;
@@ -821,22 +817,17 @@ static ssize_t writeback_store(struct de
index = pps->index;
zram_slot_lock(zram, index);
- if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
- goto next;
/*
- * Clearing ZRAM_UNDER_WB is duty of caller.
- * IOW, zram_free_page never clear it.
+ * scan_slots() sets ZRAM_PP_SLOT and relases slot lock, so
+ * slots can change in the meantime. If slots are accessed or
+ * freed they lose ZRAM_PP_SLOT flag and hence we don't
+ * post-process them.
*/
- zram_set_flag(zram, index, ZRAM_UNDER_WB);
- /* Need for hugepage writeback racing */
- zram_set_flag(zram, index, ZRAM_IDLE);
+ if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
+ goto next;
zram_slot_unlock(zram, index);
- if (zram_read_page(zram, page, index, NULL)) {
- zram_slot_lock(zram, index);
- zram_clear_flag(zram, index, ZRAM_UNDER_WB);
- zram_clear_flag(zram, index, ZRAM_IDLE);
- zram_slot_unlock(zram, index);
+ if (zram_read_page(zram, page, index, NULL)) {
release_pp_slot(zram, pps);
continue;
}
@@ -852,11 +843,6 @@ static ssize_t writeback_store(struct de
*/
err = submit_bio_wait(&bio);
if (err) {
- zram_slot_lock(zram, index);
- zram_clear_flag(zram, index, ZRAM_UNDER_WB);
- zram_clear_flag(zram, index, ZRAM_IDLE);
- zram_slot_unlock(zram, index);
-
release_pp_slot(zram, pps);
/*
* BIO errors are not fatal, we continue and simply
@@ -871,25 +857,19 @@ static ssize_t writeback_store(struct de
}
atomic64_inc(&zram->stats.bd_writes);
+ zram_slot_lock(zram, index);
/*
- * We released zram_slot_lock so need to check if the slot was
- * changed. If there is freeing for the slot, we can catch it
- * easily by zram_allocated.
- * A subtle case is the slot is freed/reallocated/marked as
- * ZRAM_IDLE again. To close the race, idle_store doesn't
- * mark ZRAM_IDLE once it found the slot was ZRAM_UNDER_WB.
- * Thus, we could close the race by checking ZRAM_IDLE bit.
+ * Same as above, we release slot lock during writeback so
+ * slot can change under us: slot_free() or slot_free() and
+ * reallocation (zram_write_page()). In both cases slot loses
+ * ZRAM_PP_SLOT flag. No concurrent post-processing can set
+ * ZRAM_PP_SLOT on such slots until current post-processing
+ * finishes.
*/
- zram_slot_lock(zram, index);
- if (!zram_allocated(zram, index) ||
- !zram_test_flag(zram, index, ZRAM_IDLE)) {
- zram_clear_flag(zram, index, ZRAM_UNDER_WB);
- zram_clear_flag(zram, index, ZRAM_IDLE);
+ if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
goto next;
- }
zram_free_page(zram, index);
- zram_clear_flag(zram, index, ZRAM_UNDER_WB);
zram_set_flag(zram, index, ZRAM_WB);
zram_set_element(zram, index, blk_idx);
blk_idx = 0;
@@ -1538,7 +1518,6 @@ out:
atomic64_dec(&zram->stats.pages_stored);
zram_set_handle(zram, index, 0);
zram_set_obj_size(zram, index, 0);
- WARN_ON_ONCE(zram->table[index].flags & ~(1UL << ZRAM_UNDER_WB));
}
/*
--- a/drivers/block/zram/zram_drv.h~zram-remove-under_wb-and-simplify-writeback
+++ a/drivers/block/zram/zram_drv.h
@@ -47,7 +47,6 @@
enum zram_pageflags {
ZRAM_SAME = ZRAM_FLAG_SHIFT, /* Page consists the same element */
ZRAM_WB, /* page is stored on backing_device */
- ZRAM_UNDER_WB, /* page is under writeback */
ZRAM_PP_SLOT, /* Selected for post-processing */
ZRAM_HUGE, /* Incompressible page */
ZRAM_IDLE, /* not accessed page since last idle marking */
_
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
