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zram: remove UNDER_WB and simplify writeback

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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@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Sergey Senozhatsky 2024-09-17 11:09:12 +09:00 committed by Andrew Morton
parent 1a1d0f8992
commit 5e99893444
2 changed files with 17 additions and 39 deletions
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55
drivers/block/zram/zram_drv.c
View File

@ -390,10 +390,7 @@ static void mark_idle(struct zram *zram, ktime_t cutoff)
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, ktime_t cutoff)
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 device *dev,
index = pps->index;
zram_slot_lock(zram, index);
/*
* 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.
*/
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.
*/
zram_set_flag(zram, index, ZRAM_UNDER_WB);
/* Need for hugepage writeback racing */
zram_set_flag(zram, index, ZRAM_IDLE);
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 device *dev,
*/
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 device *dev,
}
atomic64_inc(&zram->stats.bd_writes);
/*
* 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.
*/
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);
/*
* 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.
*/
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 @@ static void zram_free_page(struct zram *zram, size_t index)
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));
}
/*

1
drivers/block/zram/zram_drv.h
View File

@ -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 */