+ zram-cond_resched-in-writeback-loop.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: zram: cond_resched() in writeback loop
has been added to the -mm mm-unstable branch.  Its filename is
     zram-cond_resched-in-writeback-loop.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zram-cond_resched-in-writeback-loop.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@chromium.org>
Subject: zram: cond_resched() in writeback loop
Date: Wed, 18 Dec 2024 15:34:24 +0900

zram writeback is a costly operation, because every target slot (unless
ZRAM_HUGE) is decompressed before it gets written to a backing device. 
The writeback to a backing device uses submit_bio_wait() which may look
like a rescheduling point.  However, if the backing device has
BD_HAS_SUBMIT_BIO bit set __submit_bio() calls directly
disk->fops->submit_bio(bio) on the backing device and so when
submit_bio_wait() calls blk_wait_io() the I/O is already done.  On such
systems we effective end up in a loop

    for_each (target slot) {
	decompress(slot)
	__submit_bio()
	    disk->fops->submit_bio(bio)
    }

Which on PREEMPT_NONE systems triggers watchdogs (since there are no
explicit rescheduling points).  Add cond_resched() to the zram writeback
loop.

Link: https://lkml.kernel.org/r/20241218063513.297475-8-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>
---

 drivers/block/zram/zram_drv.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/block/zram/zram_drv.c~zram-cond_resched-in-writeback-loop
+++ a/drivers/block/zram/zram_drv.c
@@ -884,6 +884,8 @@ static ssize_t writeback_store(struct de
 next:
 		zram_slot_unlock(zram, index);
 		release_pp_slot(zram, pps);
+
+		cond_resched();
 	}
 
 	if (blk_idx)
_

Patches currently in -mm which might be from senozhatsky@chromium.org are

zram-free-slot-memory-early-during-write.patch
zram-remove-entry-element-member.patch
zram-factor-out-zram_same-write.patch
zram-factor-out-zram_huge-write.patch
zram-factor-out-different-page-types-read.patch
zram-use-zram_read_from_zspool-in-writeback.patch
zram-cond_resched-in-writeback-loop.patch