[PATCH 1/2] zram: use zram_read_from_zspool() in writeback

[PATCH 1/2] zram: use zram_read_from_zspool() in writeback

[Sergey Senozhatsky]

We only can read pages from zspool in writeback, zram_read_page()
is not really right in that context not only because it's a more
generic function that handles ZRAM_WB pages, but also because it
requires us to unlock slot between slot flag check and actual page
read.  Use zram_read_from_zspool() instead and do slot flags check
and page read under the same slot lock.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---

This is not -stable material.  The patch simply makes things look
more logical and corect.  The page content still can change while
we are in submit_bio_wait() and we try to handle that when
submit_bio_wait() returns.

 drivers/block/zram/zram_drv.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index d06761ad541f..0a924fae02a4 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -55,8 +55,8 @@ static size_t huge_class_size;
 static const struct block_device_operations zram_devops;
 
 static void zram_free_page(struct zram *zram, size_t index);
-static int zram_read_page(struct zram *zram, struct page *page, u32 index,
-			  struct bio *parent);
+static int zram_read_from_zspool(struct zram *zram, struct page *page,
+				 u32 index);
 
 static int zram_slot_trylock(struct zram *zram, u32 index)
 {
@@ -831,13 +831,10 @@ static ssize_t writeback_store(struct device *dev,
 		 */
 		if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
 			goto next;
+		if (zram_read_from_zspool(zram, page, index))
+			goto next;
 		zram_slot_unlock(zram, index);
 
-		if (zram_read_page(zram, page, index, NULL)) {
-			release_pp_slot(zram, pps);
-			continue;
-		}
-
 		bio_init(&bio, zram->bdev, &bio_vec, 1,
 			 REQ_OP_WRITE | REQ_SYNC);
 		bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
-- 
2.47.1.613.gc27f4b7a9f-goog

[PATCH 2/2] zram: cond_resched() in writeback loop

[Sergey Senozhatsky]

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.

Fixes: a939888ec38b ("zram: support idle/huge page writeback")
Cc: stable@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---

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

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 0a924fae02a4..f5fa3db6b4f8 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -884,6 +884,8 @@ static ssize_t writeback_store(struct device *dev,
 next:
 		zram_slot_unlock(zram, index);
 		release_pp_slot(zram, pps);
+
+		cond_resched();
 	}
 
 	if (blk_idx)
-- 
2.47.1.613.gc27f4b7a9f-goog

Re: [PATCH 1/2] zram: use zram_read_from_zspool() in writeback

[Sergey Senozhatsky]

On (24/12/11 17:25), Sergey Senozhatsky wrote:
> We only can read pages from zspool in writeback, zram_read_page()
> is not really right in that context not only because it's a more
> generic function that handles ZRAM_WB pages, but also because it
> requires us to unlock slot between slot flag check and actual page
> read.  Use zram_read_from_zspool() instead and do slot flags check
> and page read under the same slot lock.


Andrew, sorry for the noise, please ignore this mini-series.  Let me
re-send it combined with v2 of `zram: split page type read/write handling`
series [1]

[1] https://lore.kernel.org/linux-kernel/20241210105420.1888790-1-senozhatsky@chromium.org