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Message-ID: <CADyq12xHEQ4aFRdVDMkmuA1-r+FU7N4O7-8UUTpD9ACFo3tuZA@mail.gmail.com>
Date: Thu, 20 Nov 2025 11:35:58 -0500
From: Brian Geffon <bgeffon@...gle.com>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Minchan Kim <minchan@...nel.org>,
Yuwen Chen <ywen.chen@...mail.com>, Richard Chang <richardycc@...gle.com>,
Fengyu Lian <licayy@...look.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-block@...r.kernel.org
Subject: Re: [RFC PATCHv5 5/6] zram: rework bdev block allocation
On Thu, Nov 20, 2025 at 10:22 AM Sergey Senozhatsky
<senozhatsky@...omium.org> wrote:
>
> First, writeback bdev ->bitmap bits are set only from one
> context, as we can have only one single task performing
> writeback, so we cannot race with anything else. Remove
> retry path.
>
> Second, we always check ZRAM_WB flag to distinguish writtenback
> slots, so we should not confuse 0 bdev block index and 0 handle.
> We can use first bdev block (0 bit) for writeback as well.
>
> While at it, give functions slightly more accurate names, as
> we don't alloc/free anything there, we reserve a block for
> async writeback or release the block.
>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
Reviewed-by: Brian Geffon <bgeffon@...gle.com>
> ---
> drivers/block/zram/zram_drv.c | 37 +++++++++++++++++------------------
> 1 file changed, 18 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 671ef2ec9b11..ecbd9b25dfed 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -500,6 +500,8 @@ static ssize_t idle_store(struct device *dev,
> }
>
> #ifdef CONFIG_ZRAM_WRITEBACK
> +#define INVALID_BDEV_BLOCK (~0UL)
> +
> struct zram_wb_ctl {
> struct list_head idle_reqs;
> struct list_head done_reqs;
> @@ -746,23 +748,20 @@ static ssize_t backing_dev_store(struct device *dev,
> return err;
> }
>
> -static unsigned long alloc_block_bdev(struct zram *zram)
> +static unsigned long zram_reserve_bdev_block(struct zram *zram)
> {
> - unsigned long blk_idx = 1;
> -retry:
> - /* skip 0 bit to confuse zram.handle = 0 */
> - blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, blk_idx);
> - if (blk_idx == zram->nr_pages)
> - return 0;
> + unsigned long blk_idx;
>
> - if (test_and_set_bit(blk_idx, zram->bitmap))
> - goto retry;
> + blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, 0);
> + if (blk_idx == zram->nr_pages)
> + return INVALID_BDEV_BLOCK;
>
> + set_bit(blk_idx, zram->bitmap);
> atomic64_inc(&zram->stats.bd_count);
> return blk_idx;
> }
>
> -static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
> +static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
> {
> int was_set;
>
> @@ -887,7 +886,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
> * (if enabled).
> */
> zram_account_writeback_rollback(zram);
> - free_block_bdev(zram, req->blk_idx);
> + zram_release_bdev_block(zram, req->blk_idx);
> return err;
> }
>
> @@ -901,7 +900,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
> * finishes.
> */
> if (!zram_test_flag(zram, index, ZRAM_PP_SLOT)) {
> - free_block_bdev(zram, req->blk_idx);
> + zram_release_bdev_block(zram, req->blk_idx);
> goto out;
> }
>
> @@ -989,8 +988,8 @@ static int zram_writeback_slots(struct zram *zram,
> struct zram_pp_ctl *ctl,
> struct zram_wb_ctl *wb_ctl)
> {
> + unsigned long blk_idx = INVALID_BDEV_BLOCK;
> struct zram_wb_req *req = NULL;
> - unsigned long blk_idx = 0;
> struct zram_pp_slot *pps;
> int ret = 0, err = 0;
> u32 index = 0;
> @@ -1022,9 +1021,9 @@ static int zram_writeback_slots(struct zram *zram,
> ret = err;
> }
>
> - if (!blk_idx) {
> - blk_idx = alloc_block_bdev(zram);
> - if (!blk_idx) {
> + if (blk_idx == INVALID_BDEV_BLOCK) {
> + blk_idx = zram_reserve_bdev_block(zram);
> + if (blk_idx == INVALID_BDEV_BLOCK) {
> ret = -ENOSPC;
> break;
> }
> @@ -1058,7 +1057,7 @@ static int zram_writeback_slots(struct zram *zram,
> __bio_add_page(&req->bio, req->page, PAGE_SIZE, 0);
>
> zram_submit_wb_request(zram, wb_ctl, req);
> - blk_idx = 0;
> + blk_idx = INVALID_BDEV_BLOCK;
> req = NULL;
> cond_resched();
> continue;
> @@ -1365,7 +1364,7 @@ static int read_from_bdev(struct zram *zram, struct page *page,
> return -EIO;
> }
>
> -static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
> +static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
> {
> }
> #endif
> @@ -1889,7 +1888,7 @@ static void zram_free_page(struct zram *zram, size_t index)
>
> if (zram_test_flag(zram, index, ZRAM_WB)) {
> zram_clear_flag(zram, index, ZRAM_WB);
> - free_block_bdev(zram, zram_get_handle(zram, index));
> + zram_release_bdev_block(zram, zram_get_handle(zram, index));
> goto out;
> }
>
> --
> 2.52.0.rc1.455.g30608eb744-goog
>
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