linux-kernel - Re: [PATCH v4] zram: Implement multi-page write-back

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Message-ID: <htycvrcqbnkk7ldhpaqxesy7uhz3lssymwqm7nzkhyhnid3krm@mfju626njxvb>
Date: Wed, 12 Nov 2025 14:18:01 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yuwen Chen <ywen.chen@...mail.com>
Cc: axboe@...nel.dk, akpm@...ux-foundation.org, bgeffon@...gle.com, 
	licayy@...look.com, linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, liumartin@...gle.com, minchan@...nel.org, richardycc@...gle.com, 
	senozhatsky@...omium.org
Subject: Re: [PATCH v4] zram: Implement multi-page write-back

On (25/11/06 09:49), Yuwen Chen wrote:
[..]
> +	blk_start_plug(&plug);
> +	while ((req->pps = select_pp_slot(ctl))) {
>  		spin_lock(&zram->wb_limit_lock);
>  		if (zram->wb_limit_enable && !zram->bd_wb_limit) {
>  			spin_unlock(&zram->wb_limit_lock);
> @@ -774,15 +884,15 @@ static int zram_writeback_slots(struct zram *zram, struct zram_pp_ctl *ctl)
>  		}
>  		spin_unlock(&zram->wb_limit_lock);
>  
> -		if (!blk_idx) {
> -			blk_idx = alloc_block_bdev(zram);
> -			if (!blk_idx) {
> +		if (!req->blk_idx) {
> +			req->blk_idx = alloc_block_bdev(zram);
> +			if (!req->blk_idx) {
>  				ret = -ENOSPC;
>  				break;
>  			}
>  		}
>  
> -		index = pps->index;
> +		index = req->pps->index;
>  		zram_slot_lock(zram, index);
>  		/*
>  		 * scan_slots() sets ZRAM_PP_SLOT and relases slot lock, so
> @@ -792,22 +902,32 @@ static int zram_writeback_slots(struct zram *zram, struct zram_pp_ctl *ctl)
>  		 */
>  		if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
>  			goto next;
> -		if (zram_read_from_zspool(zram, page, index))
> +		if (zram_read_from_zspool(zram, req->page, index))
>  			goto next;
>  		zram_slot_unlock(zram, index);
>  
> -		bio_init(&bio, zram->bdev, &bio_vec, 1,
> +		bio_init(&req->bio, zram->bdev, &req->bio_vec, 1,
>  			 REQ_OP_WRITE | REQ_SYNC);
> -		bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
> -		__bio_add_page(&bio, page, PAGE_SIZE, 0);
> -
> -		/*
> -		 * XXX: A single page IO would be inefficient for write
> -		 * but it would be not bad as starter.
> -		 */
> -		err = submit_bio_wait(&bio);
> +		req->bio.bi_iter.bi_sector = req->blk_idx * (PAGE_SIZE >> 9);
> +		req->bio.bi_end_io = zram_writeback_endio;
> +		req->bio.bi_private = req;
> +		__bio_add_page(&req->bio, req->page, PAGE_SIZE, 0);
> +
> +		list_del_init(&req->pps->entry);
> +		submit_bio(&req->bio);
> +
> +		do {
> +			req = zram_writeback_next_request(req_pool, req_pool_cnt, &cnt_off);
> +			if (!req) {
> +				blk_finish_plug(&plug);
> +				wait_for_completion_io(&done);
> +				blk_start_plug(&plug);
> +			}
> +		} while (!req);

Why do you do this do-while loop here?