linux-kernel - Re: [PATCH 4/6] zram: support idle page writeback

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Date:   Wed, 21 Nov 2018 13:55:51 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 4/6] zram: support idle page writeback

On (11/16/18 16:20), Minchan Kim wrote:
> +		zram_set_flag(zram, index, ZRAM_UNDER_WB);
> +		zram_slot_unlock(zram, index);
> +		if (zram_bvec_read(zram, &bvec, index, 0, NULL)) {
> +			zram_slot_lock(zram, index);
> +			zram_clear_flag(zram, index, ZRAM_UNDER_WB);
> +			zram_slot_unlock(zram, index);
> +			continue;
> +		}
> +
> +		bio_init(&bio, &bio_vec, 1);
> +		bio_set_dev(&bio, zram->bdev);
> +		bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
> +		bio.bi_opf = REQ_OP_WRITE | REQ_SYNC;
> +
> +		bio_add_page(&bio, bvec.bv_page, bvec.bv_len,
> +				bvec.bv_offset);
> +		/*
> +		 * XXX: A single page IO would be inefficient for write
> +		 * but it would be not bad as starter.
> +		 */
> +		ret = submit_bio_wait(&bio);
> +		if (ret) {
> +			zram_slot_lock(zram, index);
> +			zram_clear_flag(zram, index, ZRAM_UNDER_WB);
> +			zram_slot_unlock(zram, index);
> +			continue;
> +		}

Just a thought,

I wonder if it will make sense (and if it will be possible) to writeback
idle _compressed_ objects. Right now we decompress, say, a perfectly
fine 400-byte compressed object to a PAGE_SIZE-d object and then push
it to the WB device. In this particular case it has a x10 bigger IO
pressure on flash. If we can write/read compressed object then we
will write and read 400-bytes, instead of PAGE_SIZE.

	-ss