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

Sergey Senozhatsky
2018-11-21T04:55:58Z

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