Re: [PATCH RFC 2/2] zram: support compression at the granularity of multi-pages

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (24/03/28 10:48), Barry Song wrote:
[..]
> +/*
> + * Use a temporary buffer to decompress the page, as the decompressor
> + * always expects a full page for the output.
> + */
> +static int zram_bvec_read_multi_pages_partial(struct zram *zram, struct bio_vec *bvec,
> +				  u32 index, int offset)
> +{
> +	struct page *page = alloc_pages(GFP_NOIO | __GFP_COMP, ZCOMP_MULTI_PAGES_ORDER);
> +	int ret;
> +
> +	if (!page)
> +		return -ENOMEM;
> +	ret = zram_read_multi_pages(zram, page, index, NULL);
> +	if (likely(!ret)) {
> +		atomic64_inc(&zram->stats.zram_bio_read_multi_pages_partial_count);
> +		void *dst = kmap_local_page(bvec->bv_page);
> +		void *src = kmap_local_page(page);
> +
> +		memcpy(dst + bvec->bv_offset, src + offset, bvec->bv_len);
> +		kunmap_local(src);
> +		kunmap_local(dst);
> +	}
> +	__free_pages(page, ZCOMP_MULTI_PAGES_ORDER);
> +	return ret;
> +}

[..]

> +static int zram_bvec_write_multi_pages_partial(struct zram *zram, struct bio_vec *bvec,
> +				   u32 index, int offset, struct bio *bio)
> +{
> +	struct page *page = alloc_pages(GFP_NOIO | __GFP_COMP, ZCOMP_MULTI_PAGES_ORDER);
> +	int ret;
> +	void *src, *dst;
> +
> +	if (!page)
> +		return -ENOMEM;
> +
> +	ret = zram_read_multi_pages(zram, page, index, bio);
> +	if (!ret) {
> +		src = kmap_local_page(bvec->bv_page);
> +		dst = kmap_local_page(page);
> +		memcpy(dst + offset, src + bvec->bv_offset, bvec->bv_len);
> +		kunmap_local(dst);
> +		kunmap_local(src);
> +
> +		atomic64_inc(&zram->stats.zram_bio_write_multi_pages_partial_count);
> +		ret = zram_write_page(zram, page, index);
> +	}
> +	__free_pages(page, ZCOMP_MULTI_PAGES_ORDER);
> +	return ret;
> +}

What type of testing you run on it? How often do you see partial
reads and writes? Because this looks concerning - zsmalloc memory
usage reduction is one metrics, but this also can be achieved via
recompression, writeback, or even a different compression algorithm,
but higher CPU/power usage/higher requirements for physically contig
pages cannot be offset easily. (Another corner case, assume we have
partial read requests on every CPU simultaneously.)