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Date:   Tue, 4 Apr 2017 11:17:06 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        kernel-team@....com
Subject: Re: [PATCH 2/5] zram: partial IO refactoring

Hello,

On (04/03/17 14:17), Minchan Kim wrote:
> +static bool zram_special_page_read(struct zram *zram, u32 index,
> +				struct page *page,
> +				unsigned int offset, unsigned int len)
> +{
> +	struct zram_meta *meta = zram->meta;
> +
> +	bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> +	if (unlikely(!meta->table[index].handle) ||
> +			zram_test_flag(meta, index, ZRAM_SAME)) {
> +		void *mem;
> +
> +		bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> +		mem = kmap_atomic(page);
> +		zram_fill_page(mem + offset, len, meta->table[index].element);
> +		kunmap_atomic(mem);
> +		return true;
> +	}
> +	bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> +
> +	return false;
> +}
> +
> +static bool zram_special_page_write(struct zram *zram, u32 index,
> +					struct page *page)
> +{
> +	unsigned long element;
> +	void *mem = kmap_atomic(page);
> +
> +	if (page_same_filled(mem, &element)) {
> +		struct zram_meta *meta = zram->meta;
> +
> +		kunmap_atomic(mem);
> +		/* Free memory associated with this sector now. */
> +		bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> +		zram_free_page(zram, index);
> +		zram_set_flag(meta, index, ZRAM_SAME);
> +		zram_set_element(meta, index, element);
> +		bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> +
> +		atomic64_inc(&zram->stats.same_pages);
> +		return true;
> +	}
> +	kunmap_atomic(mem);
> +
> +	return false;
> +}

zram_special_page_read() and zram_special_page_write() have a slightly
different locking semantics.

zram_special_page_read() copy-out ZRAM_SAME page having slot unlocked
(can the slot got overwritten in the meantime?), while
zram_special_page_write() keeps the slot locked through out the entire
operation.

>  static void zram_meta_free(struct zram_meta *meta, u64 disksize)
>  {
>  	size_t num_pages = disksize >> PAGE_SHIFT;
> @@ -504,169 +548,104 @@ static void zram_free_page(struct zram *zram, size_t index)
>  	zram_set_obj_size(meta, index, 0);
>  }
>  
> -static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> +static int zram_decompress_page(struct zram *zram, struct page *page, u32 index)
>  {
> -	int ret = 0;
> -	unsigned char *cmem;
> -	struct zram_meta *meta = zram->meta;
> +	int ret;
>  	unsigned long handle;
>  	unsigned int size;
> +	void *src, *dst;
> +	struct zram_meta *meta = zram->meta;
> +
> +	if (zram_special_page_read(zram, index, page, 0, PAGE_SIZE))
> +		return 0;
>  
>  	bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
>  	handle = meta->table[index].handle;
>  	size = zram_get_obj_size(meta, index);
>  
> -	if (!handle || zram_test_flag(meta, index, ZRAM_SAME)) {
> -		bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> -		zram_fill_page(mem, PAGE_SIZE, meta->table[index].element);
> -		return 0;
> -	}
> -
> -	cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_RO);
> +	src = zs_map_object(meta->mem_pool, handle, ZS_MM_RO);
>  	if (size == PAGE_SIZE) {
> -		copy_page(mem, cmem);
> +		dst = kmap_atomic(page);
> +		copy_page(dst, src);
> +		kunmap_atomic(dst);
> +		ret = 0;
>  	} else {
>  		struct zcomp_strm *zstrm = zcomp_stream_get(zram->comp);
>  
> -		ret = zcomp_decompress(zstrm, cmem, size, mem);
> +		dst = kmap_atomic(page);
> +		ret = zcomp_decompress(zstrm, src, size, dst);
> +		kunmap_atomic(dst);
>  		zcomp_stream_put(zram->comp);
>  	}
>  	zs_unmap_object(meta->mem_pool, handle);
>  	bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
>  
>  	/* Should NEVER happen. Return bio error if it does. */
> -	if (unlikely(ret)) {
> +	if (unlikely(ret))
>  		pr_err("Decompression failed! err=%d, page=%u\n", ret, index);
> -		return ret;
> -	}
>  
> -	return 0;
> +	return ret;
>  }
>  
>  static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> -			  u32 index, int offset)
> +				u32 index, int offset)
>  {
>  	int ret;
>  	struct page *page;
> -	unsigned char *user_mem, *uncmem = NULL;
> -	struct zram_meta *meta = zram->meta;
> -	page = bvec->bv_page;
>  
> -	bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> -	if (unlikely(!meta->table[index].handle) ||
> -			zram_test_flag(meta, index, ZRAM_SAME)) {
> -		bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> -		handle_same_page(bvec, meta->table[index].element);
> +	page = bvec->bv_page;
> +	if (zram_special_page_read(zram, index, page, bvec->bv_offset,
> +				bvec->bv_len))

so, I think zram_bvec_read() path calls zram_special_page_read() twice:

  a) direct zram_special_page_read() call

  b) zram_decompress_page()->zram_special_page_read()

is it supposed to be so?

>  		return 0;
> -	}
> -	bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> -
> -	if (is_partial_io(bvec))
> -		/* Use  a temporary buffer to decompress the page */
> -		uncmem = kmalloc(PAGE_SIZE, GFP_NOIO);
> -
> -	user_mem = kmap_atomic(page);
> -	if (!is_partial_io(bvec))
> -		uncmem = user_mem;
>  
> -	if (!uncmem) {
> -		pr_err("Unable to allocate temp memory\n");
> -		ret = -ENOMEM;
> -		goto out_cleanup;
> +	if (is_partial_io(bvec)) {
> +		/* Use a temporary buffer to decompress the page */
> +		page = alloc_page(GFP_NOIO|__GFP_HIGHMEM);
> +		if (!page)
> +			return -ENOMEM;
>  	}
>  
> -	ret = zram_decompress_page(zram, uncmem, index);
> -	/* Should NEVER happen. Return bio error if it does. */
> +	ret = zram_decompress_page(zram, page, index);
>  	if (unlikely(ret))
> -		goto out_cleanup;
> +		goto out;
>  
> -	if (is_partial_io(bvec))
> -		memcpy(user_mem + bvec->bv_offset, uncmem + offset,
> -				bvec->bv_len);
> +	if (is_partial_io(bvec)) {
> +		void *dst = kmap_atomic(bvec->bv_page);
> +		void *src = kmap_atomic(page);
>  
> -	flush_dcache_page(page);
> -	ret = 0;
> -out_cleanup:
> -	kunmap_atomic(user_mem);
> +		memcpy(dst + bvec->bv_offset, src + offset, bvec->bv_len);
> +		kunmap_atomic(src);
> +		kunmap_atomic(dst);
> +	}
> +out:
>  	if (is_partial_io(bvec))
> -		kfree(uncmem);
> +		__free_page(page);
> +
>  	return ret;
>  }
>  
> -static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> -			   int offset)
> +static int zram_compress(struct zram *zram, struct zcomp_strm **zstrm,
> +			struct page *page,
> +			unsigned long *out_handle, unsigned int *out_comp_len)

ok, I see why... not super happy with the `zstrm' magic, but OK. can do.

	-ss

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