Re: [PATCH] zram: use copy_page for full page copy

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jun 13, 2024 at 08:04:22AM +0800, Jisheng Zhang wrote:
> commit 42e99bd975fd ("zram: optimize memory operations with
> clear_page()/copy_page()") optimize page copy/clean operations, but
> then commit d72e9a7a93e4 ("zram: do not use copy_page with non-page
> aligned address") removes the optimization because there's memory
> corruption at that time, the reason was well explained. But after
> commit 1f7319c74275 ("zram: partial IO refactoring"), partial IO uses
> alloc_page() instead of kmalloc to allocate a page, so we can bring
> back the optimization.
> 
> commit 80ba4caf8ba9 ("zram: use copy_page for full page copy") brings
> back partial optimization, missed one point in zram_write_page().
> optimize the full page copying in zram_write_page() with copy_page()
> 
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  drivers/block/zram/zram_drv.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 3acd7006ad2c..4b2b5098062f 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -1478,11 +1478,13 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
>  	dst = zs_map_object(zram->mem_pool, handle, ZS_MM_WO);
>  
>  	src = zstrm->buffer;
> -	if (comp_len == PAGE_SIZE)
> +	if (comp_len == PAGE_SIZE) {
>  		src = kmap_local_page(page);
> -	memcpy(dst, src, comp_len);
> -	if (comp_len == PAGE_SIZE)
> +		copy_page(dst, src);
>  		kunmap_local(src);
> +	} else {
> +		memcpy(dst, src, comp_len);
> +	}

I know this is pre-existing code, but why do we need to kmap
for comp_len == PAGE_SIZE and not for the other cases?  Something
feels really obsfucated here.