Re: [PATCH v5 2/2] iomap: fault in smaller chunks for non-large folio mappings

[Matthew Wilcox <willy@infradead.org>]

On Tue, May 21, 2024 at 07:49:39PM +0800, Xu Yang wrote:
> Since commit (5d8edfb900d5 "iomap: Copy larger chunks from userspace"),
> iomap will try to copy in larger chunks than PAGE_SIZE. However, if the
> mapping doesn't support large folio, only one page of maximum 4KB will
> be created and 4KB data will be writen to pagecache each time. Then,
> next 4KB will be handled in next iteration. This will cause potential
> write performance problem.
> 
> If chunk is 2MB, total 512 pages need to be handled finally. During this
> period, fault_in_iov_iter_readable() is called to check iov_iter readable
> validity. Since only 4KB will be handled each time, below address space
> will be checked over and over again:
> 
> start         	end
> -
> buf,    	buf+2MB
> buf+4KB, 	buf+2MB
> buf+8KB, 	buf+2MB
> ...
> buf+2044KB 	buf+2MB
> 
> Obviously the checking size is wrong since only 4KB will be handled each
> time. So this will get a correct chunk to let iomap work well in non-large
> folio case.
> 
> With this change, the write speed will be stable. Tested on ARM64 device.
> 
> Before:
> 
>  - dd if=/dev/zero of=/dev/sda bs=400K  count=10485  (334 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=800K  count=5242   (278 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=1600K count=2621   (204 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=2200K count=1906   (170 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=3000K count=1398   (150 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=4500K count=932    (139 MB/s)
> 
> After:
> 
>  - dd if=/dev/zero of=/dev/sda bs=400K  count=10485  (339 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=800K  count=5242   (330 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=1600K count=2621   (332 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=2200K count=1906   (333 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=3000K count=1398   (333 MB/s)
>  - dd if=/dev/zero of=/dev/sda bs=4500K count=932    (333 MB/s)
> 
> Fixes: 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
> Cc: stable@vger.kernel.org
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> Signed-off-by: Xu Yang <xu.yang_2@nxp.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>