linux-kernel - Re: [PATCH] ublk: use copy_{to,from}

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Message-ID: <aQQwy7l_OCzG430i@fedora>
Date: Fri, 31 Oct 2025 11:45:15 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Caleb Sander Mateos <csander@...estorage.com>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ublk: use copy_{to,from}_iter() for user copy

On Thu, Oct 30, 2025 at 07:05:21PM -0600, Caleb Sander Mateos wrote:
> ublk_copy_user_pages()/ublk_copy_io_pages() currently uses
> iov_iter_get_pages2() to extract the pages from the iov_iter and
> memcpy()s between the bvec_iter and the iov_iter's pages one at a time.
> Switch to using copy_to_iter()/copy_from_iter() instead. This avoids the
> user page reference count increments and decrements and needing to split
> the memcpy() at user page boundaries. It also simplifies the code
> considerably.
> 
> Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
> ---
>  drivers/block/ublk_drv.c | 62 +++++++++-------------------------------
>  1 file changed, 14 insertions(+), 48 deletions(-)
> 
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index 0c74a41a6753..852350e639d6 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -912,58 +912,47 @@ static const struct block_device_operations ub_fops = {
>  	.open =		ublk_open,
>  	.free_disk =	ublk_free_disk,
>  	.report_zones =	ublk_report_zones,
>  };
>  
> -#define UBLK_MAX_PIN_PAGES	32
> -
>  struct ublk_io_iter {
> -	struct page *pages[UBLK_MAX_PIN_PAGES];
>  	struct bio *bio;
>  	struct bvec_iter iter;
>  };

->pages[] is actually for pinning user io pages in batch, so killing it may cause
perf drop.

This similar trick is used in direct io code path too.


Thanks 
Ming