linux-kernel - Re: [PATCH 1/2] iov: introduce ITER_BVEC_FLAG

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Date:   Wed, 9 Dec 2020 11:54:24 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>,
        Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH 1/2] iov: introduce ITER_BVEC_FLAG_FIXED

On 09/12/2020 09:06, Christoph Hellwig wrote:
> On Wed, Dec 09, 2020 at 08:36:45AM +0000, Christoph Hellwig wrote:
>> This is making the iter type even more of a mess than it already is.
>> I think we at least need placeholders for 0/1 here and an explicit
>> flags namespace, preferably after the types.
>>
>> Then again I'd much prefer if we didn't even add the flag or at best
>> just add it for a short-term transition and move everyone over to the
>> new scheme.  Otherwise the amount of different interfaces and supporting
>> code keeps exploding.

At least the flag can be ignored. Anyway sounds good to me. I'll take
your patch below to the series, thanks!

> 
> Note that the only other callers that use iov_iter_bvec and asynchronous
> read/write are loop, target and nvme-target, so this should actually
> be pretty simple.  Out of these only target needs something like this
> trivial change to keep the bvec available over the duration of the I/O,
> the other two should be fine already:
> 
> ---
> From 581a8eabbb1759e3dcfee4b1d2e419f814a8cb80 Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@....de>
> Date: Wed, 9 Dec 2020 10:05:04 +0100
> Subject: target/file: allocate the bvec array as part of struct target_core_file_cmd
> 
> This saves one memory allocation, and ensures the bvecs aren't freed
> before the AIO completion.  This will allow the lower level code to be
> optimized so that it can avoid allocating another bvec array.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  drivers/target/target_core_file.c | 20 ++++++--------------
>  1 file changed, 6 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/target/target_core_file.c b/drivers/target/target_core_file.c
> index 7143d03f0e027e..ed0c39a1f7c649 100644
> --- a/drivers/target/target_core_file.c
> +++ b/drivers/target/target_core_file.c
> @@ -241,6 +241,7 @@ struct target_core_file_cmd {
>  	unsigned long	len;
>  	struct se_cmd	*cmd;
>  	struct kiocb	iocb;
> +	struct bio_vec	bvecs[];
>  };
>  
>  static void cmd_rw_aio_complete(struct kiocb *iocb, long ret, long ret2)
> @@ -268,29 +269,22 @@ fd_execute_rw_aio(struct se_cmd *cmd, struct scatterlist *sgl, u32 sgl_nents,
>  	struct target_core_file_cmd *aio_cmd;
>  	struct iov_iter iter = {};
>  	struct scatterlist *sg;
> -	struct bio_vec *bvec;
>  	ssize_t len = 0;
>  	int ret = 0, i;
>  
> -	aio_cmd = kmalloc(sizeof(struct target_core_file_cmd), GFP_KERNEL);
> +	aio_cmd = kmalloc(struct_size(aio_cmd, bvecs, sgl_nents), GFP_KERNEL);
>  	if (!aio_cmd)
>  		return TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
>  
> -	bvec = kcalloc(sgl_nents, sizeof(struct bio_vec), GFP_KERNEL);
> -	if (!bvec) {
> -		kfree(aio_cmd);
> -		return TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
> -	}
> -
>  	for_each_sg(sgl, sg, sgl_nents, i) {
> -		bvec[i].bv_page = sg_page(sg);
> -		bvec[i].bv_len = sg->length;
> -		bvec[i].bv_offset = sg->offset;
> +		aio_cmd->bvecs[i].bv_page = sg_page(sg);
> +		aio_cmd->bvecs[i].bv_len = sg->length;
> +		aio_cmd->bvecs[i].bv_offset = sg->offset;
>  
>  		len += sg->length;
>  	}
>  
> -	iov_iter_bvec(&iter, is_write, bvec, sgl_nents, len);
> +	iov_iter_bvec(&iter, is_write, aio_cmd->bvecs, sgl_nents, len);
>  
>  	aio_cmd->cmd = cmd;
>  	aio_cmd->len = len;
> @@ -307,8 +301,6 @@ fd_execute_rw_aio(struct se_cmd *cmd, struct scatterlist *sgl, u32 sgl_nents,
>  	else
>  		ret = call_read_iter(file, &aio_cmd->iocb, &iter);
>  
> -	kfree(bvec);
> -
>  	if (ret != -EIOCBQUEUED)
>  		cmd_rw_aio_complete(&aio_cmd->iocb, ret, 0);
>  
> 

-- 
Pavel Begunkov