linux-kernel - Re: [PATCH v2 1/2] iov_iter: optimise iov_iter

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Message-ID: <e70a3c05-a968-7802-df81-0529eaa7f7b4@gmail.com>
Date:   Fri, 20 Nov 2020 02:25:08 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec

On 20/11/2020 02:22, Ming Lei wrote:
> On Fri, Nov 20, 2020 at 01:39:05AM +0000, Pavel Begunkov wrote:
>> On 20/11/2020 01:20, Matthew Wilcox wrote:
>>> On Thu, Nov 19, 2020 at 11:24:38PM +0000, Pavel Begunkov wrote:
>>>> The block layer spends quite a while in iov_iter_npages(), but for the
>>>> bvec case the number of pages is already known and stored in
>>>> iter->nr_segs, so it can be returned immediately as an optimisation
>>>
>>> Er ... no, it doesn't.  nr_segs is the number of bvecs.  Each bvec can
>>> store up to 4GB of contiguous physical memory.
>>
>> Ah, really, missed min() with PAGE_SIZE in bvec_iter_len(), then it's a
>> stupid statement. Thanks!
>>
> 
> iov_iter_npages(bvec) still can be improved a bit by the following way:

Yep, was doing exactly that, +a couple of other places that are in my way.

> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 1635111c5bd2..d85ed7acce05 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1608,17 +1608,23 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages)
>  		npages = pipe_space_for_user(iter_head, pipe->tail, pipe);
>  		if (npages >= maxpages)
>  			return maxpages;
> +	} else if (iov_iter_is_bvec(i)) {
> +		unsigned idx, offset = i->iov_offset;
> +
> +		for (idx = 0; idx < i->nr_segs; idx++) {
> +			npages += DIV_ROUND_UP(i->bvec[idx].bv_len - offset,
> +					PAGE_SIZE);
> +			offset = 0;
> +		}
> +		if (npages >= maxpages)
> +			return maxpages;
>  	} else iterate_all_kinds(i, size, v, ({
>  		unsigned long p = (unsigned long)v.iov_base;
>  		npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
>  			- p / PAGE_SIZE;
>  		if (npages >= maxpages)
>  			return maxpages;
> -	0;}),({
> -		npages++;
> -		if (npages >= maxpages)
> -			return maxpages;
> -	}),({
> +	0;}),0,({
>  		unsigned long p = (unsigned long)v.iov_base;
>  		npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
>  			- p / PAGE_SIZE;
> 

-- 
Pavel Begunkov