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Date:   Fri, 20 Nov 2020 13:00:37 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Ming Lei <ming.lei@...hat.com>, 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 12:39, Matthew Wilcox wrote:
> On Fri, Nov 20, 2020 at 08:14:29AM +0000, Christoph Hellwig wrote:
>> On Fri, Nov 20, 2020 at 02:54:57AM +0000, Matthew Wilcox wrote:
>>> On Fri, Nov 20, 2020 at 02:25:08AM +0000, Pavel Begunkov wrote:
>>>> On 20/11/2020 02:22, Ming Lei wrote:
>>>>> 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.
>>>
>>> Are you optimising the right thing here?  Assuming you're looking at
>>> the one in do_blockdev_direct_IO(), wouldn't we be better off figuring
>>> out how to copy the bvecs directly from the iov_iter into the bio
>>> rather than calling dio_bio_add_page() for each page?
>>
>> Which is most effectively done by stopping to to use *blockdev_direct_IO
>> and switching to iomap instead :)
> 
> But iomap still calls iov_iter_npages().  So maybe we need something like
> this ...

Yep, all that are not mutually exclusive optimisations.
Why `return 1`? It seems to be used later in bio_alloc(nr_pages)

> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 933f234d5bec..1c5a802a45d9 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -250,7 +250,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  	orig_count = iov_iter_count(dio->submit.iter);
>  	iov_iter_truncate(dio->submit.iter, length);
>  
> -	nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> +	nr_pages = bio_iov_iter_npages(dio->submit.iter);
>  	if (nr_pages <= 0) {
>  		ret = nr_pages;
>  		goto out;
> @@ -308,7 +308,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		dio->size += n;
>  		copied += n;
>  
> -		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> +		nr_pages = bio_iov_iter_npages(dio->submit.iter);
>  		iomap_dio_submit_bio(dio, iomap, bio, pos);
>  		pos += n;
>  	} while (nr_pages);
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index c6d765382926..86cc74f84b30 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -10,6 +10,7 @@
>  #include <linux/ioprio.h>
>  /* struct bio, bio_vec and BIO_* flags are defined in blk_types.h */
>  #include <linux/blk_types.h>
> +#include <linux/uio.h>
>  
>  #define BIO_DEBUG
>  
> @@ -447,6 +448,16 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
>  void __bio_add_page(struct bio *bio, struct page *page,
>  		unsigned int len, unsigned int off);
>  int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter);
> +
> +static inline int bio_iov_iter_npages(const struct iov_iter *i)
> +{
> +	if (!iov_iter_count(i))
> +		return 0;
> +	if (iov_iter_is_bvec(i))
> +		return 1;
> +	return iov_iter_npages(i, BIO_MAX_PAGES);
> +}
> +
>  void bio_release_pages(struct bio *bio, bool mark_dirty);
>  extern void bio_set_pages_dirty(struct bio *bio);
>  extern void bio_check_pages_dirty(struct bio *bio);
> 

-- 
Pavel Begunkov

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