[Devel] Re: [PATCH 10/11] fuse: optimize fuse_get_user_pages() - v2

[Miklos Szeredi]

Maxim Patlasov <mpatlasov at parallels.com> writes:

> Let fuse_get_user_pages() pack as many iov-s to a single fuse_req as
> possible. This is very beneficial in case of iov[] consisting of many
> iov-s of relatively small sizes (e.g. PAGE_SIZE).
>
> Changed in v2:
>  - renamed local vars in fuse_page_descs_length_init() to be more readable
>
> Signed-off-by: Maxim Patlasov <mpatlasov at parallels.com>
> ---
>  fs/fuse/file.c |   94 +++++++++++++++++++++++++++++++++++---------------------
>  1 files changed, 58 insertions(+), 36 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index db9efb5..4d30697 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1047,13 +1047,24 @@ static void fuse_release_user_pages(struct fuse_req *req, int write)
>  	}
>  }
>  
> -static inline void fuse_page_descs_length_init(struct fuse_req *req)
> +static inline void fuse_page_descs_length_init(struct fuse_req *req,
> +					       unsigned index, int nr_pages)
>  {
> -	int i;
> +	while (nr_pages-- > 0)

Please avoid such constructs.  They are hard to read and easy to mess
up.

Use an auxiliary local variable, just like you used to.

> +		req->page_descs[index + nr_pages].length = PAGE_SIZE -
> +			req->page_descs[index + nr_pages].offset;
> +}
>  
> -	for (i = 0; i < req->num_pages; i++)
> -		req->page_descs[i].length = PAGE_SIZE -
> -			req->page_descs[i].offset;
> +static inline unsigned long fuse_get_ua(const struct iovec *iov,

fuse_get_user_addr()

> +					size_t iov_offset)
> +{
> +	return (unsigned long)iov->iov_base + iov_offset;
> +}
> +
> +static inline size_t fuse_get_fr_sz(const struct iovec *iov, size_t
> iov_offset,

iov_iter_single_seg_count() is the function you are reimplementing here,
I guess.

I haven't reviewed the  as I think it will automatically get cleaner if
you switch to iov_iter.


> +				    size_t max_size)
> +{
> +	return min_t(size_t, iov->iov_len - iov_offset, max_size);
>  }
>  
>  static int fuse_get_user_pages(struct fuse_req *req,
> @@ -1062,14 +1073,12 @@ static int fuse_get_user_pages(struct fuse_req *req,
>  			       size_t *iov_offset_p,
>  			       size_t *nbytesp, int write)
>  {
> -	size_t nbytes = *nbytesp;
> -	size_t frag_size = min_t(size_t, nbytes, (*iov_pp)->iov_len - *iov_offset_p);
> -	unsigned long user_addr = (unsigned long)(*iov_pp)->iov_base + *iov_offset_p;
> -	unsigned offset = user_addr & ~PAGE_MASK;
> -	int npages;
> +	size_t nbytes = 0;  /* # bytes already packed in req */
>  
>  	/* Special case for kernel I/O: can copy directly into the buffer */
>  	if (segment_eq(get_fs(), KERNEL_DS)) {
> +		unsigned long user_addr = fuse_get_ua(*iov_pp, *iov_offset_p);
> +
>  		if (write)
>  			req->in.args[1].value = (void *) user_addr;
>  		else
> @@ -1077,42 +1086,55 @@ static int fuse_get_user_pages(struct fuse_req *req,
>  
>  		(*iov_pp)++;
>  		(*nr_segs_p)--;
> -		*nbytesp = frag_size;
> +		*nbytesp = fuse_get_fr_sz(*iov_pp, *iov_offset_p, *nbytesp);
>  		return 0;
>  	}
>  
> -	nbytes = min_t(size_t, frag_size, FUSE_MAX_PAGES_PER_REQ << PAGE_SHIFT);
> -	npages = (nbytes + offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
> -	npages = clamp(npages, 1, FUSE_MAX_PAGES_PER_REQ);
> -	npages = get_user_pages_fast(user_addr, npages, !write, req->pages);
> -	if (npages < 0)
> -		return npages;
> +	while (nbytes < *nbytesp && req->num_pages < FUSE_MAX_PAGES_PER_REQ) {
> +		int npages;
> +		unsigned long user_addr = fuse_get_ua(*iov_pp, *iov_offset_p);
> +		unsigned offset = user_addr & ~PAGE_MASK;
> +		size_t frag_size = fuse_get_fr_sz(*iov_pp, *iov_offset_p,
> +						  *nbytesp - nbytes);
>  
> -	req->num_pages = npages;
> -	req->page_descs[0].offset = offset;
> -	fuse_page_descs_length_init(req);
> +		int n = FUSE_MAX_PAGES_PER_REQ - req->num_pages;
> +		frag_size = min_t(size_t, frag_size, n << PAGE_SHIFT);
>  
> -	if (write)
> -		req->in.argpages = 1;
> -	else
> -		req->out.argpages = 1;
> +		npages = (frag_size + offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +		npages = clamp(npages, 1, n);
>  
> -	nbytes = (req->num_pages << PAGE_SHIFT) - req->page_descs[0].offset;
> +		npages = get_user_pages_fast(user_addr, npages, !write,
> +					     &req->pages[req->num_pages]);
> +		if (npages < 0)
> +			return npages;
>  
> -	if (frag_size < nbytes)
> -		req->page_descs[req->num_pages - 1].length -=
> -			nbytes - frag_size;
> +		frag_size = min_t(size_t, frag_size,
> +				  (npages << PAGE_SHIFT) - offset);
> +		nbytes += frag_size;
>  
> -	*nbytesp = min(frag_size, nbytes);
> +		if (frag_size < (*iov_pp)->iov_len - *iov_offset_p) {
> +			*iov_offset_p += frag_size;
> +		} else {
> +			(*iov_pp)++;
> +			(*nr_segs_p)--;
> +			*iov_offset_p = 0;
> +		}
>  
> -	if (*nbytesp < (*iov_pp)->iov_len - *iov_offset_p) {
> -		*iov_offset_p += *nbytesp;
> -	} else {
> -		(*iov_pp)++;
> -		(*nr_segs_p)--;
> -		*iov_offset_p = 0;
> +		req->page_descs[req->num_pages].offset = offset;
> +		fuse_page_descs_length_init(req, req->num_pages, npages);
> +
> +		req->num_pages += npages;
> +		req->page_descs[req->num_pages - 1].length -=
> +			(npages << PAGE_SHIFT) - offset - frag_size;
>  	}
>  
> +	if (write)
> +		req->in.argpages = 1;
> +	else
> +		req->out.argpages = 1;
> +
> +	*nbytesp = nbytes;
> +
>  	return 0;
>  }
>  
> @@ -1954,7 +1976,7 @@ long fuse_do_ioctl(struct file *file, unsigned int cmd, unsigned long arg,
>  	}
>  	memcpy(req->pages, pages, sizeof(req->pages[0]) * num_pages);
>  	req->num_pages = num_pages;
> -	fuse_page_descs_length_init(req);
> +	fuse_page_descs_length_init(req, 0, req->num_pages);
>  
>  	/* okay, let's send it to the client */
>  	req->in.h.opcode = FUSE_IOCTL;