Re: [PATCH 35/44] iov_iter: saner helper for page array allocation

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2022-06-22 at 05:15 +0100, Al Viro wrote:
> All call sites of get_pages_array() are essenitally identical now.
> Replace with common helper...
> 
> Returns number of slots available in resulting array or 0 on OOM;
> it's up to the caller to make sure it doesn't ask to zero-entry
> array (i.e. neither maxpages nor size are allowed to be zero).
> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  lib/iov_iter.c | 77 +++++++++++++++++++++-----------------------------
>  1 file changed, 32 insertions(+), 45 deletions(-)
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 9280f865fd6a..1c744f0c0b2c 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1187,9 +1187,20 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
>  }
>  EXPORT_SYMBOL(iov_iter_gap_alignment);
>  
> -static struct page **get_pages_array(size_t n)
> +static int want_pages_array(struct page ***res, size_t size,
> +			    size_t start, unsigned int maxpages)
>  {
> -	return kvmalloc_array(n, sizeof(struct page *), GFP_KERNEL);
> +	unsigned int count = DIV_ROUND_UP(size + start, PAGE_SIZE);
> +
> +	if (count > maxpages)
> +		count = maxpages;
> +	WARN_ON(!count);	// caller should've prevented that
> +	if (!*res) {
> +		*res = kvmalloc_array(count, sizeof(struct page *), GFP_KERNEL);
> +		if (!*res)
> +			return 0;
> +	}
> +	return count;
>  }
>  
>  static ssize_t pipe_get_pages(struct iov_iter *i,
> @@ -1197,27 +1208,20 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
>  		   size_t *start)
>  {
>  	struct pipe_inode_info *pipe = i->pipe;
> -	unsigned int npages, off;
> +	unsigned int npages, off, count;
>  	struct page **p;
>  	ssize_t left;
> -	int count;
>  
>  	if (!sanity(i))
>  		return -EFAULT;
>  
>  	*start = off = pipe_npages(i, &npages);
> -	count = DIV_ROUND_UP(maxsize + off, PAGE_SIZE);
> -	if (count > npages)
> -		count = npages;
> -	if (count > maxpages)
> -		count = maxpages;
> +	if (!npages)
> +		return -EFAULT;
> +	count = want_pages_array(pages, maxsize, off, min(npages, maxpages));
> +	if (!count)
> +		return -ENOMEM;
>  	p = *pages;
> -	if (!p) {
> -		*pages = p = get_pages_array(count);
> -		if (!p)
> -			return -ENOMEM;
> -	}
> -
>  	left = maxsize;
>  	npages = 0;
>  	if (off) {
> @@ -1280,9 +1284,8 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
>  				     struct page ***pages, size_t maxsize,
>  				     unsigned maxpages, size_t *_start_offset)
>  {
> -	unsigned nr, offset;
> -	pgoff_t index, count;
> -	size_t size = maxsize;
> +	unsigned nr, offset, count;
> +	pgoff_t index;
>  	loff_t pos;
>  
>  	pos = i->xarray_start + i->iov_offset;
> @@ -1290,16 +1293,9 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
>  	offset = pos & ~PAGE_MASK;
>  	*_start_offset = offset;
>  
> -	count = DIV_ROUND_UP(size + offset, PAGE_SIZE);
> -	if (count > maxpages)
> -		count = maxpages;
> -
> -	if (!*pages) {
> -		*pages = get_pages_array(count);
> -		if (!*pages)
> -			return -ENOMEM;
> -	}
> -
> +	count = want_pages_array(pages, maxsize, offset, maxpages);
> +	if (!count)
> +		return -ENOMEM;
>  	nr = iter_xarray_populate_pages(*pages, i->xarray, index, count);
>  	if (nr == 0)
>  		return 0;
> @@ -1348,7 +1344,7 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
>  		   struct page ***pages, size_t maxsize,
>  		   unsigned int maxpages, size_t *start)
>  {
> -	int n, res;
> +	unsigned int n;
>  
>  	if (maxsize > i->count)
>  		maxsize = i->count;
> @@ -1360,6 +1356,7 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
>  	if (likely(user_backed_iter(i))) {
>  		unsigned int gup_flags = 0;
>  		unsigned long addr;
> +		int res;
>  
>  		if (iov_iter_rw(i) != WRITE)
>  			gup_flags |= FOLL_WRITE;
> @@ -1369,14 +1366,9 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
>  		addr = first_iovec_segment(i, &maxsize);
>  		*start = addr % PAGE_SIZE;
>  		addr &= PAGE_MASK;
> -		n = DIV_ROUND_UP(maxsize + *start, PAGE_SIZE);
> -		if (n > maxpages)
> -			n = maxpages;
> -		if (!*pages) {
> -			*pages = get_pages_array(n);
> -			if (!*pages)
> -				return -ENOMEM;
> -		}
> +		n = want_pages_array(pages, maxsize, *start, maxpages);
> +		if (!n)
> +			return -ENOMEM;
>  		res = get_user_pages_fast(addr, n, gup_flags, *pages);
>  		if (unlikely(res <= 0))
>  			return res;
> @@ -1387,15 +1379,10 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
>  		struct page *page;
>  
>  		page = first_bvec_segment(i, &maxsize, start);
> -		n = DIV_ROUND_UP(maxsize + *start, PAGE_SIZE);
> -		if (n > maxpages)
> -			n = maxpages;
> +		n = want_pages_array(pages, maxsize, *start, maxpages);
> +		if (!n)
> +			return -ENOMEM;
>  		p = *pages;
> -		if (!p) {
> -			*pages = p = get_pages_array(n);
> -			if (!p)
> -				return -ENOMEM;
> -		}
>  		for (int k = 0; k < n; k++)
>  			get_page(*p++ = page++);
>  		return min_t(size_t, maxsize, n * PAGE_SIZE - *start);

Reviewed-by: Jeff Layton <jlayton@kernel.org>