kernel/fs/netfs/iterator.c

// SPDX-License-Identifier: GPL-2.0-or-later
/* Iterator helpers.
 *
 * Copyright (C) 2022 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 */

#include <linux/export.h>
#include <linux/slab.h>
#include <linux/mm.h>
#include <linux/uio.h>
#include <linux/scatterlist.h>
#include <linux/netfs.h>
#include "internal.h"

/**
 * netfs_extract_user_iter - Extract the pages from a user iterator into a bvec
 * @orig: The original iterator
 * @orig_len: The amount of iterator to copy
 * @new: The iterator to be set up
 * @extraction_flags: Flags to qualify the request
 *
 * Extract the page fragments from the given amount of the source iterator and
 * build up a second iterator that refers to all of those bits.  This allows
 * the original iterator to disposed of.
 *
 * @extraction_flags can have ITER_ALLOW_P2PDMA set to request peer-to-peer DMA be
 * allowed on the pages extracted.
 *
 * On success, the number of elements in the bvec is returned, the original
 * iterator will have been advanced by the amount extracted.
 *
 * The iov_iter_extract_mode() function should be used to query how cleanup
 * should be performed.
 */
ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
				struct iov_iter *new,
				iov_iter_extraction_t extraction_flags)
{
	struct bio_vec *bv = NULL;
	struct page **pages;
	unsigned int cur_npages;
	unsigned int max_pages;
	unsigned int npages = 0;
	unsigned int i;
	ssize_t ret;
	size_t count = orig_len, offset, len;
	size_t bv_size, pg_size;

	if (WARN_ON_ONCE(!iter_is_ubuf(orig) && !iter_is_iovec(orig)))
		return -EIO;

	max_pages = iov_iter_npages(orig, INT_MAX);
	bv_size = array_size(max_pages, sizeof(*bv));
	bv = kvmalloc(bv_size, GFP_KERNEL);
	if (!bv)
		return -ENOMEM;

	/* Put the page list at the end of the bvec list storage.  bvec
	 * elements are larger than page pointers, so as long as we work
	 * 0->last, we should be fine.
	 */
	pg_size = array_size(max_pages, sizeof(*pages));
	pages = (void *)bv + bv_size - pg_size;

	while (count && npages < max_pages) {
		ret = iov_iter_extract_pages(orig, &pages, count,
					     max_pages - npages, extraction_flags,
					     &offset);
		if (ret < 0) {
			pr_err("Couldn't get user pages (rc=%zd)\n", ret);
			break;
		}

		if (ret > count) {
			pr_err("get_pages rc=%zd more than %zu\n", ret, count);
			break;
		}

		count -= ret;
		ret += offset;
		cur_npages = DIV_ROUND_UP(ret, PAGE_SIZE);

		if (npages + cur_npages > max_pages) {
			pr_err("Out of bvec array capacity (%u vs %u)\n",
			       npages + cur_npages, max_pages);
			break;
		}

		for (i = 0; i < cur_npages; i++) {
			len = ret > PAGE_SIZE ? PAGE_SIZE : ret;
			bvec_set_page(bv + npages + i, *pages++, len - offset, offset);
			ret -= len;
			offset = 0;
		}

		npages += cur_npages;
	}

	iov_iter_bvec(new, orig->data_source, bv, npages, orig_len - count);
	return npages;
}
EXPORT_SYMBOL_GPL(netfs_extract_user_iter);

/*
 * Select the span of a bvec iterator we're going to use.  Limit it by both maximum
 * size and maximum number of segments.  Returns the size of the span in bytes.
 */
static size_t netfs_limit_bvec(const struct iov_iter *iter, size_t start_offset,
			       size_t max_size, size_t max_segs)
{
	const struct bio_vec *bvecs = iter->bvec;
	unsigned int nbv = iter->nr_segs, ix = 0, nsegs = 0;
	size_t len, span = 0, n = iter->count;
	size_t skip = iter->iov_offset + start_offset;

	if (WARN_ON(!iov_iter_is_bvec(iter)) ||
	    WARN_ON(start_offset > n) ||
	    n == 0)
		return 0;

	while (n && ix < nbv && skip) {
		len = bvecs[ix].bv_len;
		if (skip < len)
			break;
		skip -= len;
		n -= len;
		ix++;
	}

	while (n && ix < nbv) {
		len = min3(n, bvecs[ix].bv_len - skip, max_size);
		span += len;
		nsegs++;
		ix++;
		if (span >= max_size || nsegs >= max_segs)
			break;
		skip = 0;
		n -= len;
	}

	return min(span, max_size);
}

/*
 * Select the span of an xarray iterator we're going to use.  Limit it by both
 * maximum size and maximum number of segments.  It is assumed that segments
 * can be larger than a page in size, provided they're physically contiguous.
 * Returns the size of the span in bytes.
 */
static size_t netfs_limit_xarray(const struct iov_iter *iter, size_t start_offset,
				 size_t max_size, size_t max_segs)
{
	struct folio *folio;
	unsigned int nsegs = 0;
	loff_t pos = iter->xarray_start + iter->iov_offset;
	pgoff_t index = pos / PAGE_SIZE;
	size_t span = 0, n = iter->count;

	XA_STATE(xas, iter->xarray, index);

	if (WARN_ON(!iov_iter_is_xarray(iter)) ||
	    WARN_ON(start_offset > n) ||
	    n == 0)
		return 0;
	max_size = min(max_size, n - start_offset);

	rcu_read_lock();
	xas_for_each(&xas, folio, ULONG_MAX) {
		size_t offset, flen, len;
		if (xas_retry(&xas, folio))
			continue;
		if (WARN_ON(xa_is_value(folio)))
			break;
		if (WARN_ON(folio_test_hugetlb(folio)))
			break;

		flen = folio_size(folio);
		offset = offset_in_folio(folio, pos);
		len = min(max_size, flen - offset);
		span += len;
		nsegs++;
		if (span >= max_size || nsegs >= max_segs)
			break;
	}

	rcu_read_unlock();
	return min(span, max_size);
}

size_t netfs_limit_iter(const struct iov_iter *iter, size_t start_offset,
			size_t max_size, size_t max_segs)
{
	if (iov_iter_is_bvec(iter))
		return netfs_limit_bvec(iter, start_offset, max_size, max_segs);
	if (iov_iter_is_xarray(iter))
		return netfs_limit_xarray(iter, start_offset, max_size, max_segs);
	BUG();
}
EXPORT_SYMBOL(netfs_limit_iter);