Commit cae932d3 authored by David Howells's avatar David Howells
Browse files

netfs: Add func to calculate pagecount/size-limited span of an iterator 



Add a function to work out how much of an ITER_BVEC or ITER_XARRAY iterator
we can use in a pagecount-limited and size-limited span.  This will be
used, for example, to limit the number of segments in a subrequest to the
maximum number of elements that an RDMA transfer can handle.

Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
Reviewed-by: default avatarJeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
parent 7d828a06

fs/netfs/iterator.c

+97 −0
Original line number Diff line number Diff line
@@ -101,3 +101,100 @@ ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len, 
	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);

include/linux/netfs.h

+2 −0
Original line number Diff line number Diff line
@@ -321,6 +321,8 @@ void netfs_put_subrequest(struct netfs_io_subrequest *subreq, 
ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,

				struct iov_iter *new,

				iov_iter_extraction_t extraction_flags);

size_t netfs_limit_iter(const struct iov_iter *iter, size_t start_offset,

			size_t max_size, size_t max_segs);



int netfs_start_io_read(struct inode *inode);

void netfs_end_io_read(struct inode *inode);