[PATCH v4 16/39] netfs: Add func to calculate pagecount/size-limited span of an iterator

David Howells
2023-12-13T15:25:11Z

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: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
---
fs/netfs/iterator.c | 97 +++++++++++++++++++++++++++++++++++++++++++
include/linux/netfs.h | 2 +
2 files changed, 99 insertions(+)

diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
index 2ff07ba655a0..b781bbbf1d8d 100644
--- a/fs/netfs/iterator.c
+++ b/fs/netfs/iterator.c
@@ -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);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 8a5b8e7bc358..a30b47e10797 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -324,6 +324,8 @@ 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);