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: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org

diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
index 2ff07ba655a072b3c0e31c6bf473a7a80ea0c24f..b781bbbf1d8d643727e4710358e4211face70bd1 100644 (file)
--- 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 c05365e3f42813db20ec38314a93a33af9c53353..d673d0785b9d9ecaba8f606332ff5c11835357d6 100644 (file)
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -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);