netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator

Add a function to extract the pages from a user-space supplied iterator
(UBUF- or IOVEC-type) into a BVEC-type iterator, retaining the pages by
getting a pin on them (as FOLL_PIN) as we go.

This is useful in three situations:

 (1) A userspace thread may have a sibling that unmaps or remaps the
     process's VM during the operation, changing the assignment of the
     pages and potentially causing an error.  Retaining the pages keeps
     some pages around, even if this occurs; futher, we find out at the
     point of extraction if EFAULT is going to be incurred.

 (2) Pages might get swapped out/discarded if not retained, so we want to
     retain them to avoid the reload causing a deadlock due to a DIO
     from/to an mmapped region on the same file.

 (3) The iterator may get passed to sendmsg() by the filesystem.  If a
     fault occurs, we may get a short write to a TCP stream that's then
     tricky to recover from.

We don't deal with other types of iterator here, leaving it to other
mechanisms to retain the pages (eg. PG_locked, PG_writeback and the pipe
lock).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: linux-cachefs@redhat.com
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 4c76ddfb6a67..b11a84f6c32b 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -17,6 +17,7 @@
 #include <linux/workqueue.h>
 #include <linux/fs.h>
 #include <linux/pagemap.h>
+#include <linux/uio.h>
 
 enum netfs_sreq_ref_trace;
 
@@ -296,6 +297,9 @@ void netfs_get_subrequest(struct netfs_io_subrequest *subreq,
 void netfs_put_subrequest(struct netfs_io_subrequest *subreq,
 			  bool was_async, enum netfs_sreq_ref_trace what);
 void netfs_stats_show(struct seq_file *);
+ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
+				struct iov_iter *new,
+				iov_iter_extraction_t extraction_flags);
 
 /**
  * netfs_inode - Get the netfs inode context from the inode