Re: [PATCH] libceph: use iov_iter_extract_pages() in ceph_msg_data_iter_next()

[Christoph Hellwig <hch@lst.de>]

On Mon, Aug 17, 2026 at 06:16:56PM -0400, Tal Zussman wrote:
> ceph_msg_data_iter_next() gets a page reference from
> iov_iter_get_pages2() only to immediately drop it, asserting that the
> page is pinned some other way. The FIXME here predates
> iov_iter_extract_pages(), which takes no reference for kernel-backed
> iterators.
> 
> CEPH_MSG_DATA_ITER data only comes from osd_req_op_extent_osd_iter(),
> whose only caller passes the netfs read iterator, which is always
> kernel-backed. Use iov_iter_extract_pages() and remove the put and the
> assertion. The messenger still relies on the upper layers to keep the
> pages alive while it uses them, as it did before. Extracting from a
> user-backed iterator would pin pages that nothing unpins, so add a
> precautionary warn in ceph_msg_data_add_iter().

Yikes, this goes through like three layers of pointless abstraction
for a single user :(

But trying to unwind those it comes from ceph_netfs_issue_read, which
is the netfs issue_read method, which is used for all kinds of
reads, but it does seem like for direct reads it uses the kinda
interesting netfs_extract_user_iter helper to turn the user iov
into a kernel one, which makes all of this such a freakin' mess.

So yeah, the analysis is right, at the same time using
iov_iter_extract_pages is just as weird as the old version.  Someone
really needs to clean up all the mess in both netfs and ceph :(

> This removes the last caller of PageWriteback(), allowing the page
> flag accessors to be removed in a future patch.
> 
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> ---
> The assertion is the last caller of PageWriteback() in the tree. The
> removal of the PG_writeback page flag accessors will be sent
> separately.

What about just killing that assert and leaving the rest of this
mess in place until it is sorted out properly?  iov_iter_get_pages2
is a pretty good marker for that, and it would be sad to loose that.