netdev - getting rid of the last memory modifitions through gup(FOLL

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Date: Tue, 5 Sep 2023 16:16:04 +0200
From: Christoph Hellwig <hch@....de>
To: Jan Kara <jack@...e.cz>, David Howells <dhowells@...hat.com>,
	David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>
Cc: Lei Huang <lei.huang@...ux.intel.com>, miklos@...redi.hu,
	Xiubo Li <xiubli@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
	Jeff Layton <jlayton@...nel.org>,
	Trond Myklebust <trond.myklebust@...merspace.com>,
	Anna Schumaker <anna@...nel.org>,
	Latchesar Ionkov <lucho@...kov.net>,
	Dominique Martinet <asmadeus@...ewreck.org>,
	Christian Schoenebeck <linux_oss@...debyte.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	John Fastabend <john.fastabend@...il.com>,
	Jakub Sitnicki <jakub@...udflare.com>,
	Boris Pismenny <borisp@...dia.com>, linux-nfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, ceph-devel@...r.kernel.org,
	linux-mm@...ck.org, v9fs@...ts.linux.dev, netdev@...r.kernel.org
Subject: getting rid of the last memory modifitions through gup(FOLL_GET)

Hi all,

we've made some nice progress on converting code that modifies user
memory to the pin_user_pages interface, especially though the work
from David Howells on iov_iter_extract_pages.  This thread tries to
coordinate on how to finish off this work.

The obvious next step is the remaining users of iov_iter_get_pages2
and iov_iter_get_pages_alloc2.  We have three file system direct I/O
users of those left: ceph, fuse and nfs.  Lei Huang has sent patches
to convert fuse to iov_iter_extract_pages which I'd love to see merged,
and we'd need equivalent work for ceph and nfs.

The non-file system uses are in the vmsplice code, which only reads
from the pages (but would still benefit from an iov_iter_extract_pages
conversion), and in net.  Out of the users in net, all but the 9p code
appear to be for reads from memory, so they don't pin even if a
conversion would be nice to retire iov_iter_get_pages* APIs.

After that we might have to do an audit of the raw get_user_pages APIs,
but there probably aren't many that modify file backed memory.

I'm also wondering what a good debug aid would be for detecting
writes to file backed memory without a previous reservation, but
everything either involves a page flag or file system code.  But if
someone has an idea I'm all ear as something mechanical to catch these
uses would be quite helpful.