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

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Date: Sat, 9 Sep 2023 13:18:34 +0200
From: Christoph Hellwig <hch@....de>
To: David Hildenbrand <david@...hat.com>
Cc: Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>,
	David Howells <dhowells@...hat.com>, Peter Xu <peterx@...hat.com>,
	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: Re: getting rid of the last memory modifitions through
 gup(FOLL_GET)

On Fri, Sep 08, 2023 at 06:48:05PM +0200, David Hildenbrand wrote:
> vmsplice_to_pipe() -> iter_to_pipe() -> iov_iter_get_pages2()
>
> So it ends up calling get_user_pages_fast()
>
> ... and not using FOLL_PIN|FOLL_LONGTERM
>
> Why FOLL_LONGTERM? Because it's a longterm pin, where unprivileged users 
> can grab a reference on a page for all eternity, breaking CMA and memory 
> hotunplug (well, and harming compaction).
>
> Why FOLL_PIN? Well FOLL_LONGTERM only applies to FOLL_PIN. But for 
> anonymous memory, this will also take care of the last remaining hugetlb 
> COW test (trigger COW unsharing) as commented back in:
>
> https://lore.kernel.org/all/02063032-61e7-e1e5-cd51-a50337405159@redhat.com/

Well, I'm not against it.  It just isn't required for deadling with
file system writeback vs GUP modification race this thread was started
for. 

>> Can KVM page tables use file backed shared mappings?
>
> Yes, usually shmem and hugetlb. But with things like emulated 
> NVDIMMs/virtio-pmem for VMs, easily also ordinary files.
>
> But it's really not ordinary write access through GUP. It's write access 
> via a secondary page table (secondary MMU), that's synchronized to the 
> process page table -- just like if the CPU would be writing to the page 
> using the process page tables (primary MMU).

Writing through the process page tables takes a write faul when first
writing, which calls into ->page_mkwrite in the file system.  Does the
synchronization take care of that?  If not we need to add or emulate it.

> ptrace will find the pagecache page writable in the page table (PTE write 
> bit set), if it intends to write to the page (FOLL_WRITE). If it is not 
> writable, it will trigger a page fault that informs the file system.

Yes, that case is (mostly) fine.

>
> With an FS that wants writenotify, we will not map a page writable (PTE 
> write bit not set) unless it is dirty (PTE dirty bit set) IIRC.
>
> So are we concerned about a race between the filesystem removing the PTE 
> write bit (to catch next write access before it gets dirtied again) and 
> ptrace marking the page dirty?

Yes.  This is the race that we've run into with various GUP users.

> Yes. However, secondary MMU users (like KVM) would need some way to keep 
> making use of that; ideally, using a proper separate interface instead of 
> (ab)using plain GUP and confusing people :)

I'mm all for that.