linux-kernel - Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

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Date:   Fri, 8 Mar 2019 14:11:08 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Jerome Glisse <jglisse@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, peterx@...hat.com,
        linux-mm@...ck.org, Jan Kara <jack@...e.cz>
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel
 virtual address

On Fri, Mar 08, 2019 at 05:13:26PM +0800, Jason Wang wrote:
> Actually not wrapping around,  the pages for used ring was marked as 
> dirty after a round of virtqueue processing when we're sure vhost wrote 
> something there.

Thanks for the clarification. So we need to convert it to
set_page_dirty and move it to the mmu notifier invalidate but in those
cases where gup_fast was called with write=1 (1 out of 3).

If using ->invalidate_range the page pin also must be removed
immediately after get_user_pages returns (not ok to hold the pin in
vmap until ->invalidate_range is called) to avoid false positive gup
pin checks in things like KSM, or the pin must be released in
invalidate_range_start (which is called before the pin checks).

Here's why:

		/*
		 * Check that no O_DIRECT or similar I/O is in progress on the
		 * page
		 */
		if (page_mapcount(page) + 1 + swapped != page_count(page)) {
			set_pte_at(mm, pvmw.address, pvmw.pte, entry);
			goto out_unlock;
		}
		[..]
		set_pte_at_notify(mm, pvmw.address, pvmw.pte, entry);
			  ^^^^^^^ too late release the pin here, the
				  above already failed

->invalidate_range cannot be used with mutex anyway so you need to go
back with invalidate_range_start/end anyway, just the pin must be
released in _start at the latest in such case.

My prefer is generally to call gup_fast() followed by immediate
put_page() because I always want to drop FOLL_GET from gup_fast as a
whole to avoid 2 useless atomic ops per gup_fast.

I'll write more about vmap in answer to the other email.

Thanks,
Andrea