linux-kernel - Re: [PATCH] mm: do not let vdso pages into LRU rotation

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Date:	Thu, 28 Jan 2016 16:33:02 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: do not let vdso pages into LRU rotation

On Wed, Jan 27, 2016 at 12:32:16PM -0800, Andy Lutomirski wrote:
> On Wed, Jan 27, 2016 at 11:39 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> > Could the VDSO be a VM_MIXEDMAP to keep the initial unmanaged pages
> > out of the VM while allowing COW into regular anonymous pages?
> 
> Probably.  What are its limitations?  We want ptrace to work on it,
> and mprotect needs to work and allow COW.  access_process_vm should
> probably work, too.

Thanks, that's good to know.

However, after looking at this a little longer, it appears this would
need work in do_wp_page() to support non-page COW copying, then adding
vm_ops->access and complicating ->fault in all VDSO implementations.

And it looks like - at least theoretically - drivers can inject non-VM
pages into the page tables as well (comment above insert_page())

Given that this behavior has been around for a long time (the comment
at the bottom of vm_normal_page is ancient), I'll probably go with a
more conservative approach; add a comment to mark_page_accessed() and
filter out non-VM pages in the function I'm going to call from it.

Thanks!