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

From: Johannes Weiner
Date: Thu Jan 28 2016 - 16:33:37 EST

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@xxxxxxxxxxx> 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.