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

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  Re: [PATCH] mm: do not let vdso pages into LRU rotation Johannes Weiner <hannes@cmpxchg.org> - 2016-01-28 22:40 +0100
    Re: [PATCH] mm: do not let vdso pages into LRU rotation Andy Lutomirski <luto@amacapital.net> - 2016-01-29 23:30 +0100

#1321077 — 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@cmpxchg.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!

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#1322129

On Thu, Jan 28, 2016 at 1:33 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> 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@cmpxchg.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.

I just checked: in -tip, I'm creating a VM_PFNMAP (not VM_MIXEDMAP)
vma and faulting a RAM page (with struct page and all) in using
vm_insert_pfn.  Is that okay, or so I need to use VM_MIXEDMAP instead?

--Andy

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