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

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Date:	Wed, 27 Jan 2016 12:32:16 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Johannes Weiner <hannes@...xchg.org>
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 11:39 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> Hi,
>
> I noticed that vdso pages are faulted and unmapped as if they were
> regular file pages. And I'm guessing this is so that the vdso mappings
> are able to use the generic COW code in memory.c.
>
> However, it's a little unsettling that zap_pte_range() makes decisions
> based on PageAnon() and the page even reaches mark_page_accessed(), as
> that function makes several assumptions about the page being a regular
> LRU user page. It seems this isn't crashing today by sheer luck, but I
> am working on code that does when page_is_file_cache() returns garbage.
>
> I'm using this hack to work around it:
>
> diff --git a/mm/memory.c b/mm/memory.c
> index c387430f06c3..f0537c500150 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1121,7 +1121,8 @@ again:
>                                         set_page_dirty(page);
>                                 }
>                                 if (pte_young(ptent) &&
> -                                   likely(!(vma->vm_flags & VM_SEQ_READ)))
> +                                   likely(!(vma->vm_flags & VM_SEQ_READ)) &&
> +                                   !PageReserved(page))
>                                         mark_page_accessed(page);
>                                 rss[MM_FILEPAGES]--;
>                         }
>
> but I think we need a cleaner (and more robust) solution there to make
> it clearer that these pages are not regularly managed pages.
>
> 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.

>
> Are there other requirements of the VDSO that I might be missing?

There's vvar, too, on x86_64, and that mapping is really strange.
It's different in -tip than in any released kernel, too.  VM_MIXEDMAP
seems to work.

If you want to improve this, take a look at -tip -- it's cleaned up a lot.

--Andy