Re: huge zero page vs FOLL_DUMP

[Simon Jeons <simon.jeons@gmail.com>]

On Sat, 2013-01-12 at 05:36 +0200, Kirill A. Shutemov wrote:
> On Fri, Jan 11, 2013 at 03:53:34PM -0800, Michel Lespinasse wrote:
> > Hi,
> > 
> > follow_page() has code to return ERR_PTR(-EFAULT) when it encounters
> > the zero page and FOLL_DUMP flag is passed - this is used to avoid
> > dumping the zero page to disk when doing core dumps, and also by
> > munlock to avoid having potentially large number of threads trying to
> > munlock the zero page at once, which we can't reclaim anyway.
> > 
> > We don't have the corresponding logic when follow_page() encounters a
> > huge zero page. I think we should, preferably before 3.8. However, I
> > am slightly confused as to what to do for the munlock case, as the
> > huge zero page actually does seem to be reclaimable. My guess is that
> > we could still skip the munlocks, until the zero page is actually
> > reclaimed at which point we should check if we can munlock it.
> > 
> > Kirill, is this something you would have time to look into ?
> 
> Nice catch! Thank you.
> 
> I don't think we should do anything about mlock(). Huge zero page cannot
> be mlocked -- it will not pass page->mapping check in

Hi Kirill,

What's store in page->mapping of huge zero page?

> follow_trans_huge_pmd(). And it's not reclaimable if it's mapped to
> anywhere.
> 
> Could you tese the patch?
> 
> From 062a9b670ede9fe5fca1d1947b42990b6b0642a4 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Date: Sat, 12 Jan 2013 05:18:58 +0200
> Subject: [PATCH] thp: Avoid dumping huge zero page
> 
> No reason to preserve huge zero page in core dump.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reported-by: Michel Lespinasse <walken@google.com>
> ---
>  mm/huge_memory.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 6001ee6..b5783d8 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1257,6 +1257,10 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>  	if (flags & FOLL_WRITE && !pmd_write(*pmd))
>  		goto out;
>  
> +	/* Avoid dumping huge zero page */
> +	if ((flags & FOLL_DUMP) && is_huge_zero_pmd(*pmd))
> +		return ERR_PTR(-EFAULT);
> +
>  	page = pmd_page(*pmd);
>  	VM_BUG_ON(!PageHead(page));
>  	if (flags & FOLL_TOUCH) {
> -- 
> 1.8.1
> 

-- 
Simon Jeons <simon.jeons@gmail.com>

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