Re: huge zero page vs FOLL_DUMP

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

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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
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

-- 
 Kirill A. Shutemov

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