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SubjectRe: [PATCH] mm: PageBuddy and mapcount underflows robustness
On Mon, Mar 14, 2011 at 10:30:11AM -0700, Linus Torvalds wrote:
> On Mon, Mar 14, 2011 at 9:59 AM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> >
> > +#define PAGE_BUDDY_MAPCOUNT_VALUE (-1024*1024)
>
> I realize that this is a nitpick, but from a code generation
> standpoint, large random constants like these are just nasty.
>
> I would suggest aiming for constants that are easy to generate and/or
> fit better in the code stream. In many encoding schemes (eg x86), -128
> is much easier to generate, since it fits in a signed byte and allows
> small instructions etc. And in most RISC encodings, 8- or 16-bit
> constants can be encoded much more easily than something like your
> current one, and bigger ones often end up resulting in a load from
> memory or at least several immediate-building instructions.

-128 sure is ok with me. It's extremely unlikely to be off by 127
times, if we're off by more than 127 times we're still going to detect
the error.

> Also, this is just disgusting. It adds no safety here to have that
> VM_BUG_ON(), so it's just unnecessary code generation to do this.
> Also, we don't even WANT to do that stupid "__ClearPageBuddy()" in the
> first place! What those two code-sites actually want are just a simple
>
> reset_page_mapcount(page);
>
> which does the right thing in _general_, and not just for the buddy
> case - we want to reset the mapcount for other reasons than just
> pagebuddy (ie the underflow/overflow case).

Agreed.

> And it avoids the VM_BUG_ON() too, making the crazy conditionals be not needed.

Well using reset_page_mapcount in the two error sites, didn't require
me to remove the VM_BUG_ON from __ClearPageBuddy so I left it
there...

===
Subject: mm: PageBuddy and mapcount robustness

From: Andrea Arcangeli <aarcange@redhat.com>

Change the _mapcount value indicating PageBuddy from -2 to -128 for
more robusteness against page_mapcount() undeflows.

Use reset_page_mapcount instead of __ClearPageBuddy in bad_page to
ignore the previous retval of PageBuddy().

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Hugh Dickins <hughd@google.com>
---
include/linux/mm.h | 11 +++++++++--
mm/page_alloc.c | 4 ++--
2 files changed, 11 insertions(+), 4 deletions(-)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -402,16 +402,23 @@ static inline void init_page_count(struc
/*
* PageBuddy() indicate that the page is free and in the buddy system
* (see mm/page_alloc.c).
+ *
+ * PAGE_BUDDY_MAPCOUNT_VALUE must be <= -2 but better not too close to
+ * -2 so that an underflow of the page_mapcount() won't be mistaken
+ * for a genuine PAGE_BUDDY_MAPCOUNT_VALUE. -128 can be created very
+ * efficiently by most CPU architectures.
*/
+#define PAGE_BUDDY_MAPCOUNT_VALUE (-128)
+
static inline int PageBuddy(struct page *page)
{
- return atomic_read(&page->_mapcount) == -2;
+ return atomic_read(&page->_mapcount) == PAGE_BUDDY_MAPCOUNT_VALUE;
}

static inline void __SetPageBuddy(struct page *page)
{
VM_BUG_ON(atomic_read(&page->_mapcount) != -1);
- atomic_set(&page->_mapcount, -2);
+ atomic_set(&page->_mapcount, PAGE_BUDDY_MAPCOUNT_VALUE);
}

static inline void __ClearPageBuddy(struct page *page)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -286,7 +286,7 @@ static void bad_page(struct page *page)

/* Don't complain about poisoned pages */
if (PageHWPoison(page)) {
- __ClearPageBuddy(page);
+ reset_page_mapcount(page); /* remove PageBuddy */
return;
}

@@ -317,7 +317,7 @@ static void bad_page(struct page *page)
dump_stack();
out:
/* Leave bad fields for debug, except PageBuddy could make trouble */
- __ClearPageBuddy(page);
+ reset_page_mapcount(page); /* remove PageBuddy */
add_taint(TAINT_BAD_PAGE);
}


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