[PATCH] mm/slab: remove duplicate check

[PATCH] mm/slab: remove duplicate check

[Gavin Shan]

While allocateing pages using buddy allocator, the compound page
is probably split up to free pages. Under the circumstance, the
compound page should be destroied by function destroy_compound_page().
However, there has duplicate check to judge if the page is compound
one.

The patch removes the duplicate check since the function compound_order()
will returns 0 while the page hasn't PG_head set in function destroy_compound_page().
That's to say, the function destroy_compound_page() needn't check
PG_head any more through function PageHead().

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
---
 mm/page_alloc.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a712fb9..1277632 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -363,8 +363,7 @@ static int destroy_compound_page(struct page *page, unsigned long order)
 	int nr_pages = 1 << order;
 	int bad = 0;
 
-	if (unlikely(compound_order(page) != order) ||
-	    unlikely(!PageHead(page))) {
+	if (unlikely(compound_order(page) != order)) {
 		bad_page(page);
 		bad++;
 	}
-- 
1.7.5.4

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Re: [PATCH] mm/slab: remove duplicate check

[Johannes Weiner]

On Fri, May 11, 2012 at 05:16:09PM +0800, Gavin Shan wrote:
> While allocateing pages using buddy allocator, the compound page
> is probably split up to free pages. Under the circumstance, the
> compound page should be destroied by function destroy_compound_page().
> However, there has duplicate check to judge if the page is compound
> one.
> 
> The patch removes the duplicate check since the function compound_order()
> will returns 0 while the page hasn't PG_head set in function destroy_compound_page().
> That's to say, the function destroy_compound_page() needn't check
> PG_head any more through function PageHead().
> 
> Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>

Looks good!

But the slab in the subject suggests it would not affect other parts
of mm, while it actually affects THP, too.  Should probably be
removed?

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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Re: [PATCH] mm/slab: remove duplicate check

[Gavin Shan]

>> While allocateing pages using buddy allocator, the compound page
>> is probably split up to free pages. Under the circumstance, the
>> compound page should be destroied by function destroy_compound_page().
>> However, there has duplicate check to judge if the page is compound
>> one.
>> 
>> The patch removes the duplicate check since the function compound_order()
>> will returns 0 while the page hasn't PG_head set in function destroy_compound_page().
>> That's to say, the function destroy_compound_page() needn't check
>> PG_head any more through function PageHead().
>> 
>> Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
>
>Looks good!
>
>But the slab in the subject suggests it would not affect other parts
>of mm, while it actually affects THP, too.  Should probably be
>removed?
>
>Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>

Thanks for looking into this, Johannes :-)

I'm not sure I should change the subject to "mm: xxx" and resend it
since it has been put into linux-mm next tree as the maillist mm-commits
told me. By the way, I even don't know how linux-mm next got sychronized
with linux mainline yet :-)

Thanks,
Gavin

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