Re: [PATCH 20/25] Do not check for compound pages during the page allocator sanity checks

Mel Gorman
2009-04-22T10:09:55Z

On Wed, Apr 22, 2009 at 09:20:40AM +0900, KOSAKI Motohiro wrote:
> > A number of sanity checks are made on each page allocation and free
> > including that the page count is zero. page_count() checks for
> > compound pages and checks the count of the head page if true. However,
> > in these paths, we do not care if the page is compound or not as the
> > count of each tail page should also be zero.
> >
> > This patch makes two changes to the use of page_count() in the free path. It
> > converts one check of page_count() to a VM_BUG_ON() as the count should
> > have been unconditionally checked earlier in the free path. It also avoids
> > checking for compound pages.
> >
> > [mel@csn.ul.ie: Wrote changelog]
> > Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
> > Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
> > ---
> > mm/page_alloc.c | 6 +++---
> > 1 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index ec01d8f..376d848 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -425,7 +425,7 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
> > return 0;
> >
> > if (PageBuddy(buddy) && page_order(buddy) == order) {
> > - BUG_ON(page_count(buddy) != 0);
> > + VM_BUG_ON(page_count(buddy) != 0);
> > return 1;
> > }
> > return 0;
> >
>
> Looks good.
>
>
> > @@ -501,7 +501,7 @@ static inline int free_pages_check(struct page *page)
> > {
> > if (unlikely(page_mapcount(page) |
> > (page->mapping != NULL) |
> > - (page_count(page) != 0) |
> > + (atomic_read(&page->_count) != 0) |
> > (page->flags & PAGE_FLAGS_CHECK_AT_FREE))) {
> > bad_page(page);
> > return 1;
> > @@ -646,7 +646,7 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
> > {
> > if (unlikely(page_mapcount(page) |
> > (page->mapping != NULL) |
> > - (page_count(page) != 0) |
> > + (atomic_read(&page->_count) != 0) |
> > (page->flags & PAGE_FLAGS_CHECK_AT_PREP))) {
> > bad_page(page);
> > return 1;
>
>
> inserting VM_BUG_ON(PageTail(page)) is better?
>

We already go one further with

#define PAGE_FLAGS_CHECK_AT_PREP ((1 << NR_PAGEFLAGS) - 1)

...

if (.... | (page->flags & PAGE_FLAGS_CHECK_AT_PREP))
bad_page(page);

PG_tail is in PAGE_FLAGS_CHECK_AT_PREP so we're already checking for it
and calling bad_page() if set.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab