On Tue, Apr 29, 2014 at 05:42:07PM +0800, Jianyu Zhan wrote: > Currently, put_compound_page should carefully handle tricky case > to avoid racing with compound page releasing or spliting, which > makes it growing quite lenthy(about 200+ lines) and need deep > tab indention, which makes it quite hard to follow and maintain. > > This patch(and the next patch) tries to refactor this function. > It is a prepared patch. > > Based on the code skeleton of put_compound_page: > > put_compound_pge: Typo. > if !PageTail(page) > put head page fastpath; > return; > > /* else PageTail */ > page_head = compound_head(page) > if !__compound_tail_refcounted(page_head) > put head page optimal path; <---(1) > return; > else > put head page slowpath; <--- (2) > return; > > This patch introduces two helpers, put_[un]refcounted_compound_page, > handling the code path (1) and code path (2), respectively. They both > are tagged __always_inline, thus it elmiates function call overhead, > making them operating the same way as before. > > They are almost copied verbatim(except one place, a "goto out_put_single" > is expanded), with some comments rephrasing. > > Signed-off-by: Jianyu Zhan <nasa4836@xxxxxxxxx> > --- > mm/swap.c | 142 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 142 insertions(+) > > diff --git a/mm/swap.c b/mm/swap.c > index c0cd7d0..a576449 100644 > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -79,6 +79,148 @@ static void __put_compound_page(struct page *page) > (*dtor)(page); > } > > +/** > + * Two special cases here: we could avoid taking compound_lock_irqsave > + * and could skip the tail refcounting(in _mapcount). > + * > + * 1. Hugetlbfs page: > + * > + * PageHeadHuge will remain true until the compound page > + * is released and enters the buddy allocator, and it could > + * not be split by __split_huge_page_refcount(). > + * > + * So if we see PageHeadHuge set, and we have the tail page pin, > + * then we could safely put head page. > + * > + * 2. Slab THP page: There's no such thing. It called Slab compound page. > + * > + * PG_slab is cleared before the slab frees the head page, and > + * tail pin cannot be the last reference left on the head page, > + * because the slab code is free to reuse the compound page > + * after a kfree/kmem_cache_free without having to check if > + * there's any tail pin left. In turn all tail pinsmust be always > + * released while the head is still pinned by the slab code > + * and so we know PG_slab will be still set too. > + * > + * So if we see PageSlab set, and we have the tail page pin, > + * then we could safely put head page. > + */ -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>