Re: [PATCHv2 4/4] mm: make compound_head() robust
From: Kirill A. Shutemov <hidden>
Date: 2015-08-18 18:24:46
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On Tue, Aug 18, 2015 at 06:41:13PM +0200, Michal Hocko wrote:
On Tue 18-08-15 13:20:22, Michal Hocko wrote:quoted
On Mon 17-08-15 18:09:05, Kirill A. Shutemov wrote:quoted
Hugh has pointed that compound_head() call can be unsafe in some context. There's one example: CPU0 CPU1 isolate_migratepages_block() page_count() compound_head() !!PageTail() == true put_page() tail->first_page = NULL head = tail->first_page alloc_pages(__GFP_COMP) prep_compound_page() tail->first_page = head __SetPageTail(p); !!PageTail() == true <head == NULL dereferencing> The race is pure theoretical. I don't it's possible to trigger it in practice. But who knows. We can fix the race by changing how encode PageTail() and compound_head() within struct page to be able to update them in one shot. The patch introduces page->compound_head into third double word block in front of compound_dtor and compound_order. That means it shares storage space with: - page->lru.next; - page->next; - page->rcu_head.next; - page->pmd_huge_pte; That's too long list to be absolutely sure, but looks like nobody uses bit 0 of the word. It can be used to encode PageTail(). And if the bit set, rest of the word is pointer to head page.I didn't look too closely but the general idea makes sense to me and the overal code simplification is sound. I will give it more detailed review after I sort out other stuff.AFICS page::first_page wasn't used outside of compound page logic so you should remove it in this patch. The rest looks good to me.
I missed it by accident during rework for v2. Will fix.
Acked-by: Michal Hocko <mhocko@suse.com>
Thanks! -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>