From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 8435C8D0040 for ; Fri, 25 Mar 2011 12:48:53 -0400 (EDT) Date: Fri, 25 Mar 2011 17:48:47 +0100 From: Andrea Arcangeli Subject: Re: get_page() vs __split_huge_page_refcount() Message-ID: <20110325164847.GD431@random.random> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Michel Lespinasse Cc: linux-mm Hi Michel, On Thu, Mar 24, 2011 at 10:00:16PM -0700, Michel Lespinasse wrote: > My question is this: After someone obtains a page reference using > get_user_pages(), what prevents them from getting additional > references with get_page() ? I always thought it was legal to > duplicate references that way, but now I don't see how it'd be safe > doing so on anon pages with THP enabled. It's not legal anymore as you noticed, but I'm not aware of anything doing that. I don't see an useful case where a driver could need to take one extra refcount after GUP returned. The normal API is GUP/put_page. We could make it legal again by taking the compound_lock after a PageCompound check though. I hope it's not needed though. It's unavoidable in put_page because put_page will run out of order with regard to __split_huge_page_refcount. But serializing get_page in GUP against __split_huge_page_refcount is automatic through the pmd_trans_splitting bit and needed for all page table walkers anyway. Maybe it's good idea to add a comment to transhuge.txt about that? I don't think I added it. Grepping for get_page in drivers doesn't show too many, they mostly run through the vm_ops->fault handler. Most important I can't see how possibly it could be useful to run a get_page after get_user_pages(FOLL_GET) returns. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org