From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx156.postini.com [74.125.245.156]) by kanga.kvack.org (Postfix) with SMTP id 3861B6B004D for ; Thu, 2 Aug 2012 13:10:28 -0400 (EDT) Date: Thu, 2 Aug 2012 19:10:19 +0200 From: Johannes Weiner Subject: Re: [PATCH] slub: use free_page instead of put_page for freeing kmalloc allocation Message-ID: <20120802171019.GA1239@cmpxchg.org> References: <1343913065-14631-1-git-send-email-glommer@parallels.com> <20120802164203.GA30111@cmpxchg.org> <501AB013.1090607@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <501AB013.1090607@parallels.com> Sender: owner-linux-mm@kvack.org List-ID: To: Glauber Costa Cc: Christoph Lameter , linux-kernel@vger.kernel.org, Andrew Morton , linux-mm@kvack.org, David Rientjes , Pekka Enberg , Andrea Arcangeli On Thu, Aug 02, 2012 at 08:51:31PM +0400, Glauber Costa wrote: > On 08/02/2012 08:42 PM, Johannes Weiner wrote: > > On Thu, Aug 02, 2012 at 09:06:41AM -0500, Christoph Lameter wrote: > >> On Thu, 2 Aug 2012, Glauber Costa wrote: > >> > >>> diff --git a/mm/slub.c b/mm/slub.c > >>> index e517d43..9ca4e20 100644 > >>> --- a/mm/slub.c > >>> +++ b/mm/slub.c > >>> @@ -3453,7 +3453,7 @@ void kfree(const void *x) > >>> if (unlikely(!PageSlab(page))) { > >>> BUG_ON(!PageCompound(page)); > >>> kmemleak_free(x); > >>> - put_page(page); > >>> + __free_pages(page, compound_order(page)); > >> > >> Hmmm... put_page would have called put_compound_page(). which would have > >> called the dtor function. dtor is set to __free_pages() ok which does > >> mlock checks and verifies that the page is in a proper condition for > >> freeing. Then it calls free_one_page(). > >> > >> __free_pages() decrements the refcount and then calls __free_pages_ok(). > >> > >> So we loose the checking and the dtor stuff with this patch. Guess that is > >> ok? > > > > The changelog is not correct, however. People DO get pages underlying > > slab objects and even free the slab objects before returning the page. > > See recent fix: > > Well, yes, in the sense that slab objects are page-backed. > > The point is that a user of kmalloc/kfree should not treat a memory area > as if it were a page, even if it is page-sized. I whole-heartedly agree. But it's hard to verify there aren't any doing that. And even though it's ugly to do, it's technically working, no? No longer supporting it would be a regression. > If it is just the Changelog you are unhappy about, I can do another > submission rewording it. __free_pages still respects the refcount, so I think the Changelog is not actually appropriate for the change you're making. You're just changing what Christoph outlined above, the compound page handling. -- 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: email@kvack.org