From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx201.postini.com [74.125.245.201]) by kanga.kvack.org (Postfix) with SMTP id AD37F6B0033 for ; Mon, 17 Jun 2013 05:17:43 -0400 (EDT) Received: by mail-ie0-f169.google.com with SMTP id 10so6462413ied.28 for ; Mon, 17 Jun 2013 02:17:43 -0700 (PDT) Message-ID: <51BED42F.9000507@ozlabs.ru> Date: Mon, 17 Jun 2013 19:17:35 +1000 From: Alexey Kardashevskiy MIME-Version: 1.0 Subject: Re: [PATCH 2/4] powerpc: Prepare to support kernel handling of IOMMU map/unmap References: <1370412673-1345-1-git-send-email-aik@ozlabs.ru> <1370412673-1345-3-git-send-email-aik@ozlabs.ru> <1371356818.21896.114.camel@pasglop> In-Reply-To: <1371356818.21896.114.camel@pasglop> Content-Type: text/plain; charset=KOI8-R Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Benjamin Herrenschmidt Cc: linuxppc-dev@lists.ozlabs.org, David Gibson , Alexander Graf , Paul Mackerras , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, kvm-ppc@vger.kernel.org, "linux-mm@kvack.org" On 06/16/2013 02:26 PM, Benjamin Herrenschmidt wrote: >> +#if defined(CONFIG_SPARSEMEM_VMEMMAP) || defined(CONFIG_FLATMEM) >> +int realmode_get_page(struct page *page) >> +{ >> + if (PageCompound(page)) >> + return -EAGAIN; >> + >> + get_page(page); >> + >> + return 0; >> +} >> +EXPORT_SYMBOL_GPL(realmode_get_page); >> + >> +int realmode_put_page(struct page *page) >> +{ >> + if (PageCompound(page)) >> + return -EAGAIN; >> + >> + if (!atomic_add_unless(&page->_count, -1, 1)) >> + return -EAGAIN; >> + >> + return 0; >> +} >> +EXPORT_SYMBOL_GPL(realmode_put_page); >> +#endif > > Several worries here, mostly that if the generic code ever changes > (something gets added to get_page() that makes it no-longer safe for use > in real mode for example, or some other condition gets added to > put_page()), we go out of sync and potentially end up with very hard and > very subtle bugs. > > It might be worth making sure that: > > - This is reviewed by some generic VM people (and make sure they > understand why we need to do that) > > - A comment is added to get_page() and put_page() to make sure that if > they are changed in any way, dbl check the impact on our > realmode_get_page() (or "ping" us to make sure things are still ok). After changing get_page() to get_page_unless_zero(), the get_page API I use is: get_page_unless_zero() - basically atomic_inc_not_zero() atomic_add_unless() - just operated with the counter PageCompound() - check if it is a huge page. No usage of get_page or put_page. If any of those changes, I would expect it to hit us immediately, no? So it may only make sense to add a comment to PageCompound(). But the comment says "PageCompound is generally not used in hot code paths", and our path is hot. Heh. diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 6d53675..c70a654 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -329,7 +329,8 @@ static inline void set_page_writeback(struct page *page) * System with lots of page flags available. This allows separate * flags for PageHead() and PageTail() checks of compound pages so that bit * tests can be used in performance sensitive paths. PageCompound is - * generally not used in hot code paths. + * generally not used in hot code paths except arch/powerpc/mm/init_64.c + * which uses it to detect huge pages and avoid handling those in real mode. */ __PAGEFLAG(Head, head) CLEARPAGEFLAG(Head, head) __PAGEFLAG(Tail, tail) So? -- Alexey -- 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