From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-iy0-f179.google.com (mail-iy0-f179.google.com [209.85.210.179]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (verified OK)) by ozlabs.org (Postfix) with ESMTPS id AA093B6F82 for ; Sun, 17 Jul 2011 01:36:35 +1000 (EST) Received: by iyn35 with SMTP id 35so1945131iyn.38 for ; Sat, 16 Jul 2011 08:36:32 -0700 (PDT) Message-ID: <4E21AFF3.20607@gmail.com> Date: Sat, 16 Jul 2011 11:36:19 -0400 From: Shan Hai MIME-Version: 1.0 To: Peter Zijlstra Subject: Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core References: <1310717238-13857-1-git-send-email-haishan.bai@gmail.com> <1310717238-13857-2-git-send-email-haishan.bai@gmail.com> <1310725418.2586.309.camel@twins> <4E205A63.90401@gmail.com> <1310743477.2586.358.camel@twins> In-Reply-To: <1310743477.2586.358.camel@twins> Content-Type: text/plain; charset=UTF-8; format=flowed Cc: tony.luck@intel.com, linux-kernel@vger.kernel.org, cmetcalf@tilera.com, dhowells@redhat.com, paulus@samba.org, tglx@linutronix.de, walken@google.com, linuxppc-dev@lists.ozlabs.org, akpm@linux-foundation.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 07/15/2011 11:24 AM, Peter Zijlstra wrote: > On Fri, 2011-07-15 at 11:18 -0400, Shan Hai wrote: > >>>> + vma = find_vma(mm, address); >>> Uhm, find_vma() needs mmap_sem, and futex_atomic_cmpxchg_inatomic() is >>> most certainly not called with that lock held. >>> >> My fault, that will be fixed in the V2 patch. > But you cannot, the function isn't called _atomic_ just for kicks, its > used while holding spinlocks. > Yes we can do that, _atomic_ here is just atomic for cmpxchg implemented by the combination of 'lwarx' and 'stwcx.' instructions as done in the spin lock implementation, so even we hold the mmap_sem that has no impact on the _atomic_ feature of the futex_atomic_cmpxchg_inatomic(). >>>> + if (likely(vma)) { >>>> + /* only fixup present page */ >>>> + if (follow_page(vma, address, FOLL_WRITE)) { >>>> + handle_mm_fault(mm, vma, address, FAULT_FLAG_WRITE); >>> So how can this toggle your sw dirty/young tracking, that's pretty much >>> what gup(.write=1) does too! >>> >> because of the kernel read only permission of the page is transparent >> to the follow_page(), the handle_mm_fault() is not to be activated >> in the __get_use_pages(), so the gup(.write=1) could not help to fixup >> the write permission. > So why do you need the vma? Is it like I wrote earlier that you don't > have spare PTE bits and need the vma flags to see if it may become > writable? > Need vma for the reason to call handle_mm_fault(), that's all. > gup(.write=1) not triggering this is a serious problem though, not > something you can just paper over. I wouldn't be at all surprised to > find there's more things broken because of that. In my opinion another solution might be check the read only for kernel feature of a page in the follow_page() on gup(.write=1) to avoid this problem on all architectures. Thanks Shan Hai