From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-f71.google.com (mail-wr1-f71.google.com [209.85.221.71]) by kanga.kvack.org (Postfix) with ESMTP id BF6506B000A for ; Thu, 4 Oct 2018 08:35:49 -0400 (EDT) Received: by mail-wr1-f71.google.com with SMTP id d16-v6so7958791wrr.17 for ; Thu, 04 Oct 2018 05:35:49 -0700 (PDT) Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk. [2001:4d48:ad52:3201:214:fdff:fe10:1be6]) by mx.google.com with ESMTPS id v104-v6si4077450wrc.162.2018.10.04.05.35.47 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 04 Oct 2018 05:35:48 -0700 (PDT) Date: Thu, 4 Oct 2018 13:34:01 +0100 From: Russell King - ARM Linux Subject: Re: [PATCH v2] mm: Introduce new function vm_insert_kmem_page Message-ID: <20181004123400.GC30658@n2100.armlinux.org.uk> References: <20181003185854.GA1174@jordon-HP-15-Notebook-PC> <20181003200003.GA9965@bombadil.infradead.org> <20181003221444.GZ30658@n2100.armlinux.org.uk> 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: Souptick Joarder Cc: Matthew Wilcox , Miguel Ojeda , robin@protonic.nl, stefanr@s5r6.in-berlin.de, hjc@rock-chips.com, Heiko Stuebner , airlied@linux.ie, robin.murphy@arm.com, iamjoonsoo.kim@lge.com, Andrew Morton , Marek Szyprowski , Kees Cook , treding@nvidia.com, Michal Hocko , Dan Williams , "Kirill A. Shutemov" , Mark Rutland , aryabinin@virtuozzo.com, Dmitry Vyukov , Kate Stewart , tchibo@google.com, riel@redhat.com, Minchan Kim , Peter Zijlstra , "Huang, Ying" , ak@linux.intel.com, rppt@linux.vnet.ibm.com, linux@dominikbrodowski.net, Arnd Bergmann , cpandya@codeaurora.org, hannes@cmpxchg.org, Joe Perches , mcgrof@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux1394-devel@lists.sourceforge.net, dri-devel@lists.freedesktop.org, linux-rockchip@lists.infradead.org, Linux-MM On Thu, Oct 04, 2018 at 05:45:13PM +0530, Souptick Joarder wrote: > On Thu, Oct 4, 2018 at 3:45 AM Russell King - ARM Linux > wrote: > > > > On Wed, Oct 03, 2018 at 01:00:03PM -0700, Matthew Wilcox wrote: > > > On Thu, Oct 04, 2018 at 12:28:54AM +0530, Souptick Joarder wrote: > > > > These are the approaches which could have been taken to handle > > > > this scenario - > > > > > > > > * Replace vm_insert_page with vmf_insert_page and then write few > > > > extra lines of code to convert VM_FAULT_CODE to errno which > > > > makes driver users more complex ( also the reverse mapping errno to > > > > VM_FAULT_CODE have been cleaned up as part of vm_fault_t migration , > > > > not preferred to introduce anything similar again) > > > > > > > > * Maintain both vm_insert_page and vmf_insert_page and use it in > > > > respective places. But it won't gurantee that vm_insert_page will > > > > never be used in #PF context. > > > > > > > > * Introduce a similar API like vm_insert_page, convert all non #PF > > > > consumer to use it and finally remove vm_insert_page by converting > > > > it to vmf_insert_page. > > > > > > > > And the 3rd approach was taken by introducing vm_insert_kmem_page(). > > > > > > > > In short, vmf_insert_page will be used in page fault handlers > > > > context and vm_insert_kmem_page will be used to map kernel > > > > memory to user vma outside page fault handlers context. > > > > > > As far as I can tell, vm_insert_kmem_page() is line-for-line identical > > > with vm_insert_page(). Seriously, here's a diff I just did: > > > > > > -static int insert_page(struct vm_area_struct *vma, unsigned long addr, > > > - struct page *page, pgprot_t prot) > > > +static int insert_kmem_page(struct vm_area_struct *vma, unsigned long addr, > > > + struct page *page, pgprot_t prot) > > > - /* Ok, finally just insert the thing.. */ > > > -int vm_insert_page(struct vm_area_struct *vma, unsigned long addr, > > > +int vm_insert_kmem_page(struct vm_area_struct *vma, unsigned long addr, > > > - return insert_page(vma, addr, page, vma->vm_page_prot); > > > + return insert_kmem_page(vma, addr, page, vma->vm_page_prot); > > > -EXPORT_SYMBOL(vm_insert_page); > > > +EXPORT_SYMBOL(vm_insert_kmem_page); > > > > > > What on earth are you trying to do? > > > > > Reading the commit log, it seems that the intention is to split out > > vm_insert_page() used outside of page-fault handling with the use > > within page-fault handling, so that different return codes can be > > used. > > > > I don't see that justifies the code duplication - can't > > vm_insert_page() and vm_insert_kmem_page() use the same mechanics > > to do their job, and just translate the error code from the most- > > specific to the least-specific error code? Do we really need two > > copies of the same code just to return different error codes. > > Sorry about it. > can I take below approach in a patch series -> > > create a wrapper function vm_insert_kmem_page using vm_insert_page. > Convert all the non #PF users to use it. > Then make vm_insert_page static and convert inline vmf_insert_page to caller. I'm confused, what are you trying to do? It seems that we already have: vm_insert_page() - returns an errno vmf_insert_page() - returns a VM_FAULT_* code >>From what I _think_ you're saying, you're trying to provide vm_insert_kmem_page() as a direct replacement for the existing vm_insert_page(), and then make vm_insert_page() behave as per vmf_insert_page(), so we end up with: vm_insert_kmem_page() - returns an errno vm_insert_page() - returns a VM_FAULT_* code vmf_insert_page() - returns a VM_FAULT_* code and is identical to vm_insert_page() Given that the documentation for vm_insert_page() says: * Usually this function is called from f_op->mmap() handler * under mm->mmap_sem write-lock, so it can change vma->vm_flags. * Caller must set VM_MIXEDMAP on vma if it wants to call this * function from other places, for example from page-fault handler. this says that the "usual" use method for vm_insert_page() is _outside_ of page fault handling - if it is used _inside_ page fault handling, then it states that additional fixups are required on the VMA. So I don't get why your patch commentry seems to be saying that users of vm_insert_page() outside of page fault handling all need to be patched - isn't this the use case that this function is defined to be handling? If you're going to be changing the semantics, doesn't this create a flag day where we could get new users of vm_insert_page() using the _existing_ semantics merged after you've changed its semantics (eg, the return code)? Maybe I don't understand fully what you're trying to achieve here. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up