linux-kernel - Re: [PATCH v2] x86/vdso: Change return type to vm_fault

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Message-ID: <20180703121122.GB3544@bombadil.infradead.org>
Date:   Tue, 3 Jul 2018 05:11:22 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        linux-kernel@...r.kernel.org, brajeswar.linux@...il.com,
        Sabyasachi Gupta <sabyasachi.linux@...il.com>
Subject: Re: [PATCH v2] x86/vdso: Change return type to vm_fault_t for fault
 handlers

On Tue, Jul 03, 2018 at 05:18:30PM +0530, Souptick Joarder wrote:
> On Tue, Jul 3, 2018 at 4:47 PM, Matthew Wilcox <willy@...radead.org> wrote:
> > On Mon, Jun 25, 2018 at 11:27:37PM +0530, Souptick Joarder wrote:
> >> Use new return type vm_fault_t for both fault handler
> >> vdso_fault() and vvar_fault(). Previously vm_insert_pfn()
> >> returns err which has to mapped into VM_FAULT_* type.
> >> The new function vmf_insert_pfn() will replace this
> >> inefficiency by returning correct VM_FAULT_* type.
> >
> >> @@ -105,10 +105,10 @@ static int vvar_fault(const struct vm_special_mapping *sm,
> >>        * the page past the end of the vvar mapping.
> >>        */
> >>       if (sym_offset == 0)
> >> -             return VM_FAULT_SIGBUS;
> >> +             return ret;
> >>
> >>       if (sym_offset == image->sym_vvar_page) {
> >> -             ret = vm_insert_pfn(vma, vmf->address,
> >> +             ret = vmf_insert_pfn(vma, vmf->address,
> >>                                   __pa_symbol(&__vvar_page) >> PAGE_SHIFT);
> >>       } else if (sym_offset == image->sym_pvclock_page) {
> >>               struct pvclock_vsyscall_time_info *pvti =
> >> @@ -124,14 +124,11 @@ static int vvar_fault(const struct vm_special_mapping *sm,
> >
> > Haven't you missed converting vm_insert_pfn_prot() at line 117?
> > Did you test-compile this?
> >
> 
> I left it intentionally in this patch. When we will be replacing
> vm_insert_foo() with new API vmf_insert_foo(), vm_insert_pfn_prot
> need to be changed to return vm_fault_t type.
> 
> I will change the return type of vm_insert_pfn_prot() in that patch.

What?!  That's just broken.  vm_insert_pfn_prot returns an errno, so
this patch introduces a bug where sometimes 'ret' contains a vm_fault_t
and sometimes contains an errno.  That's exactly the kind of thing this
patch series is supposed to be *preventing*, not introducing!

I'll send a replacement patch series to do this properly.