linux-kernel - Re: [PATCH 14/19] mm: Add user_landing in mm

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Date:   Mon, 9 Nov 2020 01:25:20 +0000
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Andy Lutomirski <luto@...nel.org>, Dmitry Safonov <dima@...sta.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Guo Ren <guoren@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will@...nel.org>, X86 ML <x86@...nel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>
Subject: Re: [PATCH 14/19] mm: Add user_landing in mm_struct

On 11/8/20 7:04 PM, Andy Lutomirski wrote:
> On Sat, Nov 7, 2020 at 9:18 PM Dmitry Safonov <dima@...sta.com> wrote:
>>
>> Instead of having every architecture to define vdso_base/vdso_addr etc,
>> provide a generic mechanism to track landing in userspace.
>> It'll minimize per-architecture difference, the number of callbacks to
>> provide.
>>
>> Originally, it started from thread [1] where the need for .close()
>> callback on vm_special_mapping was pointed, this generic code besides
>> removing duplicated .mremap() callbacks provides a cheaper way to
>> support munmap() on vdso mappings without introducing .close() callbacks
>> for every architecture (with would bring even more code duplication).
> 
> I find the naming odd.  It's called "user_landing", which is
> presumably a hard-to-understand shorthand for "user mode landing pad
> for return from a signal handler if SA_RESTORER is not set".  But,
> looking at the actual code, it's not this at all -- it's just the vDSO
> base address.

Agree. Originally, I tried to track the actual landing address on the
vdso, but .mremap() seemed simpler when tracking the vma base.

> So how about just calling it vdso_base?  I'm very much in favor of
> consolidating and cleaning up, and improving the vdso remap/unmap
> code, but I'm not convinced that we should call it anything other than
> the vdso base.

Sure.

Thanks,
         Dmitry