linux-kernel - Re: [PATCH v8 2/8] powerpc/vdso: Remove __kernel_datapage_offset and simplify __get

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Message-ID: <6862421a-5a14-2e38-b825-e39e6ad3d51d@csgroup.eu>
Date:   Tue, 25 Aug 2020 16:15:26 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, nathanl@...ux.ibm.com
Cc:     linux-arch@...r.kernel.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        vincenzo.frascino@....com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v8 2/8] powerpc/vdso: Remove __kernel_datapage_offset and
 simplify __get_datapage()



Le 04/08/2020 à 13:17, Christophe Leroy a écrit :
> 
> 
> On 07/16/2020 02:59 AM, Michael Ellerman wrote:
>> Christophe Leroy <christophe.leroy@....fr> writes:
>>> The VDSO datapage and the text pages are always located immediately
>>> next to each other, so it can be hardcoded without an indirection
>>> through __kernel_datapage_offset
>>>
>>> In order to ease things, move the data page in front like other
>>> arches, that way there is no need to know the size of the library
>>> to locate the data page.
>>>
[...]

>>
>> I merged this but then realised it breaks the display of the vdso in 
>> /proc/self/maps.
>>
>> ie. the vdso vma gets no name:
>>
>>    # cat /proc/self/maps

[...]

>>
>>
>> And it's also going to break the logic in arch_unmap() to detect if
>> we're unmapping (part of) the VDSO. And it will break arch_remap() too.
>>
>> And the logic to recognise the signal trampoline in
>> arch/powerpc/perf/callchain_*.c as well.
> 
> I don't think it breaks that one, because ->vdsobase is still the start 
> of text.
> 
>>
>> So I'm going to rebase and drop this for now.
>>
>> Basically we have a bunch of places that assume that vdso_base is == the
>> start of the VDSO vma, and also that the code starts there. So that will
>> need some work to tease out all those assumptions and make them work
>> with this change.
> 
> Ok, one day I need to look at it in more details and see how other 
> architectures handle it etc ...
> 

I just sent out a series which switches powerpc to the new 
_install_special_mapping() API, the one powerpc uses being deprecated 
since commit a62c34bd2a8a ("x86, mm: Improve _install_special_mapping
and fix x86 vdso naming")

arch_remap() gets replaced by vdso_remap()

For arch_unmap(), I'm wondering how/what other architectures do, because 
powerpc seems to be the only one to erase the vdso context pointer when 
unmapping the vdso. So far I updated it to take into account the pages 
switch.

Everything else is not impacted because our vdso_base is still the base 
of the text and that's what those things (signal trampoline, callchain, 
...) expect.

Maybe we should change it to 'void *vdso' in the same way as other 
architectures, as it is not anymore the exact vdso_base but the start of 
VDSO text.

Note that the series applies on top of the generic C VDSO implementation 
series. However all but the last commit cleanly apply without that 
series. As that last commit is just an afterwork cleanup, it can come in 
a second step.

Christophe