Re: [PATCH v8 2/8] powerpc/vdso: Remove __kernel_datapage_offset and simplify __get_datapage()

[Christophe Leroy]

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@xxxxxx> 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