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From: KOSAKI Motohiro on 25 Jan 2010 21:50
> Peter Zijlstra wrote:
> > On Mon, 2010-01-25 at 16:27 +0900, KOSAKI Motohiro wrote:
> <snip>
> >> Futex should work both file anon anon. however I personally think
> >> vdso is not file nor anon. it is special mappings. nobody defined
> >> futex spec on special mappings. (yes, undefined).
> >>
> >> Personally, I think EINVAL or EFAULT are best result of vdso futexing, like as
> >> futexing againt kernel address. but I guess another person have another thinking.
> >>
> >> I'd like to hear futex folks's opinion.
> >
> > Well, my opinion is we should remove the vdso, its ugly as hell :-)
> >
> > But I think it would make most sense to extend its definition in the
> > direction of it being a file (for all intents and purposes its a special
> > DSO -- which unfortunately isn't present in any filesystem).
> >
> > [ For all intents and purposes processes can already communicate through
> > futexes on the libc space, so being able to do so through the vsdo
> > really doesn't add anything ]
> >
> > So the problem is that the VDSO pages do not have a page->mapping
> > because they lack the actual filesystem part of files, so even if (with
> > the recent zero-page patch from Kosaki-san) you make private COWs of the
> > VDSO, you'll get stuck in that loop.
> >
> > So the prettiest solution is to simply place the vdso in an actual
> > filesystem and slowly migrate towards letting userspace map it as a
> > regular DSO -- /sys/lib{32,64}/libkernel.so like.
> >
> > [ that has the bonus of getting rid of install_special_mapping() ]
> >
> > The ugly solution is special casing the vdso in get_futex_key().
>
> I like the creating-a-real-file solution. However, for now (and for
> stable), I think Kosaki's suggestion of EINVAL or EFAULT is a good
> stop-gap. EINVAL might play the best with existing glibc implementations.

May I confirm your mention?

If we can accept EFAULT, we don't need any change. my previous futex patch
already did. because 1) VDSO is alwasys read-only mapped 2) write mode
get_user_pages_fast() against read-only pte/vma return EFAULT.

Current linus and stable tree don't cause Mark's original problem. instead, just
return EFAULT. (Well, I'm sorry. my previous mail was unclear. I wrote v2.6.31 test
result)

If you can't accept EFAULT, we need to add vdso specific logic into get_futex_key().
Is this your intention?



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From: Peter Zijlstra on 26 Jan 2010 03:00
On Tue, 2010-01-26 at 11:41 +0900, KOSAKI Motohiro wrote:
> > Peter Zijlstra wrote:
> > > On Mon, 2010-01-25 at 16:27 +0900, KOSAKI Motohiro wrote:
> > <snip>
> > >> Futex should work both file anon anon. however I personally think
> > >> vdso is not file nor anon. it is special mappings. nobody defined
> > >> futex spec on special mappings. (yes, undefined).
> > >>
> > >> Personally, I think EINVAL or EFAULT are best result of vdso futexing, like as
> > >> futexing againt kernel address. but I guess another person have another thinking.
> > >>
> > >> I'd like to hear futex folks's opinion.
> > >
> > > Well, my opinion is we should remove the vdso, its ugly as hell :-)
> > >
> > > But I think it would make most sense to extend its definition in the
> > > direction of it being a file (for all intents and purposes its a special
> > > DSO -- which unfortunately isn't present in any filesystem).
> > >
> > > [ For all intents and purposes processes can already communicate through
> > > futexes on the libc space, so being able to do so through the vsdo
> > > really doesn't add anything ]
> > >
> > > So the problem is that the VDSO pages do not have a page->mapping
> > > because they lack the actual filesystem part of files, so even if (with
> > > the recent zero-page patch from Kosaki-san) you make private COWs of the
> > > VDSO, you'll get stuck in that loop.
> > >
> > > So the prettiest solution is to simply place the vdso in an actual
> > > filesystem and slowly migrate towards letting userspace map it as a
> > > regular DSO -- /sys/lib{32,64}/libkernel.so like.
> > >
> > > [ that has the bonus of getting rid of install_special_mapping() ]
> > >
> > > The ugly solution is special casing the vdso in get_futex_key().
> >
> > I like the creating-a-real-file solution. However, for now (and for
> > stable), I think Kosaki's suggestion of EINVAL or EFAULT is a good
> > stop-gap. EINVAL might play the best with existing glibc implementations.
>
> May I confirm your mention?
>
> If we can accept EFAULT, we don't need any change. my previous futex patch
> already did. because 1) VDSO is alwasys read-only mapped 2) write mode
> get_user_pages_fast() against read-only pte/vma return EFAULT.
>
> Current linus and stable tree don't cause Mark's original problem. instead, just
> return EFAULT. (Well, I'm sorry. my previous mail was unclear. I wrote v2.6.31 test
> result)
>
> If you can't accept EFAULT, we need to add vdso specific logic into get_futex_key().
> Is this your intention?

Oh, right you are, I mixed up the force and write arguments. Yes I tihnk
we're good.


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From: Thomas Gleixner on 26 Jan 2010 03:40
On Tue, 26 Jan 2010, KOSAKI Motohiro wrote:
> > Peter Zijlstra wrote:
> > I like the creating-a-real-file solution. However, for now (and for
> > stable), I think Kosaki's suggestion of EINVAL or EFAULT is a good
> > stop-gap. EINVAL might play the best with existing glibc implementations.
>
> May I confirm your mention?
>
> If we can accept EFAULT, we don't need any change. my previous futex patch
> already did. because 1) VDSO is alwasys read-only mapped 2) write mode
> get_user_pages_fast() against read-only pte/vma return EFAULT.
>
> Current linus and stable tree don't cause Mark's original problem. instead, just
> return EFAULT. (Well, I'm sorry. my previous mail was unclear. I wrote v2.6.31 test
> result)
>
> If you can't accept EFAULT, we need to add vdso specific logic into get_futex_key().

EFAULT is perfectly fine. No need for any special tricks.

Thanks,

tglx

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From: Darren Hart on 26 Jan 2010 09:30
KOSAKI Motohiro wrote:
>> Peter Zijlstra wrote:
>>> On Mon, 2010-01-25 at 16:27 +0900, KOSAKI Motohiro wrote:
>> <snip>
>>>> Futex should work both file anon anon. however I personally think
>>>> vdso is not file nor anon. it is special mappings. nobody defined
>>>> futex spec on special mappings. (yes, undefined).
>>>>
>>>> Personally, I think EINVAL or EFAULT are best result of vdso futexing, like as
>>>> futexing againt kernel address. but I guess another person have another thinking.
>>>>
>>>> I'd like to hear futex folks's opinion.
>>> Well, my opinion is we should remove the vdso, its ugly as hell :-)
>>>
>>> But I think it would make most sense to extend its definition in the
>>> direction of it being a file (for all intents and purposes its a special
>>> DSO -- which unfortunately isn't present in any filesystem).
>>>
>>> [ For all intents and purposes processes can already communicate through
>>> futexes on the libc space, so being able to do so through the vsdo
>>> really doesn't add anything ]
>>>
>>> So the problem is that the VDSO pages do not have a page->mapping
>>> because they lack the actual filesystem part of files, so even if (with
>>> the recent zero-page patch from Kosaki-san) you make private COWs of the
>>> VDSO, you'll get stuck in that loop.
>>>
>>> So the prettiest solution is to simply place the vdso in an actual
>>> filesystem and slowly migrate towards letting userspace map it as a
>>> regular DSO -- /sys/lib{32,64}/libkernel.so like.
>>>
>>> [ that has the bonus of getting rid of install_special_mapping() ]
>>>
>>> The ugly solution is special casing the vdso in get_futex_key().
>> I like the creating-a-real-file solution. However, for now (and for
>> stable), I think Kosaki's suggestion of EINVAL or EFAULT is a good
>> stop-gap. EINVAL might play the best with existing glibc implementations.
>
> May I confirm your mention?
>
> If we can accept EFAULT, we don't need any change. my previous futex patch
> already did. because 1) VDSO is alwasys read-only mapped 2) write mode
> get_user_pages_fast() against read-only pte/vma return EFAULT.
>
> Current linus and stable tree don't cause Mark's original problem. instead, just
> return EFAULT. (Well, I'm sorry. my previous mail was unclear. I wrote v2.6.31 test
> result)
>
> If you can't accept EFAULT, we need to add vdso specific logic into get_futex_key().
> Is this your intention?

That was my intention, but after looking at the glibc source, I don't
see any reason for EINVAL over EFAULT. I apparently mis-remembered
something there. EFAULT is fine.

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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