Re: [PATCH 0/5] VA allocator fixes

[Nicholas Piggin <npiggin@gmail.com>]

On Mon, 6 Nov 2017 16:16:07 +0100
Florian Weimer <fweimer@redhat.com> wrote:

> On 11/06/2017 11:03 AM, Nicholas Piggin wrote:
> > Florian found a nasty corner case with the VA allocation logic
> > for crossing from 128TB to 512TB limit on hash, and made a
> > really superb report of the problem -- traces, reproducer recipes,
> > analysis, etc. which already mostly solved it.
> > 
> > The first patch in the series should solve Florian's particular
> > case, the next 3 are other issues with addr_limit. The last
> > patch is technically a cleanup but I think it's fairly important
> > in terms of understanding the code and also enabling some BUG
> > checks (when addr_limit == 0).
> > 
> > I have not tested these exactly on Florian's test case, but
> > some tests of my own behave better afterwards. Hopefully he has
> > time to re-test. Some careful review would be welcome too.  
> 
> I think I have applied the five patches you posted, but I still get a 
> brk value above 128 TiB:
> 
> # /lib64/ld64.so.1 ./a.out
> initial brk value: 0x7fffde960000
> probing at 0x80000001fffc
> 
> I assumed you wanted to reject those?

It was difficult to understand what the intended semantics are, but I
think brk should succeed (it is implemented with MAP_FIXED). Of course
it should not succeed then segfault when you try to access it.

> 
> In either case, I recommend to tweak the VM layout, so that ld.so does 
> not land closely to to the 128 TiB limit, so that the brk failure or 
> returning of 48-bit addresses is avoided.

Yeah well that's yet another issue. I was not really involved with the
address space extension work. Anees, Kirill, was the intention for the
128T->512T extension logic to be a no-op for all address space allocaiton
except those with explicit addresses?

Thanks,
Nick