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SubjectRe: frequent lockups in 3.18rc4
On Fri, Nov 21, 2014 at 10:22 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Nov 21, 2014 at 9:22 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> Both mystify me. Why does the 32-bit version walk down the hierarchy
>> at all instead of just touching the top level?
>
> Quite frankly, I think it's just due to historical reasons, and should
> be removed.
>
> But the historical reasons are that with the aliasing of the PUD and
> PMD entries in the PGD, it's all fairly confusing. So I think we only
> used to do the top level, but then when we expanded from two levels to
> three, that "top level" became the pmd, and then when we expanded from
> three to four, the pmd was actually two levels down. So it's all
> basically mindless work.
>
> So I do think we could simplify and unify things.
>
> In 32-bit mode, we actually have two different cases:
>
> - in PAE, there's the magic top-level 4-entry PGD that always *has*
> to be present (the P bit isn't actually checked by hardware)
>
> As a result, in PAE mode, the top PGD entries always exist, and
> are always prepopulated, and for the kernel area (including obviously
> the vmalloc space) always points to the init_pgd[] entry.
>
> Ergo, in PAE mode, I don't think we should ever hit this case in
> the first place.
>
> - in non-PAE mode, we should just copy the top-level entry, and return.
>
> And in 64-bit more, we only have the "copy the top-level entry" case.
>
> So I think we should
>
> (a) remove the 32-bit vs 64-bit difference, because that's not actually valid
>
> (b) make it a PAE vs non-PAE difference
>
> (c) the PAE case is a no-op
>
> (d) the non-PAE case would look something like this:
>
> static noinline int vmalloc_fault(unsigned long address)
> {
> unsigned index;
> pgd_t *pgd_dst, pgd_entry;
>
> /* Make sure we are in vmalloc area: */
> if (!(address >= VMALLOC_START && address < VMALLOC_END))
> return -1;
>
> index = pgd_index(address);
> pgd_entry = init_mm.pgd[index];
> if (!pgd_present(pgd_entry))
> return -1;
>
> pgd_dst = __va(PAGE_MASK & read_cr3());
> if (pgd_present(pgd_dst[index]))
> return -1;
>
> ACCESS_ONCE(pgd_dst[index]) = pgd_entry;
> return 0;
> }
> NOKPROBE_SYMBOL(vmalloc_fault);
>
> and it's done.
>
> Would anybody be willing to actually *test* something like the above?
> The above may compile, but that's all the "testing" it got.
>

I'd be happy to test it (i.e. boot it and try to use my computer), but
I have nowhere near enough RAM to do it right.

Is there any easy way to get the vmalloc code to randomize enough bits
to exercise this?

--Andy


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