Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync_(un)mappings()

Andy Lutomirski
2020-05-10T05:06:00Z

On Sat, May 9, 2020 at 2:57 PM Joerg Roedel <joro@8bytes.org> wrote:
>
> Hi Andy,
>
> On Sat, May 09, 2020 at 12:05:29PM -0700, Andy Lutomirski wrote:

> > So, unless I'm missing something here, there is an absolute maximum of
> > 512 top-level entries that ever need to be synchronized.
>
> And here is where your assumption is wrong. On 32-bit PAE systems it is
> not the top-level entries that need to be synchronized for vmalloc, but
> the second-level entries. And dependent on the kernel configuration,
> there are (in total, not only vmalloc) 1536, 1024, or 512 of these
> second-level entries. How much of them are actually used for vmalloc
> depends on the size of the system RAM (but is at least 64), because
> the vmalloc area begins after the kernel direct-mapping (with an 8MB
> unmapped hole).

I spent some time looking at the code, and I'm guessing you're talking
about the 3-level !SHARED_KERNEL_PMD case. I can't quite figure out
what's going on.

Can you explain what is actually going on that causes different
mms/pgds to have top-level entries in the kernel range that point to
different tables? Because I'm not seeing why this makes any sense.

>
> > Now, there's an additional complication. On x86_64, we have a rule:
> > those entries that need to be synced start out null and may, during
> > the lifetime of the system, change *once*. They are never unmapped or
> > modified after being allocated. This means that those entries can
> > only ever point to a page *table* and not to a ginormous page. So,
> > even if the hardware were to support ginormous pages (which, IIRC, it
> > doesn't), we would be limited to merely immense and not ginormous
> > pages in the vmalloc range. On x86_32, I don't think we have this
> > rule right now. And this means that it's possible for one of these
> > pages to be unmapped or modified.
>
> The reason for x86-32 being different is that the address space is
> orders of magnitude smaller than on x86-64. We just have 4 top-level
> entries with PAE paging and can't afford to partition kernel-adress
> space on that level like we do on x86-64. That is the reason the address
> space is partitioned on the second (PMD) level, which is also the reason
> vmalloc synchronization needs to happen on that level. And because
> that's not enough yet, its also the page-table level to map huge-pages.

Why does it need to be partitioned at all? The only thing that comes
to mind is that the LDT range is per-mm. So I can imagine that the
PAE case with a 3G user / 1G kernel split has to have the vmalloc
range and the LDT range in the same top-level entry. Yuck.

>
> > So my suggestion is that just apply the x86_64 rule to x86_32 as well.
> > The practical effect will be that 2-level-paging systems will not be
> > able to use huge pages in the vmalloc range, since the rule will be
> > that the vmalloc-relevant entries in the top-level table must point to
> > page *tables* instead of huge pages.
>
> I could very well live with prohibiting huge-page ioremap mappings for
> x86-32. But as I wrote before, this doesn't solve the problems I am
> trying to address with this patch-set, or would only address them if
> significant amount of total system memory is used.
>
> The pre-allocation solution would work for x86-64, it would only need
> 256kb of preallocated memory for the vmalloc range to never synchronize
> or fault again. I have thought about that and did the math before
> writing this patch-set, but doing the math for 32 bit drove me away from
> it for reasons written above.
>

If it's *just* the LDT that's a problem, we could plausibly shrink the
user address range a little bit and put the LDT in the user portion.
I suppose this could end up creating its own set of problems involving
tracking which code owns which page tables.

> And since a lot of the vmalloc_sync_(un)mappings problems I debugged
> were actually related to 32-bit, I want a solution that works for 32 and
> 64-bit x86 (at least until support for x86-32 is removed). And I think
> this patch-set provides a solution that works well for both.

I'm not fundamentally objecting to your patch set, but I do want to
understand what's going on that needs this stuff.

>
>
> Joerg