Re: [PATCH -tip, v2] x86/mm/32: Sync only to VMALLOC_END in vmalloc_sync_all()

Ingo Molnar
2019-11-26T17:14:27Z

* Joerg Roedel <jroedel@suse.de> wrote:

> Hi Ingo,
>
> On Tue, Nov 26, 2019 at 12:11:19PM +0100, Ingo Molnar wrote:
> > The vmalloc_sync_all() also iterating over the LDT range is buggy,
> > because for the LDT the mappings are *intentionally* and fundamentally
> > different between processes, i.e. not synchronized.
>
> Yes, you are right, your patch description is much better, thanks for
> making it more clear and correct.
>
> > Furthermore I'm not sure we need to iterate over the PKMAP range either:
> > those are effectively permanent PMDs as well, and they are not part of
> > the vmalloc.c lazy deallocation scheme in any case - they are handled
> > entirely separately in mm/highmem.c et al.
>
> I looked a bit at that, and I didn't find an explict place where the
> PKMAP PMD gets established. It probably happens implicitly on the first
> kmap() call, so we are safe as long as the first call to kmap happens
> before the kernel starts the first userspace process.

No, it happens during early boot, in permanent_kmaps_init():

vaddr = PKMAP_BASE;
page_table_range_init(vaddr, vaddr + PAGE_SIZE*LAST_PKMAP, pgd_base);

That page_table_range_init() will go from PKMAP_BASE to the last PKMAP,
which on PAE kernels is typically 0xff600000...0xff800000, 2MB in size,
taking up exactly one PMD entry.

This single pagetable page, covering 2MB of virtual memory via 4K
entries, gets passed on to the mm/highmem.c code via:

pkmap_page_table = pte;

The pkmap_page_table is mapped early on into init_mm, every task started
after that with a new pgd inherits it, and the pmd entry never changes,
so there's nothing to synchronize.

The pte entries within this single pagetable page do change frequently
according to the kmap() code, but since the pagetable page is shared
between all tasks and the TLB flushes are SMP safe, it's all synchronized
by only modifying pkmap_page_table, as it should.

> But that is not an issue that should be handled by vmalloc_sync_all(),
> as the name already implies that it only cares about the vmalloc range.

Well, hypothetically it could *accidentally* have some essentially effect
on bootstrapping the PKMAP pagetables - I don't think that's so, based on
the reading of the code, but only testing will tell for sure.

> So your change to only iterate to VMALLOC_END makes sense and we should
> establish the PKMAP PMD at a defined place to make sure it exists when
> we start the first process.

I believe that's done in permanent_kmaps_init().

> > Note that this is *completely* untested - I might have wrecked PKMAP in
> > my ignorance. Mind giving it a careful review and a test?
>
> My testing environment for 32 bit is quite limited these days, but I
> tested it in my PTI-x32 environment and the patch below works perfectly
> fine there and still fixes the ldt_gdt selftest.

Cool, thanks! I'll apply it with your Tested-by.

Ingo