Re: [PATCH v2 2/2] x86/mm: implement free pmd/pte page interfaces

From: joro@xxxxxxxxxx
Date: Thu Apr 26 2018 - 13:23:39 EST

On Thu, Apr 26, 2018 at 04:21:19PM +0000, Kani, Toshi wrote:
> All pages under the pmd had been unmapped and then lazy TLB purged with
> INVLPG before coming to this code path. Speculation is not allowed to
> pages without mapping.

CPUs have not only TLBs, but also page-walk caches which cache
intermediary results of page-table walks and which is flushed together
with the TLB.

So the PMD entry you clear can still be in a page-walk cache and this
needs to be flushed too before you can free the PTE page. Otherwise
page-walks might still go to the page you just freed. That is especially
bad when the page is already reallocated and filled with other data.

> > Further this needs synchronization with other page-tables in the system
> > when the kernel PMDs are not shared between processes. In x86-32 with
> > PAE this causes a BUG_ON() being triggered at arch/x86/mm/fault.c:268
> > because the page-tables are not correctly synchronized.
> I think this is an issue with pmd mapping support on x86-32-PAE, not
> with this patch. I think the code needed to be updated to sync at the
> pud level.

It is an issue with this patch, because this patch is for x86 and on x86
every change to the kernel page-tables potentially needs to by
synchronized to the other page-tables. And this patch doesn't implement
it, which triggers a BUG_ON() under certain conditions.