Re: [RFC PATCH 13/37] mm: implement speculative handling in __handle_mm_fault().

[Andy Lutomirski]

On Wed, Apr 28, 2021 at 8:05 AM Michel Lespinasse <michel@xxxxxxxxxxxxxx> wrote:
>
> On Wed, Apr 07, 2021 at 08:36:01AM -0700, Andy Lutomirski wrote:
> > On 4/6/21 6:44 PM, Michel Lespinasse wrote:
> > > The page table tree is walked with local irqs disabled, which prevents
> > > page table reclamation (similarly to what fast GUP does). The logic is
> > > otherwise similar to the non-speculative path, but with additional
> > > restrictions: in the speculative path, we do not handle huge pages or
> > > wiring new pages tables.
> >
> > Not on most architectures.  Quoting the actual comment in mm/gup.c:
> >
> > >  * Before activating this code, please be aware that the following assumptions
> > >  * are currently made:
> > >  *
> > >  *  *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to
> > >  *  free pages containing page tables or TLB flushing requires IPI broadcast.
> >
> > On MMU_GATHER_RCU_TABLE_FREE architectures, you cannot make the
> > assumption that it is safe to dereference a pointer in a page table just
> > because irqs are off.  You need RCU protection, too.
> >
> > You have the same error in the cover letter.
>
> Hi Andy,
>
> Thanks for your comment. At first I thought did not matter, because we
> only enable ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT on selected
> architectures, and I thought MMU_GATHER_RCU_TABLE_FREE is not set on
> these. But I was wrong - MMU_GATHER_RCU_TABLE_FREE is enabled on X86
> with paravirt. So I took another look at fast GUP to make sure I
> actually understand it.
>
> This brings a question about lockless_pages_from_mm() - I see it
> disabling interrupts, which it explains is necessary for disabling THP
> splitting IPIs, but I do not see it taking an RCU read lock as would
> be necessary for preventing paga table freeing on
> MMU_GATHER_RCU_TABLE_FREE configs. I figure local_irq_save()
> indirectly takes an rcu read lock somehow ? I think this is something
> I should also mention in my explanation, and I have not seen a good
> description of this on the fast GUP side...

Sounds like a bug!  That being said, based on my extremely limited
understanding of how the common RCU modes work, local_irq_save()
probably implies an RCU lock in at least some cases.  Hi Paul!

--Andy