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

[Michel Lespinasse]

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...

Thanks,

--
Michel "walken" Lespinasse