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

[Andy Lutomirski]

On Wed, Apr 28, 2021 at 5:02 PM Michel Lespinasse <michel@xxxxxxxxxxxxxx> wrote:
>
> On Wed, Apr 28, 2021 at 09:11:08AM -0700, Paul E. McKenney wrote:
> > On Wed, Apr 28, 2021 at 08:13:53AM -0700, Andy Lutomirski wrote:
> > > 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!
> >
> > In modern kernels, local_irq_save() does have RCU reader semantics,
> > meaning that synchronize_rcu() will wait for pre-exiting irq-disabled
> > regions.  It will also wait for pre-existing bh-disable, preempt-disable,
> > and of course rcu_read_lock() sections of code.
>
> Thanks Paul for confirming / clarifying this. BTW, it would be good to
> add this to the rcu header files, just so people have something to
> reference to when they depend on such behavior (like fast GUP
> currently does).

Or, even better, fast GUP could add an explicit RCU read lock.

>
> Going back to my patch. I don't need to protect against THP splitting
> here, as I'm only handling the small page case. So when
> MMU_GATHER_RCU_TABLE_FREE is enabled, I *think* I could get away with
> using only an rcu read lock, instead of disabling interrupts which
> implicitly creates the rcu read lock. I'm not sure which way to go -
> fast GUP always disables interrupts regardless of the
> MMU_GATHER_RCU_TABLE_FREE setting, and I think there is a case to be
> made for following the fast GUP stes rather than trying to be smarter.

How about adding some little helpers:

lockless_page_walk_begin();

lockless_page_walk_end();

these turn into RCU read locks if MMU_GATHER_RCU_TABLE_FREE and into
irqsave otherwise.  And they're somewhat self-documenting.

--Andy