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

From: Michel Lespinasse <michel@lespinasse.org>
To: Andy Lutomirski <luto@kernel.org>, Paul McKenney <paulmck@kernel.org>
Cc: Michel Lespinasse <michel@lespinasse.org>,
	Linux-MM <linux-mm@kvack.org>,
	Laurent Dufour <ldufour@linux.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Michal Hocko <mhocko@suse.com>,
	Matthew Wilcox <willy@infradead.org>,
	Rik van Riel <riel@surriel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Joel Fernandes <joelaf@google.com>,
	Rom Lemarchand <romlem@google.com>,
	Linux-Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 13/37] mm: implement speculative handling in __handle_mm_fault().
Date: Wed, 28 Apr 2021 07:58:23 -0700	[thread overview]
Message-ID: <20210428145823.GA856@lespinasse.org> (raw)
In-Reply-To: <eee7431c-3dc8-ca3c-02fb-9e059d30e951@kernel.org>

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