Re: [PATCH] mm/arm: pgtable: remove young bit check for pte_valid_user

[Will Deacon]

On Fri, Apr 10, 2026 at 02:01:41PM +0300, Brian Ruley wrote:
> On Apr 09, Russell King (Oracle) wrote:
> > 
> > On Thu, Apr 09, 2026 at 06:17:36PM +0300, Brian Ruley wrote:
> > > However, in the case I describe, if VA_B is mapped immediately to pfn_q
> > > after it been has unmapped and freed for VA_A, then it's quite possible
> > > that the page is still indexed in the cache.
> > 
> > True.
> > 
> > > The hypothesis is that if
> > > VA_A and VA_B land in the same I-cache set and VA_A old cache entry
> > > still exists (tagged with pfn_q), then the CPU can fetch stale
> > > instructions because the tag will match. That's one reason why we need
> > > to invalidate the cache, but that will be skipped in the path:
> > >
> > >     migrate_pages
> > >      migrate_pages_batch
> > >       migrate_folio_move
> > >        remove_migration_ptes
> > >         remove_migration_pte
> > >          set_pte_at
> > >           set_ptes
> > >            __sync_icache_dcache  (skipped if !young)
> > >             set_pte_ext
> > 
> > In this case, if the old PTE was marked !young, then the new PTE will
> > have:
> >         pte = pte_mkold(pte);
> > 
> > on it, which marks it !young. As you say, __sync_icache_dcache() will
> > be skipped. While a PTE entry will be set for the kernel, the code in
> > set_pte_ext() will *not* establish a hardware PTE entry. For the
> > 2-level pte code:
> > 
> >         tst     r1, #L_PTE_YOUNG        @ <- results in Z being set
> >         tstne   r1, #L_PTE_VALID        @ <- not executed
> >         eorne   r1, r1, #L_PTE_NONE     @ <- not executed
> >         tstne   r1, #L_PTE_NONE         @ <- not executed
> >         moveq   r3, #0                  @ <- hardware PTE value
> >  ARM(   str     r3, [r0, #2048]! )      @ <- writes hardware PTE
> > 
> > So, for a !young PTE, the hardware PTE entry is written as zero,
> > which means accesses should fault, which will then cause the PTE to
> > be marked young.
> > 
> > For the 3-level case, the L_PTE_YOUNG bit corresponds with the AF bit
> > in the PTE, and there aren't split Linux / hardware PTE entries. AF
> > being clear should result in a page fault being generated for the
> > kernel to handle making the PTE young.
> > 
> > In both of these cases, set_ptes() will need to be called with the
> > updated PTE which will now be marked young, and that will result in
> > the I-cache being flushed.
> 
> Hi Russell,
> 
> Thank you for the clarification, this is very educational for me.
> I understand your scepticism, and I can't explain what's going on based
> on what you replied. However, I do honestly believe there is a problem
> here. I'll share the exact testing details and the instrumentation
> we added that convinced us to reach out at the end. One idea we also
> had was that could cache aliasing be happening here.

I thought a bit more about this over the weekend and started to wonder
if there's a potential race where multiple CPUs try to write the same
PTE and don't synchronise properly on the cache-maintenance.

In particular, PG_dcache_clean is manipulated with a test_and_set_bit()
operation _before_ the cache maintenance is performed, so there's a
small window where the flag is set but the page is _not_ clean. I don't
think that matters with regards to invalid migration entries, but
perhaps the migration just means that we end up putting down a bunch of
'old' entries and are then more likely to see concurrent faults trying
to make the thing young again, potentially hitting this race.

Looking at arm64 this morning, I noticed that we split the flag
manipulation so that it's set with a set_bit() after the maintenance has
been performed. Git then points to 588a513d3425 ("arm64: Fix race
condition on PG_dcache_clean in __sync_icache_dcache()") which seems to
talk about the same race. In fact, the mailing list posting:

  https://lore.kernel.org/all/20210514095001.13236-1-catalin.marinas@xxxxxxx/

points out that arch/arm/ is also affected but we forgot to CC Russell
because I think this all came out of the MTE-enablement work [1] and it
sounds like Catalin was trying to fix it in the core mprotect() code.

Brian, can you try something like 588a513d3425?

Will

[1] https://lore.kernel.org/all/YJGHApOCXl811VK3@xxxxxxx/