Re: [RFC PATCH] mm: Enable CONFIG_PT_RECLAIM on all architectures

[Dev Jain]

On 03/11/25 12:33 pm, Qi Zheng wrote:
> Hi Dev,
>
> On 11/3/25 2:37 PM, Dev Jain wrote:
> > The implementation of CONFIG_PT_RECLAIM is completely contained in 
> > generic
> > mm code. It depends on the RCU callback which will reclaim the 
> > pagetables -
> > there is nothing arch-specific about that. So, enable this config for
> > all architectures.
>
> Thanks for doing this!
>
> But unfortunately, not all architectures call tlb_remove_ptdesc() in
> __pte_free_tlb(). Some architectures directly call pte_free() to
> free PTE pages (without RCU).

Thanks! This was not obvious to figure out.

Is there an arch bottleneck because of which they do this? I mean to say,

is something stopping us from simply redirecting __pte_free_tlb to 
tlb_remove_ptdesc

or pte_free_defer?


I am looking to enable this config at least on arm64 by default, I 
believe it will be legal

to do this at least here.


> We need to modify these architectures first, otherwise it will
> lead to UAF. This approach is feasible because Hugh provides similar
> support in pte_free_defer().
>
> Enabling PT_RECLAIM on all architecture has always been on my
> TODO list, but it's been blocked by other things. :(
>
> Thanks,
> Qi
>
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > ---
> >   arch/x86/Kconfig | 1 -
> >   mm/Kconfig       | 5 +----
> >   mm/pt_reclaim.c  | 2 +-
> >   3 files changed, 2 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index fa3b616af03a..5681308a5650 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -327,7 +327,6 @@ config X86
> >       select FUNCTION_ALIGNMENT_4B
> >       imply IMA_SECURE_AND_OR_TRUSTED_BOOT    if EFI
> >       select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
> > -    select ARCH_SUPPORTS_PT_RECLAIM        if X86_64
> >       select ARCH_SUPPORTS_SCHED_SMT        if SMP
> >       select SCHED_SMT            if SMP
> >       select ARCH_SUPPORTS_SCHED_CLUSTER    if SMP
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 0e26f4fc8717..903c37d02555 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -1355,13 +1355,10 @@ config ARCH_HAS_USER_SHADOW_STACK
> >         The architecture has hardware support for userspace shadow call
> >             stacks (eg, x86 CET, arm64 GCS or RISC-V Zicfiss).
> >   -config ARCH_SUPPORTS_PT_RECLAIM
> > -    def_bool n
> > -
> >   config PT_RECLAIM
> >       bool "reclaim empty user page table pages"
> >       default y
> > -    depends on ARCH_SUPPORTS_PT_RECLAIM && MMU && SMP
> > +    depends on MMU && SMP
> >       select MMU_GATHER_RCU_TABLE_FREE
> >       help
> >         Try to reclaim empty user page table pages in paths other 
> > than munmap
> > diff --git a/mm/pt_reclaim.c b/mm/pt_reclaim.c
> > index 7e9455a18aae..049e17f08c6a 100644
> > --- a/mm/pt_reclaim.c
> > +++ b/mm/pt_reclaim.c
> > @@ -1,6 +1,6 @@
> >   // SPDX-License-Identifier: GPL-2.0
> >   #include <linux/hugetlb.h>
> > -#include <asm-generic/tlb.h>
> > +#include <asm/tlb.h>
> >   #include <asm/pgalloc.h>
> >     #include "internal.h"