Re: [PATCH v9 16/17] mm: make vma cache SLAB_TYPESAFE_BY_RCU

[Mateusz Guzik <mjguzik@gmail.com>]

On Wed, Jan 15, 2025 at 6:47 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Tue, Jan 14, 2025 at 8:00 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
> >
> > On Wed, Jan 15, 2025 at 4:15 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > On Tue, Jan 14, 2025 at 6:27 PM Wei Yang <richard.weiyang@gmail.com> wrote:
> > > >
> > > > On Fri, Jan 10, 2025 at 08:26:03PM -0800, Suren Baghdasaryan wrote:
> > > >
> > > > >diff --git a/kernel/fork.c b/kernel/fork.c
> > > > >index 9d9275783cf8..151b40627c14 100644
> > > > >--- a/kernel/fork.c
> > > > >+++ b/kernel/fork.c
> > > > >@@ -449,6 +449,42 @@ struct vm_area_struct *vm_area_alloc(struct mm_struct *mm)
> > > > >       return vma;
> > > > > }
> > > > >
> > > > >+static void vm_area_init_from(const struct vm_area_struct *src,
> > > > >+                            struct vm_area_struct *dest)
> > > > >+{
> > [snip]
> > > > Would this be difficult to maintain? We should make sure not miss or overwrite
> > > > anything.
> > >
> > > Yeah, it is less maintainable than a simple memcpy() but I did not
> > > find a better alternative. I added a warning above the struct
> > > vm_area_struct definition to update this function every time we change
> > > that structure. Not sure if there is anything else I can do to help
> > > with this.
> > >
> >
> > Bare minimum this could have a BUILD_BUG_ON in below the func for the
> > known-covered size. But it would have to be conditional on arch and
> > some macros, somewhat nasty.
> >
> > KASAN or KMSAN (I don't remember which) can be used to find missing
> > copies. To that end the target struct could be marked as fully
> > uninitialized before copy and have a full read performed from it
> > afterwards -- guaranteed to trip over any field which any field not
> > explicitly covered (including padding though). I don't know what magic
> > macros can be used to do in Linux, I am saying the support to get this
> > result is there. I understand most people don't use this, but this
> > still should be enough to trip over buggy patches in -next.
>
> If my previous suggestion does not fly I'll start digging into KASAN
> to see how we can use it. Thanks for the tip.
>
> >
> > Finally, the struct could have macros delimiting copy/non-copy
> > sections (with macros expanding to field names), for illustrative
> > purposes:
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 332cee285662..25063a3970c8 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -677,6 +677,7 @@ struct vma_numab_state {
> >   * getting a stable reference.
> >   */
> >  struct vm_area_struct {
> > +#define vma_start_copy0 vm_rcu
> >         /* The first cache line has the info for VMA tree walking. */
> >
> >         union {
> > @@ -731,6 +732,7 @@ struct vm_area_struct {
> >         /* Unstable RCU readers are allowed to read this. */
> >         struct vma_lock *vm_lock;
> >  #endif
> > +#define vma_end_copy1 vm_lock
> >
> >         /*
> >          * For areas with an address space and backing store,
> >
> > then you would do everything with a series of calls
>
> I'm not sure... My proposed approach with offsetof() I think is a bit
> cleaner than adding macros to denote copy sections. WDYT?
>

another non-copy field may show up down the road and then the person
adding it is going to be a sad panda. wont happen if the "infra" is
there.

but I concede this is not a big deal and i'm not going to bikeshed about it.

-- 
Mateusz Guzik <mjguzik gmail.com>