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

[Suren Baghdasaryan]

On Tue, Jan 14, 2025 at 11:58 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 1/15/25 04:15, Suren Baghdasaryan wrote:
> > On Tue, Jan 14, 2025 at 6:27 PM Wei Yang <richard.weiyang@xxxxxxxxx> 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)
> >> >+{
> >> >+      dest->vm_mm = src->vm_mm;
> >> >+      dest->vm_ops = src->vm_ops;
> >> >+      dest->vm_start = src->vm_start;
> >> >+      dest->vm_end = src->vm_end;
> >> >+      dest->anon_vma = src->anon_vma;
> >> >+      dest->vm_pgoff = src->vm_pgoff;
> >> >+      dest->vm_file = src->vm_file;
> >> >+      dest->vm_private_data = src->vm_private_data;
> >> >+      vm_flags_init(dest, src->vm_flags);
> >> >+      memcpy(&dest->vm_page_prot, &src->vm_page_prot,
> >> >+             sizeof(dest->vm_page_prot));
> >> >+      /*
> >> >+       * src->shared.rb may be modified concurrently when called from
> >> >+       * dup_mmap(), but the clone will reinitialize it.
> >> >+       */
> >> >+      data_race(memcpy(&dest->shared, &src->shared, sizeof(dest->shared)));
> >> >+      memcpy(&dest->vm_userfaultfd_ctx, &src->vm_userfaultfd_ctx,
> >> >+             sizeof(dest->vm_userfaultfd_ctx));
> >> >+#ifdef CONFIG_ANON_VMA_NAME
> >> >+      dest->anon_name = src->anon_name;
> >> >+#endif
> >> >+#ifdef CONFIG_SWAP
> >> >+      memcpy(&dest->swap_readahead_info, &src->swap_readahead_info,
> >> >+             sizeof(dest->swap_readahead_info));
> >> >+#endif
> >> >+#ifndef CONFIG_MMU
> >> >+      dest->vm_region = src->vm_region;
> >> >+#endif
> >> >+#ifdef CONFIG_NUMA
> >> >+      dest->vm_policy = src->vm_policy;
> >> >+#endif
> >> >+}
> >>
> >> 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.
>
> Willy knows one but refuses to share it :(

Ah, that reminds me why I dropped this approach :) But to be honest,
back then we also had vma_clear() and that added to the ugliness. Now
I could simply to this without all those macros:

static inline void vma_copy(struct vm_area_struct *new,
                                            struct vm_area_struct *orig)
{
        /* Copy the vma while preserving vma->vm_lock */
        data_race(memcpy(new, orig, offsetof(struct vm_area_struct, vm_lock)));
        data_race(memcpy(new + offsetofend(struct vm_area_struct, vm_lock),
                orig + offsetofend(struct vm_area_struct, vm_lock),
                sizeof(struct vm_area_struct) -
                offsetofend(struct vm_area_struct, vm_lock));
}

Would that be better than the current approach?

>
> > 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.
> >
> >>
> >> --
> >> Wei Yang
> >> Help you, Help me
>