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

Suren Baghdasaryan
2025-02-13T22:56:13Z

On Wed, Jan 15, 2025 at 7:10 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Tue, Jan 14, 2025 at 11:58 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> >
> > On 1/15/25 04:15, Suren Baghdasaryan 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)
> > >> >+{
> > >> >+ 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 discussed proposed alternatives with Willy and he prefers the
current field-by-field copy approach. I also tried using
kmsan_check_memory() to check for uninitialized memory in the
vm_area_struct but unfortunately KMSAN stumbles on the holes in this
structure and there are 4 of them (I attached pahole output at the end
of this email). I tried unpoisoning holes but that gets very ugly very
fast. So, I posted v10
(https://lore.kernel.org/all/20250213224655.1680278-18-surenb@google.com/)
without changing this part.

struct vm_area_struct {
union {
struct {
unsigned long vm_start; /* 0 8 */
unsigned long vm_end; /* 8 8 */
}; /* 0 16 */
freeptr_t vm_freeptr; /* 0 8 */
}; /* 0 16 */
union {
struct {
unsigned long vm_start; /*
0 8 */
unsigned long vm_end; /*
8 8 */
}; /*
0 16 */
freeptr_t vm_freeptr; /*
0 8 */
};

struct mm_struct * vm_mm; /* 16 8 */
pgprot_t vm_page_prot; /* 24 8 */
union {
const vm_flags_t vm_flags; /* 32 8 */
vm_flags_t __vm_flags; /* 32 8 */
}; /* 32 8 */
union {
const vm_flags_t vm_flags; /*
0 8 */
vm_flags_t __vm_flags; /*
0 8 */
};

unsigned int vm_lock_seq; /* 40 4 */

/* XXX 4 bytes hole, try to pack */

struct list_head anon_vma_chain; /* 48 16 */
/* --- cacheline 1 boundary (64 bytes) --- */
struct anon_vma * anon_vma; /* 64 8 */
const struct vm_operations_struct * vm_ops; /* 72 8 */
unsigned long vm_pgoff; /* 80 8 */
struct file * vm_file; /* 88 8 */
void * vm_private_data; /* 96 8 */
atomic_long_t swap_readahead_info; /* 104 8 */
struct mempolicy * vm_policy; /* 112 8 */

/* XXX 8 bytes hole, try to pack */

/* --- cacheline 2 boundary (128 bytes) --- */
refcount_t vm_refcnt
__attribute__((__aligned__(64))); /* 128 4 */

/* XXX 4 bytes hole, try to pack */

struct {
struct rb_node rb __attribute__((__aligned__(8)));
/* 136 24 */
unsigned long rb_subtree_last; /* 160 8 */
} shared; /* 136 32 */
struct {
struct rb_node rb
__attribute__((__aligned__(8))); /* 0 24 */
unsigned long rb_subtree_last; /*
24 8 */

/* size: 32, cachelines: 1, members: 2 */
/* forced alignments: 1 */
/* last cacheline: 32 bytes */
};

struct vm_userfaultfd_ctx vm_userfaultfd_ctx; /* 168 0 */

/* size: 192, cachelines: 3, members: 16 */
/* sum members: 152, holes: 3, sum holes: 16 */
/* padding: 24 */
/* forced alignments: 1, forced holes: 1, sum forced holes: 8 */
};

>
> >
> > > 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
> >