Re: [PATCH 4/4] mm: move lesser used vma_area_struct members into the last cacheline

Suren Baghdasaryan
2024-11-12T15:45:29Z

On Tue, Nov 12, 2024 at 2:07 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 11/11/24 21:55, Suren Baghdasaryan wrote:
> > Move several vma_area_struct members which are rarely or never used
> > during page fault handling into the last cacheline to better pack
> > vm_area_struct. As a result vm_area_struct will fit into 3 cachelines
> > as opposed to 4 cachelines before this change. New vm_area_struct layout:
> >
> > struct vm_area_struct {
> > union {
> > struct {
> > long unsigned int vm_start; /* 0 8 */
> > long unsigned int vm_end; /* 8 8 */
> > }; /* 0 16 */
> > struct callback_head vm_rcu ; /* 0 16 */
> > } __attribute__((__aligned__(8))); /* 0 16 */
> > 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 */
> > bool detached; /* 40 1 */
> >
> > /* XXX 3 bytes hole, try to pack */
> >
> > unsigned int vm_lock_seq; /* 44 4 */
> > 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 */
> > long unsigned int 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) --- */
> > struct vma_lock vm_lock (__aligned__(64)); /* 128 4 */
> >
> > /* XXX 4 bytes hole, try to pack */
> >
> > struct {
> > struct rb_node rb (__aligned__(8)); /* 136 24 */
> > long unsigned int rb_subtree_last; /* 160 8 */
> > } __attribute__((__aligned__(8))) shared; /* 136 32 */
> > struct vm_userfaultfd_ctx vm_userfaultfd_ctx; /* 168 0 */
>
> I don't see anon_name in the output, I thought it was added for Android? :)

Yes, this output is generated with defconfig. That's why you see some
holes in this structure. On my x86 machine I have non-zero
vm_userfaultfd_ctx and numab_state, on Android I have
vm_userfaultfd_ctx and anon_name.

>
> >
> > /* size: 192, cachelines: 3, members: 17 */
> > /* sum members: 153, holes: 3, sum holes: 15 */
> > /* padding: 24 */
>
> Instead you seem to have padding so an attempt to use SLAB_TYPESAFE_BY_RCU
> should use that and not add more up to 256 pages.

Yes, thanks for the tip about SLAB_TYPESAFE_BY_RCU freelist. In actual
configurations where I saw SLAB_TYPESAFE_BY_RCU causing this structure
to grow I had less padding at the end.

> Perhaps this pahole output wasn't generated with a fully representative config?

You are right. I'll replace it with the actual output from my x86
setup (Android probably has a smaller interested audience).

>
> > /* forced alignments: 3, forced holes: 2, sum forced holes: 12 */
> > } __attribute__((__aligned__(64)));
> >
> >
> > Memory consumption per 1000 VMAs becomes 48 pages:
> >
> > slabinfo after vm_area_struct changes:
> > <name> ... <objsize> <objperslab> <pagesperslab> : ...
> > vm_area_struct ... 192 42 2 : ...
> >
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> > include/linux/mm_types.h | 37 ++++++++++++++++++-------------------
> > 1 file changed, 18 insertions(+), 19 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 789bccc05520..c3755b680911 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -733,16 +733,6 @@ struct vm_area_struct {
> > unsigned int vm_lock_seq;
> > #endif
> >
> > - /*
> > - * For areas with an address space and backing store,
> > - * linkage into the address_space->i_mmap interval tree.
> > - *
> > - */
> > - struct {
> > - struct rb_node rb;
> > - unsigned long rb_subtree_last;
> > - } shared;
> > -
> > /*
> > * A file's MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
> > * list, after a COW of one of the file pages. A MAP_SHARED vma
> > @@ -762,14 +752,6 @@ struct vm_area_struct {
> > struct file * vm_file; /* File we map to (can be NULL). */
> > void * vm_private_data; /* was vm_pte (shared mem) */
> >
> > -#ifdef CONFIG_ANON_VMA_NAME
> > - /*
> > - * For private and shared anonymous mappings, a pointer to a null
> > - * terminated string containing the name given to the vma, or NULL if
> > - * unnamed. Serialized by mmap_lock. Use anon_vma_name to access.
> > - */
> > - struct anon_vma_name *anon_name;
> > -#endif
> > #ifdef CONFIG_SWAP
> > atomic_long_t swap_readahead_info;
> > #endif
> > @@ -782,11 +764,28 @@ struct vm_area_struct {
> > #ifdef CONFIG_NUMA_BALANCING
> > struct vma_numab_state *numab_state; /* NUMA Balancing state */
> > #endif
> > - struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> > #ifdef CONFIG_PER_VMA_LOCK
> > /* Unstable RCU readers are allowed to read this. */
> > struct vma_lock vm_lock ____cacheline_aligned_in_smp;
> > #endif
> > + /*
> > + * For areas with an address space and backing store,
> > + * linkage into the address_space->i_mmap interval tree.
> > + *
> > + */
> > + struct {
> > + struct rb_node rb;
> > + unsigned long rb_subtree_last;
> > + } shared;
> > +#ifdef CONFIG_ANON_VMA_NAME
> > + /*
> > + * For private and shared anonymous mappings, a pointer to a null
> > + * terminated string containing the name given to the vma, or NULL if
> > + * unnamed. Serialized by mmap_lock. Use anon_vma_name to access.
> > + */
> > + struct anon_vma_name *anon_name;
> > +#endif
> > + struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> > } __randomize_layout;
> >
> > #ifdef CONFIG_NUMA
>