Re: [PATCH v2 4/4] mm/vmalloc: Hugepage vmalloc mappings

Will Deacon
2020-04-15T10:49:01Z

Hi Nick,

On Mon, Apr 13, 2020 at 10:53:03PM +1000, Nicholas Piggin wrote:
> For platforms that define HAVE_ARCH_HUGE_VMAP and support PMD vmap mappings,
> have vmalloc attempt to allocate PMD-sized pages first, before falling back
> to small pages. Allocations which use something other than PAGE_KERNEL
> protections are not permitted to use huge pages yet, not all callers expect
> this (e.g., module allocations vs strict module rwx).
>
> This gives a 6x reduction in dTLB misses for a `git diff` (of linux), from
> 45600 to 6500 and a 2.2% reduction in cycles on a 2-node POWER9.

I wonder if it's worth extending vmap() to handle higher order pages in
a similar way? That might be helpful for tracing PMUs such as Arm SPE,
where the CPU streams tracing data out to a virtually addressed buffer
(see rb_alloc_aux_page()).

> This can result in more internal fragmentation and memory overhead for a
> given allocation. It can also cause greater NUMA unbalance on hashdist
> allocations.
>
> There may be other callers that expect small pages under vmalloc but use
> PAGE_KERNEL, I'm not sure if it's feasible to catch them all. An
> alternative would be a new function or flag which enables large mappings,
> and use that in callers.
>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
> include/linux/vmalloc.h | 2 +
> mm/vmalloc.c | 135 +++++++++++++++++++++++++++++-----------
> 2 files changed, 102 insertions(+), 35 deletions(-)
>
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 291313a7e663..853b82eac192 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -24,6 +24,7 @@ struct notifier_block; /* in notifier.h */
> #define VM_UNINITIALIZED 0x00000020 /* vm_struct is not fully initialized */
> #define VM_NO_GUARD 0x00000040 /* don't add guard page */
> #define VM_KASAN 0x00000080 /* has allocated kasan shadow memory */
> +#define VM_HUGE_PAGES 0x00000100 /* may use huge pages */

Please can you add a check for this in the arm64 change_memory_common()
code? Other architectures might need something similar, but we need to
forbid changing memory attributes for portions of the huge page.

In general, I'm a bit wary of software table walkers tripping over this.
For example, I don't think apply_to_existing_page_range() can handle
huge mappings at all, but the one user (KASAN) only ever uses page mappings
so it's ok there.

> @@ -2325,9 +2356,11 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
> if (unlikely(!size))
> return NULL;
>
> - if (flags & VM_IOREMAP)
> - align = 1ul << clamp_t(int, get_count_order_long(size),
> - PAGE_SHIFT, IOREMAP_MAX_ORDER);
> + if (flags & VM_IOREMAP) {
> + align = max(align,
> + 1ul << clamp_t(int, get_count_order_long(size),
> + PAGE_SHIFT, IOREMAP_MAX_ORDER));
> + }

I don't follow this part. Please could you explain why you're potentially
aligning above IOREMAP_MAX_ORDER? It doesn't seem to follow from the rest
of the patch.

Cheers,

Will