Re: [PATCH v5 7/8] execmem: add support for cache of large ROX pages

[Kees Bakker]

Op 09-10-2024 om 20:08 schreef Mike Rapoport:
> From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>
>
> Using large pages to map text areas reduces iTLB pressure and improves
> performance.
>
> Extend execmem_alloc() with an ability to use huge pages with ROX
> permissions as a cache for smaller allocations.
>
> To populate the cache, a writable large page is allocated from vmalloc with
> VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
> ROX.
>
> Portions of that large page are handed out to execmem_alloc() callers
> without any changes to the permissions.
>
> When the memory is freed with execmem_free() it is invalidated again so
> that it won't contain stale instructions.
>
> The cache is enabled when an architecture sets EXECMEM_ROX_CACHE flag in
> definition of an execmem_range.
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> ---
>   include/linux/execmem.h |   2 +
>   mm/execmem.c            | 317 +++++++++++++++++++++++++++++++++++++++-
>   mm/internal.h           |   1 +
>   mm/vmalloc.c            |   5 +
>   4 files changed, 320 insertions(+), 5 deletions(-)
> [...]
> +static void execmem_cache_clean(struct work_struct *work)
> +{
> +	struct maple_tree *free_areas = &execmem_cache.free_areas;
> +	struct mutex *mutex = &execmem_cache.mutex;
> +	MA_STATE(mas, free_areas, 0, ULONG_MAX);
> +	void *area;
> +
> +	mutex_lock(mutex);
> +	mas_for_each(&mas, area, ULONG_MAX) {
> +		size_t size;
> +
No need to check for !area, because it is already guaranteed by the 
while loop condition (mas_for_each)
> +		if (!area)
> +			continue;
> +
> +		size = mas_range_len(&mas);
> +
> +		if (IS_ALIGNED(size, PMD_SIZE) &&
> +		    IS_ALIGNED(mas.index, PMD_SIZE)) {
> +			struct vm_struct *vm = find_vm_area(area);
> +
> +			execmem_set_direct_map_valid(vm, true);
> +			mas_store_gfp(&mas, NULL, GFP_KERNEL);
> +			vfree(area);
> +		}
> +	}
> +	mutex_unlock(mutex);
> +}