Re: [PATCH v3 7/8] execmem: add support for cache of large ROX pages

[Ard Biesheuvel <ardb@kernel.org>]

Hi Mike,

On Mon, 9 Sept 2024 at 08:51, Mike Rapoport <rppt@kernel.org> wrote:
>
> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>
> 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@kernel.org>
> ---
>  include/linux/execmem.h |   2 +
>  mm/execmem.c            | 289 +++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 286 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/execmem.h b/include/linux/execmem.h
> index dfdf19f8a5e8..7436aa547818 100644
> --- a/include/linux/execmem.h
> +++ b/include/linux/execmem.h
> @@ -77,12 +77,14 @@ struct execmem_range {
>
>  /**
>   * struct execmem_info - architecture parameters for code allocations
> + * @fill_trapping_insns: set memory to contain instructions that will trap
>   * @ranges: array of parameter sets defining architecture specific
>   * parameters for executable memory allocations. The ranges that are not
>   * explicitly initialized by an architecture use parameters defined for
>   * @EXECMEM_DEFAULT.
>   */
>  struct execmem_info {
> +       void (*fill_trapping_insns)(void *ptr, size_t size, bool writable);
>         struct execmem_range    ranges[EXECMEM_TYPE_MAX];
>  };
>
> diff --git a/mm/execmem.c b/mm/execmem.c
> index 0f6691e9ffe6..f547c1f3c93d 100644
> --- a/mm/execmem.c
> +++ b/mm/execmem.c
> @@ -7,28 +7,88 @@
>   */
>
>  #include <linux/mm.h>
> +#include <linux/mutex.h>
>  #include <linux/vmalloc.h>
>  #include <linux/execmem.h>
> +#include <linux/maple_tree.h>
>  #include <linux/moduleloader.h>
>  #include <linux/text-patching.h>
>
> +#include <asm/tlbflush.h>
> +
> +#include "internal.h"
> +
>  static struct execmem_info *execmem_info __ro_after_init;
>  static struct execmem_info default_execmem_info __ro_after_init;
>
> -static void *__execmem_alloc(struct execmem_range *range, size_t size)
> +#ifdef CONFIG_MMU
> +struct execmem_cache {
> +       struct mutex mutex;
> +       struct maple_tree busy_areas;
> +       struct maple_tree free_areas;
> +};
> +
> +static struct execmem_cache execmem_cache = {
> +       .mutex = __MUTEX_INITIALIZER(execmem_cache.mutex),
> +       .busy_areas = MTREE_INIT_EXT(busy_areas, MT_FLAGS_LOCK_EXTERN,
> +                                    execmem_cache.mutex),
> +       .free_areas = MTREE_INIT_EXT(free_areas, MT_FLAGS_LOCK_EXTERN,
> +                                    execmem_cache.mutex),
> +};
> +
> +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;
> +
> +               if (!xa_is_value(area))
> +                       continue;
> +
> +               size = xa_to_value(area);
> +
> +               if (IS_ALIGNED(size, PMD_SIZE) &&
> +                   IS_ALIGNED(mas.index, PMD_SIZE)) {
> +                       void *ptr = (void *)mas.index;
> +
> +                       mas_erase(&mas);
> +                       vfree(ptr);
> +               }
> +       }
> +       mutex_unlock(mutex);
> +}
> +
> +static DECLARE_WORK(execmem_cache_clean_work, execmem_cache_clean);
> +
> +static void execmem_fill_trapping_insns(void *ptr, size_t size, bool writable)
> +{
> +       if (execmem_info->fill_trapping_insns)
> +               execmem_info->fill_trapping_insns(ptr, size, writable);
> +       else
> +               memset(ptr, 0, size);

Does this really have to be a function pointer with a runtime check?

This could just be a __weak definition, with the arch providing an
override if the memset() is not appropriate.