Re: [PATCH v7 7/8] execmem: add support for cache of large ROX pages

[Ryan Roberts <ryan.roberts@arm.com>]

Hi Mike,

Drive by review comments below...


On 23/10/2024 17:27, Mike Rapoport 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.
> 
> The direct map alias of that large page is exculded from the direct map.
> 
> 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.
> 
> An architecture has to implement execmem_fill_trapping_insns() callback
> and select ARCH_HAS_EXECMEM_ROX configuration option to be able to use
> the ROX cache.
> 
> The cache is enabled on per-range basis when an architecture sets
> EXECMEM_ROX_CACHE flag in definition of an execmem_range.
> 
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
> Tested-by: kdevops <kdevops@lists.linux.dev>
> ---

[...]

> +
> +static int execmem_cache_populate(struct execmem_range *range, size_t size)
> +{
> +	unsigned long vm_flags = VM_ALLOW_HUGE_VMAP;
> +	unsigned long start, end;
> +	struct vm_struct *vm;
> +	size_t alloc_size;
> +	int err = -ENOMEM;
> +	void *p;
> +
> +	alloc_size = round_up(size, PMD_SIZE);
> +	p = execmem_vmalloc(range, alloc_size, PAGE_KERNEL, vm_flags);

Shouldn't this be passing PAGE_KERNEL_ROX? Otherwise I don't see how the
allocated memory is ROX? I don't see any call below where you change the permission.

Given the range has the pgprot in it, you could just drop passing the pgprot
explicitly here and have execmem_vmalloc() use range->pgprot directly?

Thanks,
Ryan

> +	if (!p)
> +		return err;
> +
> +	vm = find_vm_area(p);
> +	if (!vm)
> +		goto err_free_mem;
> +
> +	/* fill memory with instructions that will trap */
> +	execmem_fill_trapping_insns(p, alloc_size, /* writable = */ true);
> +
> +	start = (unsigned long)p;
> +	end = start + alloc_size;
> +
> +	vunmap_range(start, end);
> +
> +	err = execmem_set_direct_map_valid(vm, false);
> +	if (err)
> +		goto err_free_mem;
> +
> +	err = vmap_pages_range_noflush(start, end, range->pgprot, vm->pages,
> +				       PMD_SHIFT);
> +	if (err)
> +		goto err_free_mem;
> +
> +	err = execmem_cache_add(p, alloc_size);
> +	if (err)
> +		goto err_free_mem;
> +
> +	return 0;
> +
> +err_free_mem:
> +	vfree(p);
> +	return err;
> +}

[...]