Re: [RFC PATCH 3/4] mm/page_alloc: introduce __GFP_PTE_MAPPED flag to allocate pte-mapped pages

[David Hildenbrand]

On 23.08.21 15:25, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
>
> When __GFP_PTE_MAPPED flag is passed to an allocation request of order 0,
> the allocated page will be mapped at PTE level in the direct map.
>
> To reduce the direct map fragmentation, maintain a cache of 4K pages that
> are already mapped at PTE level in the direct map. Whenever the cache
> should be replenished, try to allocate 2M page and split it to 4K pages
> to localize shutter of the direct map. If the allocation of 2M page fails,
> fallback to a single page allocation at expense of the direct map
> fragmentation.
>
> The cache registers a shrinker that releases free pages from the cache to
> the page allocator.
>
> The __GFP_PTE_MAPPED and caching of 4K pages are enabled only if an
> architecture selects ARCH_WANTS_PTE_MAPPED_CACHE in its Kconfig.
>
> [
> cache management are mostly copied from
> https://lore.kernel.org/lkml/20210505003032.489164-4-rick.p.edgecombe@xxxxxxxxx/
> ]
>
> Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> ---
>   arch/Kconfig                    |   8 +
>   arch/x86/Kconfig                |   1 +
>   include/linux/gfp.h             |  11 +-
>   include/linux/mm.h              |   2 +
>   include/linux/pageblock-flags.h |  26 ++++
>   init/main.c                     |   1 +
>   mm/internal.h                   |   3 +-
>   mm/page_alloc.c                 | 261 +++++++++++++++++++++++++++++++-
>   8 files changed, 309 insertions(+), 4 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 129df498a8e1..2db95331201b 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -243,6 +243,14 @@ config ARCH_HAS_SET_MEMORY
>   config ARCH_HAS_SET_DIRECT_MAP
>   	bool

[...]

> +static int __pte_mapped_cache_init(struct pte_mapped_cache *cache)
> +{
> +	int err;
> +
> +	err = list_lru_init(&cache->lru);
> +	if (err)
> +		return err;
> +
> +	cache->shrinker.count_objects = pte_mapped_cache_shrink_count;
> +	cache->shrinker.scan_objects = pte_mapped_cache_shrink_scan;
> +	cache->shrinker.seeks = DEFAULT_SEEKS;
> +	cache->shrinker.flags = SHRINKER_NUMA_AWARE;
> +
> +	err = register_shrinker(&cache->shrinker);
> +	if (err)
> +		goto err_list_lru_destroy;

With a shrinker in place, it really does somewhat feel like this should 
be a cache outside of the buddy. Or at least moved outside of 
page_alloc.c with a clean interface to work with the buddy.

But I only had a quick glimpse over this patch.

--
Thanks,

David / dhildenb