Re: [PATCH v1] mm:free unused pages in kmalloc_order

[Matthew Wilcox <willy@infradead.org>]

On Sat, Jun 27, 2020 at 04:55:07AM +0000, Long Li wrote:
> Environment using the slub allocator, 1G memory in my ARM32.
> kmalloc(1024, GFP_HIGHUSER) can allocate memory normally,
> kmalloc(64*1024, GFP_HIGHUSER) will cause a memory leak, because
> alloc_pages returns highmem physical pages, but it cannot be directly
> converted into a virtual address and return NULL, the pages has not
> been released. Usually driver developers will not use the
> GFP_HIGHUSER flag to allocate memory in kmalloc, but I think this
> memory leak is not perfect, it is best to be fixed. This is the
> first time I have posted a patch, there may be something wrong.

Slab used to disallow GFP_HIGHMEM allocations earlier than this,
so you'd never get here.  See one moron's patch here, 20 years ago ...
https://lore.kernel.org/lkml/20001228145801.C19693@parcelfarce.linux.theplanet.co.uk/

Things changed a bit since then.  SLAB_LEVEL_MASK became GFP_SLAB_BUG_MASK,
then GFP_SLAB_BUG_MASK moved to mm/internal.h.  Nowadays, GFP_SLAB_BUG_MASK
is checked only in new_slab(), and it is definitely skipped when we go through
the kmalloc_large() patch.

Could you send a replacement patch which checks GFP_SLAB_BUG_MASK before
calling alloc_pages()?

> +++ b/mm/slab_common.c
> @@ -819,8 +819,12 @@ void *kmalloc_order(size_t size, gfp_t flags, unsigned int order)
>  	page = alloc_pages(flags, order);
>  	if (likely(page)) {
>  		ret = page_address(page);
> -		mod_node_page_state(page_pgdat(page), NR_SLAB_UNRECLAIMABLE_B,
> -				    PAGE_SIZE << order);
> +		if (ret)
> +			mod_node_page_state(page_pgdat(page),
> +					NR_SLAB_UNRECLAIMABLE_B,
> +					PAGE_SIZE << order);
> +		else
> +			__free_pages(page, order);
>  	}
>  	ret = kasan_kmalloc_large(ret, size, flags);
>  	/* As ret might get tagged, call kmemleak hook after KASAN. */
> -- 
> 2.17.1
> 
>