Re: [PATCH v2] mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()

[Michal Hocko <mhocko@suse.com>]

On Thu 14-03-19 10:42:49, Vlastimil Babka wrote:
> alloc_pages_exact*() allocates a page of sufficient order and then splits it
> to return only the number of pages requested. That makes it incompatible with
> __GFP_COMP, because compound pages cannot be split.
> 
> As shown by [1] things may silently work until the requested size (possibly
> depending on user) stops being power of two. Then for CONFIG_DEBUG_VM, BUG_ON()
> triggers in split_page(). Without CONFIG_DEBUG_VM, consequences are unclear.
> 
> There are several options here, none of them great:
> 
> 1) Don't do the spliting when __GFP_COMP is passed, and return the whole
> compound page. However if caller then returns it via free_pages_exact(),
> that will be unexpected and the freeing actions there will be wrong.
> 
> 2) Warn and remove __GFP_COMP from the flags. But the caller wanted it, so
> things may break later somewhere.
> 
> 3) Warn and return NULL. However NULL may be unexpected, especially for
> small sizes.
> 
> This patch picks option 3, as it's best defined.

The question is whether callers of alloc_pages_exact do have any
fallback because if they don't then this is forcing an always fail path
and I strongly suspect this is not really what users want. I would
rather go with 2) because "callers wanted it" is much less probable than
"caller is simply confused and more gfp flags is surely better than
fewer".

> [1] https://lore.kernel.org/lkml/20181126002805.GI18977@shao2-debian/T/#u
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> Sent v1 before amending commit, sorry.
> 
>  mm/page_alloc.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0b9f577b1a2a..dd3f89e8f88d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4752,7 +4752,7 @@ static void *make_alloc_exact(unsigned long addr, unsigned int order,
>  /**
>   * alloc_pages_exact - allocate an exact number physically-contiguous pages.
>   * @size: the number of bytes to allocate
> - * @gfp_mask: GFP flags for the allocation
> + * @gfp_mask: GFP flags for the allocation, must not contain __GFP_COMP
>   *
>   * This function is similar to alloc_pages(), except that it allocates the
>   * minimum number of pages to satisfy the request.  alloc_pages() can only
> @@ -4768,6 +4768,10 @@ void *alloc_pages_exact(size_t size, gfp_t gfp_mask)
>  	unsigned long addr;
>  
>  	addr = __get_free_pages(gfp_mask, order);
> +
> +	if (WARN_ON_ONCE(gfp_mask & __GFP_COMP))
> +		return NULL;
> +
>  	return make_alloc_exact(addr, order, size);
>  }
>  EXPORT_SYMBOL(alloc_pages_exact);
> @@ -4777,7 +4781,7 @@ EXPORT_SYMBOL(alloc_pages_exact);
>   *			   pages on a node.
>   * @nid: the preferred node ID where memory should be allocated
>   * @size: the number of bytes to allocate
> - * @gfp_mask: GFP flags for the allocation
> + * @gfp_mask: GFP flags for the allocation, must not contain __GFP_COMP
>   *
>   * Like alloc_pages_exact(), but try to allocate on node nid first before falling
>   * back.
> @@ -4785,7 +4789,12 @@ EXPORT_SYMBOL(alloc_pages_exact);
>  void * __meminit alloc_pages_exact_nid(int nid, size_t size, gfp_t gfp_mask)
>  {
>  	unsigned int order = get_order(size);
> -	struct page *p = alloc_pages_node(nid, gfp_mask, order);
> +	struct page *p;
> +
> +	if (WARN_ON_ONCE(gfp_mask & __GFP_COMP))
> +		return NULL;
> +
> +	p = alloc_pages_node(nid, gfp_mask, order);
>  	if (!p)
>  		return NULL;
>  	return make_alloc_exact((unsigned long)page_address(p), order, size);
> -- 
> 2.20.1

-- 
Michal Hocko
SUSE Labs