Re: [PATCH v3] mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()

[Michal Hocko <mhocko@suse.com>]

On Mon 18-03-19 13:21:59, Vlastimil Babka wrote:
> OK here's a new version that changes the patch to remove __GFP_COMP per
> the v2 discussion, and also fixes the bug Kirill spotted (thanks!).
> 
> ----8<----
> >From 1fbc84c208573b885f51818ed823f89b3aa1e0ae Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@suse.cz>
> Date: Thu, 14 Mar 2019 10:19:30 +0100
> Subject: [PATCH v3] mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()
> 
> 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 may have really
> 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 2, because as Michal Hocko put it: "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>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  mm/page_alloc.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0b9f577b1a2a..123d9a407599 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
> @@ -4767,6 +4767,9 @@ void *alloc_pages_exact(size_t size, gfp_t gfp_mask)
>  	unsigned int order = get_order(size);
>  	unsigned long addr;
>  
> +	if (WARN_ON_ONCE(gfp_mask & __GFP_COMP))
> +		gfp_mask &= ~__GFP_COMP;
> +
>  	addr = __get_free_pages(gfp_mask, order);
>  	return make_alloc_exact(addr, order, size);
>  }
> @@ -4777,7 +4780,7 @@ 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 +4788,12 @@ 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))
> +		gfp_mask &= ~__GFP_COMP;
> +
> +	p = alloc_pages_node(nid, gfp_mask, order);
>  	if (!p)
>  		return NULL;
>  	return make_alloc_exact((unsigned long)page_address(p), order, size);
> -- 
> 2.21.0
> 

-- 
Michal Hocko
SUSE Labs