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

[Vlastimil Babka <vbabka@suse.cz>]

On 3/14/19 11:15 AM, Michal Hocko wrote:
> 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".

I initially went with 2 as well, as you can see from v1 :) but then I looked at
the commit [2] mentioned in [1] and I think ALSA legitimaly uses __GFP_COMP so
that the pages are then mapped to userspace. Breaking that didn't seem good.

The point is that with the warning in place, A developer will immediately know
that they did something wrong, regardless if the size is power-of-two or not.
But yeah, if it's adding of __GFP_COMP that is not deterministic, a bug can
still sit silently for a while.

But maybe we could go with 1) if free_pages_exact() is also adjusted to check
for CompoundPage and free it properly?

>> [1] https://lore.kernel.org/lkml/20181126002805.GI18977@shao2-debian/T/#u

[2]
https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/commit/?id=3a6d1980fe96dbbfe3ae58db0048867f5319cdbf

>> 
>> 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
>