Re: [PATCH v3 0/4] mm: clarify nofail memory allocation

[Vlastimil Babka <vbabka@suse.cz>]

On 8/30/24 09:24, Michal Hocko wrote:
> On Fri 30-08-24 10:31:14, Barry Song wrote:
>> > > > Patch 4/4: We will move the order > 1 check from the current fast path
>> > > > to the slow path and extend
>> > > >                  the check of gfp_direct_reclaim flag also in the slow path.
>> > >
>> > > OK, let's have that go in now as well.
>> 
>> Hi Michal and Vlastimil,
>> Could you please review the changes below before I send v4 for patch 4/4?
>> 
>> 1. We should consolidate all warnings in one place. Currently, the order > 1 warning is
>> in the hotpath, while others are in less likely scenarios. Moving all warnings to the
>> slowpath will reduce the overhead for order > 1 and increase the visibility of other
>> warnings.
>> 
>> 2. We currently have two warnings for order: one for order > 1 in the hotpath and another
>> for order > costly_order in the laziest path. I suggest standardizing on order > 1 since
>> it’s been in use for a long time.
>> 
>> 3.I don't think we need to check for __GFP_NOWARN in this case. __GFP_NOWARN is
>> meant to suppress allocation failure reports, but here we're dealing with bug detection, not
>> allocation failures.

Ack. __GFP_NOWARN is to suppress warnings in case the allocation has a less
expensive fallback to the current attempt, which logically means the current
attempt can't be a __GFP_NOFAIL one. So having both is a bug itself (not
worth reporting) so we can just ignore __GFP_NOWARN.

>> So I'd rather use WARN_ON_ONCE than WARN_ON_ONCE_GFP.
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index c81ee5662cc7..0d3dd679d0ab 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3033,12 +3033,6 @@ struct page *rmqueue(struct zone *preferred_zone,
>>  {
>>  	struct page *page;
>>  
>> -	/*
>> -	 * We most definitely don't want callers attempting to
>> -	 * allocate greater than order-1 page units with __GFP_NOFAIL.
>> -	 */
>> -	WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
>> -
>>  	if (likely(pcp_allowed_order(order))) {
>>  		page = rmqueue_pcplist(preferred_zone, zone, order,
>>  				       migratetype, alloc_flags);
>> @@ -4174,6 +4168,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>  						struct alloc_context *ac)
>>  {
>>  	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
>> +	bool nofail = gfp_mask & __GFP_DIRECT_RECLAIM;

__GFP_NOFAIL

>>  	bool can_compact = gfp_compaction_allowed(gfp_mask);
>>  	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
>>  	struct page *page = NULL;
>> @@ -4187,6 +4182,25 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>  	unsigned int zonelist_iter_cookie;
>>  	int reserve_flags;
>>  
>> +	if (nofail) {

Could add unlikely() to put it off the instruction cache hotpath.

>> +		/*
>> +		 * We most definitely don't want callers attempting to
>> +		 * allocate greater than order-1 page units with __GFP_NOFAIL.
>> +		 */
>> +		WARN_ON_ONCE(order > 1);
>> +		/*
>> +		 * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,
>> +		 * otherwise, we may result in lockup.
>> +		 */
>> +		WARN_ON_ONCE(!can_direct_reclaim);
>> +		/*
>> +		 * PF_MEMALLOC request from this context is rather bizarre
>> +		 * because we cannot reclaim anything and only can loop waiting
>> +		 * for somebody to do a work for us.
>> +		 */
>> +		WARN_ON_ONCE(current->flags & PF_MEMALLOC);
>> +	}
> 
> Yes, this makes sense. Any reason you have not put that int the nofail
> branch below?

Because that branch is executed only when we're already so depleted we gave
up retrying, and we want to warn about the buggy users more reliably (see
point 1 above).

>> +
>>  restart:
>>  	compaction_retries = 0;
>>  	no_progress_loops = 0;
>> @@ -4404,29 +4418,15 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>  	 * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
>>  	 * we always retry
>>  	 */
>> -	if (gfp_mask & __GFP_NOFAIL) {
>> +	if (nofail) {
>>  		/*
>> -		 * All existing users of the __GFP_NOFAIL are blockable, so warn
>> -		 * of any new users that actually require GFP_NOWAIT
>> +		 * Lacking direct_reclaim we can't do anything to reclaim memory,
>> +		 * we disregard these unreasonable nofail requests and still
>> +		 * return NULL
>>  		 */
>> -		if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
>> +		if (!can_direct_reclaim)
>>  			goto fail;
>>  
>> -		/*
>> -		 * PF_MEMALLOC request from this context is rather bizarre
>> -		 * because we cannot reclaim anything and only can loop waiting
>> -		 * for somebody to do a work for us
>> -		 */
>> -		WARN_ON_ONCE_GFP(current->flags & PF_MEMALLOC, gfp_mask);
>> -
>> -		/*
>> -		 * non failing costly orders are a hard requirement which we
>> -		 * are not prepared for much so let's warn about these users
>> -		 * so that we can identify them and convert them to something
>> -		 * else.
>> -		 */
>> -		WARN_ON_ONCE_GFP(costly_order, gfp_mask);
>> -
>>  		/*
>>  		 * Help non-failing allocations by giving some access to memory
>>  		 * reserves normally used for high priority non-blocking
>> 
>> > >
>> > > --
>> > > Michal Hocko
>> > > SUSE Labs
>> 
>> Thanks
>> Barry
>