Re: [PATCH] mm/page_alloc: don't warn about large allocations with __GFP_NOFAIL

[Baokun Li <libaokun@huaweicloud.com>]

On 2025-11-05 16:05, Michal Hocko wrote:
> On Wed 05-11-25 15:41:06, libaokun@huaweicloud.com wrote:
>> From: Baokun Li <libaokun1@huawei.com>
>>
>> Filesystems use __GFP_NOFAIL to allocate block-sized folios for metadata
>> reads at critical points, since they cannot afford to go read-only,
>> shut down, or enter an inconsistent state due to memory pressure.
>>
>> Currently, attempting to allocate page units greater than order-1 with
>> the __GFP_NOFAIL flag triggers a WARN_ON() in __alloc_pages_slowpath().
>> However, filesystems supporting large block sizes (blocksize > PAGE_SIZE)
>> can easily require allocations larger than order-1.
>>
>> As Matthew Wilcox noted, if we have a filesystem with 64KiB sectors, there
>> will be many clean folios in the page cache that are 64KiB or larger.
>>
>> With gfp flags and order already included in the OOM report, both
>> Vlastimil Babka and Michal Hocko suggested that we can take the risk of
>> removing this warning first and then observe whether a large number of
>> related OOM reports appear.
>>
>> If that happens, we can consider adding special handling in other places.
>>
>> Suggested-by: Matthew Wilcox <willy@infradead.org>
>> Link: https://lore.kernel.org/all/aQPX1-XWQjKaMTZB@casper.infradead.org
>> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
>> Link: https://lore.kernel.org/all/188a95ba-6384-4319-bb74-c0d9ec6c4079@suse.cz
>> Suggested-by: Michal Hocko <mhocko@suse.com>
>> Link: https://lore.kernel.org/all/aQotQBjnDDeL_wHx@tiehlicka
>> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> Thanks for referencing above links which are providing a useful insight
> for future reference. I would just add a link to Matthew explanation why
> kvmalloc is not an option
> Link: https://lore.kernel.org/all/aQTHMI3t5mNXp0M1@casper.infradead.org/T/#u
> Acked-by: Michal Hocko <mhocko@suse.com>
Okay, I will add the link in the next version.

Thanks for your review!


Cheers,
Baokun

>> ---
>>
>> RFC: https://lore.kernel.org/all/20251031061350.2052509-1-libaokun@huaweicloud.com
>>
>>  mm/page_alloc.c | 5 -----
>>  1 file changed, 5 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index fb91c566327c..e4efda1158b2 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4683,11 +4683,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>  	int reserve_flags;
>>  
>>  	if (unlikely(nofail)) {
>> -		/*
>> -		 * 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.
>> -- 
>> 2.46.1

-- 
With Best Regards,
Baokun Li