Re: [PATCH v2 2/3] vmalloc: Optimize vfree

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 3/20/26 09:39, David Hildenbrand (Arm) wrote:
> On 3/16/26 16:49, Vlastimil Babka wrote:
>>>  mm/vmalloc.c | 34 +++++++++++++++++++++++++---------
>>>  1 file changed, 25 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index c607307c657a6..8b935395fb068 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -3459,18 +3459,34 @@ void vfree(const void *addr)
>>>  
>>>  	if (unlikely(vm->flags & VM_FLUSH_RESET_PERMS))
>>>  		vm_reset_perms(vm);
>>> -	for (i = 0; i < vm->nr_pages; i++) {
>>> -		struct page *page = vm->pages[i];
>>> +
>>> +	if (vm->nr_pages) {
>>> +		bool account = !(vm->flags & VM_MAP_PUT_PAGES);
>>> +		unsigned long start_pfn, pfn;
>>> +		struct page *page = vm->pages[0];
>>> +		int nr = 1;
>>>  
>>>  		BUG_ON(!page);
>>> -		/*
>>> -		 * High-order allocs for huge vmallocs are split, so
>>> -		 * can be freed as an array of order-0 allocations
>>> -		 */
>>> -		if (!(vm->flags & VM_MAP_PUT_PAGES))
>>> +		start_pfn = page_to_pfn(page);
>>> +		if (account)
>>>  			mod_lruvec_page_state(page, NR_VMALLOC, -1);
>>> -		__free_page(page);
>>> -		cond_resched();
>>> +
>>> +		for (i = 1; i < vm->nr_pages; i++) {
>>> +			page = vm->pages[i];
>>> +			BUG_ON(!page);
>> 
>> We shouldn't be adding BUG_ON()'s. Rather demote also the pre-existing one
>> to VM_WARN_ON_ONCE() and skip gracefully.
>> 
>>> +			if (account)
>>> +				mod_lruvec_page_state(page, NR_VMALLOC, -1);
>> 
>> I think we should be able to batch this too to use "nr"?
> 
> Are we sure that pages cannot cross nodes etc? It could happen that we
> have a contig range that spans zones/nodes/etc ...

Hmm single order-3 allocation can't but we could be unlucky and get the last
order-3 from zone X and first order-3 from adjacent zone Y.
In that case the loop would need to also check same zone/node.

> Anyhow, should we try to decouple both things, providing a
> core-mm function to do the page freeing?
> 
> We do have something similar, optimized unpinning of large folios,
> in unpin_user_pages_dirty_lock(). This here is a bit different.
> 
> 
> So what I am thinking about for this code here to do:
> 
> 	if (!(vm->flags & VM_MAP_PUT_PAGES)) {
> 		for (i = 0; i < vm->nr_pages; i++)
> 			mod_lruvec_page_state(vm->pages[i], NR_VMALLOC, -1);
> 	}
> 	free_pages_bulk(vm->pages, vm->nr_pages);
> 
> 
> We could optimize the first loop to do batching where possible as well.
> 
> 
> free_pages_bulk() would match alloc_pages_bulk()
> 
> 	void free_pages_bulk(struct page **page_array, unsigned long nr_pages)
> 
> Internally we'd do the contig handling.
> 
> Was that already discussed?

AFAIU some of Zi's replies hinted at this direction. It would make sense, yeah.