[PATCH v3 4/6] mm/vmalloc: make set_area_direct_map HUGE_VMAP friendly

[David Hildenbrand (Arm)]

On 9/8/26 13:12, Mike Rapoport wrote:
> On Tue, Sep 08, 2026 at 12:41:48PM +0200, David Hildenbrand (Arm) wrote:
>> On 9/3/26 11:28, Mike Rapoport (Microsoft) wrote:
>>> set_area_direct_map() always updates direct map alias permissions in
>>> single page increments.
>>>
>>> For HUGE_VMAP areas it's suboptimal. Not only the loop in
>>> set_area_direct_map() needlessly has more iterations (e.g times 512 on
>>> x86), but it also causes fragmentation of the direct map that could be
>>> avoided for the HUGE_VMAP areas populated with large pages.
>>>
>>> All pages in an area are always of the same order: either same-order
>>> large pages when VM_ALLOW_HUGE_VMAP is set and all huge pages were
>>> successfully allocated, or order-0 page when VM_ALLOW_HUGE_VMAP is
>>> cleared or when huge pages allocation fails and fallback path is taken.
>>>
>>> Instead of updating the direct map permissions for every order-0 page in
>>> an area, use the area's page_order as the loop increment and update the
>>> large pages in one call to set_direct_map_{invalid,default}_noflush().
>>>
>>> Signed-off-by: Mike Rapoport (Microsoft) <rppt at kernel.org>
>>> ---
>>>  mm/vmalloc.c | 13 ++++++++-----
>>>  1 file changed, 8 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index 5506b180f5c27..6ed6c160abed7 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -3367,12 +3367,15 @@ static inline void set_area_direct_map(const struct vm_struct *area,
>>>  				       int (*set_direct_map)(struct page *page,
>>>  							     unsigned int nr))
>>>  {
>>> -	unsigned long i;
>>> +	unsigned int nr = (1U << vm_area_page_order(area));
>>> +
>>> +	for (unsigned long i = 0; i < area->nr_pages; i += nr) {
>>> +		if (page_address(area->pages[i])) {
>>> +			int err = set_direct_map(area->pages[i], nr);
>>
>> Do we really expect some pages to have a directmap and others not?
> 
> Every page in the area can be allocated separately and
> __vmalloc_area_node() has
> 
> 	if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
> 		gfp_mask |= __GFP_HIGHMEM;
> 
> so generally speaking yes :)

Ah, I was thinking in terms of HUGE_VMAP, because there we sure would expect
all-or-nothing, right?

-- 
Cheers,

David