Re: [PATCH v4 2/3] mm/vmalloc: free unused pages on vrealloc() shrink

[Shivam Kalra <shivamkalra98@zohomail.in>]

On 16/03/26 22:42, Uladzislau Rezki wrote:
> On Sat, Mar 14, 2026 at 02:34:14PM +0530, Shivam Kalra via B4 Relay wrote:
>> From: Shivam Kalra <shivamkalra98@zohomail.in>
>>
>> When vrealloc() shrinks an allocation and the new size crosses a page
>> boundary, unmap and free the tail pages that are no longer needed. This
>> reclaims physical memory that was previously wasted for the lifetime
>> of the allocation.
>>
>> The heuristic is simple: always free when at least one full page becomes
>> unused. Huge page allocations (page_order > 0) are skipped, as partial
>> freeing would require splitting.
>>
>> The virtual address reservation (vm->size / vmap_area) is intentionally
>> kept unchanged, preserving the address for potential future grow-in-place
>> support.
>>
>> Fix the grow-in-place check to compare against vm->nr_pages rather than
>> get_vm_area_size(), since the latter reflects the virtual reservation
>> which does not shrink. Without this fix, a grow after shrink would
>> access freed pages.
>>
>> Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
>> ---
>>  mm/vmalloc.c | 19 ++++++++++++++-----
>>  1 file changed, 14 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index b29bf58c0e3f..2c455f2038f6 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -4345,14 +4345,23 @@ void *vrealloc_node_align_noprof(const void *p, size_t size, unsigned long align
>>  			goto need_realloc;
>>  	}
>>  
>> -	/*
>> -	 * TODO: Shrink the vm_area, i.e. unmap and free unused pages. What
>> -	 * would be a good heuristic for when to shrink the vm_area?
>> -	 */
>>  	if (size <= old_size) {
>> +		unsigned int new_nr_pages = PAGE_ALIGN(size) >> PAGE_SHIFT;
>> +
>>  		/* Zero out "freed" memory, potentially for future realloc. */
>>  		if (want_init_on_free() || want_init_on_alloc(flags))
>>  			memset((void *)p + size, 0, old_size - size);
>> +
>> +		/* Free tail pages when shrink crosses a page boundary. */
>> +		if (new_nr_pages < vm->nr_pages && !vm_area_page_order(vm)) {
>> +			unsigned long addr = (unsigned long)p;
>> +
>> +			vunmap_range(addr + (new_nr_pages << PAGE_SHIFT),
>> +				     addr + (vm->nr_pages << PAGE_SHIFT));
>> +
>> +			vm_area_free_pages(vm, new_nr_pages, vm->nr_pages);
>> +			vm->nr_pages = new_nr_pages;
>> +		}
>>  		vm->requested_size = size;
>>  		kasan_vrealloc(p, old_size, size);
>>  		return (void *)p;
>> @@ -4361,7 +4370,7 @@ void *vrealloc_node_align_noprof(const void *p, size_t size, unsigned long align
>>  	/*
>>  	 * We already have the bytes available in the allocation; use them.
>>  	 */
>> -	if (size <= alloced_size) {
>> +	if (size <= (size_t)vm->nr_pages << PAGE_SHIFT) {
>>  		/*
>>  		 * No need to zero memory here, as unused memory will have
>>  		 * already been zeroed at initial allocation time or during
>>
>> -- 
>> 2.43.0
>>
>>
> Do we perform vm_reset_perms(vm) for tail pages? As i see you update the
> vm->nr_pages when shrinking. Then on vfree() we have:
> 
> <snip>
> /*
>  * Flush the vm mapping and reset the direct map.
>  */
> static void vm_reset_perms(struct vm_struct *area)
> {
> 	unsigned long start = ULONG_MAX, end = 0;
> 	unsigned int page_order = vm_area_page_order(area);
> 	int flush_dmap = 0;
> 	int i;
> 
> 	/*
> 	 * Find the start and end range of the direct mappings to make sure that
> 	 * the vm_unmap_aliases() flush includes the direct map.
> 	 */
> 	for (i = 0; i < area->nr_pages; i += 1U << page_order) {
> ...
> <snip>
> 
> i.e. tail pages go back to the page allocator without resetting permission.
> 
> --
> Uladzslau Rezki
Hi Uladzislau,

Good catch, thank you for spotting this. You are absolutely right-we are
currently returning the tail pages to the page allocator without
resetting their direct-map permissions if VM_FLUSH_RESET_PERMS was set.

While my specific use case doesn't utilize VM_FLUSH_RESET_PERMS,
vrealloc needs to safely handle all vmalloc flags as a generic API.

I will fix this in the next version (v5). I plan to add a helper
function to perform the permission reset specifically for the range of
tail pages being freed during the shrink.

Thanks,
Shivam