Re: [PATCH v5 2/3] mm/vmalloc: free unused pages on vrealloc() shrink

["Danilo Krummrich" <dakr@kernel.org>]

On Tue Mar 17, 2026 at 3:39 PM CET, Alice Ryhl wrote:
> On Tue, Mar 17, 2026 at 01:47:34PM +0530, Shivam Kalra wrote:
>> 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. Allocations with VM_FLUSH_RESET_PERMS
>> are also skipped, as their direct-map permissions must be reset before
>> pages are returned to the page allocator, which is handled by
>> vm_reset_perms() during vfree().
>> 
>> 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>

Feel free to add

Suggested-by: Danilo Krummrich <dakr@kernel.org>

>> ---
>>  mm/vmalloc.c | 20 +++++++++++++++-----
>>  1 file changed, 15 insertions(+), 5 deletions(-)
>> 
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index b29bf58c0e3f..f3820c6712c1 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -4345,14 +4345,24 @@ 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) &&
>> +		    !(vm->flags & VM_FLUSH_RESET_PERMS)) {
>> +			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 +4371,7 @@ 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
>
> Hmm. So what happened here is that it has previously always been the
> case that get_vm_area_size(area) == vm->nr_pages << PAGE_SHIFT, so these
> constants were interchangable. But now that is no longer the case.
>
> For example, 'remap_vmalloc_range_partial' compares the vm area size
> with the range being mapped, and then proceeds to look up the pages and
> map them. But now those pages may be missing.
>
> I can't really tell if there are other places in this file that need to
> be updated too.

This may well be possible. I remember that when I added vrealloc() and looked
into growing and shrinking, I concluded that it might need a bit of rework in
terms of tracking the sizes of the different layers. Unfortunately, I don't
remember the details anymore, but I'm quite sure there were some subtleties
along the lines of what Alice points out, so I recommend to double check.