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

[Alice Ryhl]

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@xxxxxxxxxxx>
> ---
>  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.

Alice