Re: [PATCH v13 0/5] mm/vmalloc: free unused pages on vrealloc() shrink

[Shivam Kalra]

On 13/05/26 03:31, Andrew Morton wrote:
> On Mon, 11 May 2026 13:47:28 +0530 Shivam Kalra via B4 Relay <devnull+shivamkalra98.zohomail.in@xxxxxxxxxx> wrote:
> 
>> This series implements the TODO in vrealloc() to unmap and free unused
>> pages when shrinking across a page boundary.
> 
> Thanks.  AI review asked a few questions.  Does anything here look
> legitimate?
> 
> 	https://sashiko.dev/#/patchset/20260511-vmalloc-shrink-v13-0-643b7ec277a9@xxxxxxxxxxx

Thanks for flagging this. I went through the Sashiko review, and,
one of the findings is legitimate and needs a fix.

1) Patch 3 - vmap() mappings with nr_pages == 0 (LEGITIMATE)
This is correct. For vmap() areas created without VM_MAP_PUT_PAGES,
nr_pages is never set and stays 0 (vm_struct is kzalloc'd). With the
current patch, vread_iter() would compute size = 0 for these areas,
skip them entirely, and zero-fill the region in /proc/kcore.

Ulad, this is the approach you suggested during v10 review [1]; we
both checked whether nr_pages could be 0 and concluded it couldn't,
but we only considered the VM_ALLOC (vmalloc) path. vmap() areas also
have vm != NULL but never set nr_pages unless VM_MAP_PUT_PAGES is
passed, which most callers don't use.

Since vrealloc() only operates on VM_ALLOC areas, I think we can
scope the nr_pages path to just that:
	if (vm)
		/*
		 * For VM_ALLOC areas, use nr_pages rather than
		 * get_vm_area_size() because vrealloc() may shrink
		 * the mapping without updating area->size. Other
		 * mapping types (vmap, ioremap) don't set nr_pages.
		 */
		size = (vm->flags & VM_ALLOC) ?
			(vm->nr_pages << PAGE_SHIFT) :
			get_vm_area_size(vm);
	else
		size = va_size(va);
Ulad, does this approach look right to you, or would you prefer to
initialize nr_pages unconditionally in vmap() instead?

[1] https://lore.kernel.org/lkml/aeYBYqPSEJRC8mjh@milan/