Re: [PATCH v4] mm/vmalloc: make vm_struct.nr_pages an unsigned long

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Aug 03, 2026 at 05:39:35PM -0700, Andrew Morton wrote:
> On Sun, 2 Aug 2026 17:52:26 +0200 Uladzislau Rezki <urezki@gmail.com> wrote:
> 
> > On Sat, Aug 01, 2026 at 11:52:02AM -0700, Andrew Morton wrote:
> > > On Sat,  1 Aug 2026 14:49:15 +0300 Artem Lytkin <iprintercanon@gmail.com> wrote:
> > > 
> >
> > ...
> >
> > > Ulad, AI review suggests that vrealloc() has an issue handling
> > > __GFP_ZERO.  Can you please check?
> > > 
> > > 	https://sashiko.dev/#/patchset/20260801114915.115224-1-iprintercanon@gmail.com
> > > 
> > I have checked. I think the AI is missing at least one point.
> > AI argument which is:
> > 
> > <snip>
> > If a driver initially allocates memory using vmalloc() without __GFP_ZERO 
> > (leaving spare page capacity uninitialized), and then grows the allocation 
> > using vrealloc() with __GFP_ZERO, the caller expects the newly exposed bytes 
> > to be zeroed.
> > <snip>
> > 
> > In the vrealloc_node_align_noprof() header documentation there is a statement:
> > 
> > <snip>
> >  * If __GFP_ZERO logic is requested, callers must ensure that, starting with the
> >  * initial memory allocation, every subsequent call to this API for the same
> >  * memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
> >  * __GFP_ZERO is not fully honored by this API.
> > <snip>
> > 
> > AI argument violates the documentation, i.e. mixing __GFP_ZERO is not allowed.
> 
> Sashiko is talking about the initial allocation not using __GFP_ZERO
> but vrealloc() *does* use __GFP_ZERO.  The documentation you quoted
> doesn't address that case?
> 
But this is not allowed according to doc :)

<snip>
...
callers must ensure that, starting with the initial memory allocation
...
<snip>

Also, AI describes the situation like:

vmalloc()
vrealloc(grow, since need more) 

i.e. from the description:
<snip>
and then grows the allocation using vrealloc() with __GFP_ZERO, the caller
expects the newly exposed bytes to be zeroed.
<snip>

and it will be zeroed in fact, because the path would be:

<snip>
need_realloc:
	/* TODO: Grow the vm_area, i.e. allocate and map additional pages. */
	n = __vmalloc_node_noprof(size, align, flags, nid, __builtin_return_address(0));

	if (!n)
		return NULL;
<snip>

and not the one which AI pointed to.

The real scenario is:

1. vmalloc(!GFP_ZERO) - alloc size 10
2. vrealloc(!GFP_ZERO) - realloc to size 5
3. vrealloc(GFP_ZERO) - realloc back to 10.

3 - will not zeroed. As noted in doc ZERO should be used starting
from the beginning [1]. But in __most__ cases it will be zeroed
anyway because we free/unmap tail pages and if:

<snip>
		/*
		 * Free tail pages when shrink crosses a page boundary.
		 *
		 * Skip huge page allocations (page_order > 0) as partial
		 * freeing would require splitting.
		 *
		 * Skip VM_FLUSH_RESET_PERMS, as direct-map permissions must
		 * be reset before pages are returned to the allocator.
		 *
		 * Skip VM_USERMAP, as remap_vmalloc_range_partial() validates
		 * mapping requests against the unchanged vm->size; freeing
		 * tail pages would cause vmalloc_to_page() to return NULL for
		 * the unmapped range.
		 *
		 * Skip if either GFP_NOFS or GFP_NOIO are used.
		 * kmemleak_free_part() internally allocates with
		 * GFP_KERNEL, which could trigger a recursive deadlock
		 * if we are under filesystem or I/O reclaim.
		 */
		if (new_nr_pages < vm->nr_pages && !vm_area_page_order(vm) &&
		    !(vm->flags & (VM_FLUSH_RESET_PERMS | VM_USERMAP)) &&
		    gfp_has_io_fs(flags)) {
<snip>

is true the next grow with GFP_ZERO will be zeroed.

For huge alloc it will not be zeroed and for other conditions.

> Also, developers don't read documentation ;) What happens if some
> caller *does* use vmalloc(!__GFP_ZERO) then vrealloc(__GFP_ZERO)? 
> Silent misbehavior would be bad - it would be good if vrealloc() were
> to drop a WARN() then ignore the __GFP_ZERO.
> 
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -4294,11 +4294,6 @@ EXPORT_SYMBOL(vzalloc_node_noprof);
> >   * __GFP_THISNODE flag should be set, otherwise the function will try to avoid
> >   * reallocation and possibly disregard the specified @nid.
> >   *
> > - * If __GFP_ZERO logic is requested, callers must ensure that, starting with the
> > - * initial memory allocation, every subsequent call to this API for the same
> > - * memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
> > - * __GFP_ZERO is not fully honored by this API.
> > - *
> >   * Requesting an alignment that is bigger than the alignment of the existing
> >   * allocation will fail.
> >   *
> > @@ -4415,13 +4410,12 @@ 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 <= 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
> > -		 * realloc shrink time.
> > -		 */
> > -		vm->requested_size = size;
> >  		kasan_vrealloc(p, old_size, size);
> > +
> > +		if (want_init_on_alloc(flags))
> > +			memset((void *)p + old_size, 0, size - old_size);
> > +
> > +		vm->requested_size = size;
> >  		return (void *)p;
> >  	}
> 
> OK, thanks, I'll assume you'll prepare this for real when convenient.
>
OK. I will prepare something and send out the patch after testing.

--
Uladzislau Rezki