Re: [PATCH v6 1/5] mm/zone_device: Reinitialize large zone device private folios

[Matthew Brost <matthew.brost@intel.com>]

On Fri, Jan 16, 2026 at 08:17:22PM +0100, Vlastimil Babka wrote:
> On 1/16/26 18:49, Jason Gunthorpe wrote:
> > On Fri, Jan 16, 2026 at 12:10:16PM +0100, Francois Dugast wrote:
> >> -void zone_device_page_init(struct page *page, unsigned int order)
> >> +void zone_device_page_init(struct page *page, struct dev_pagemap *pgmap,
> >> +			   unsigned int order)
> >>  {
> >> +	struct page *new_page = page;
> >> +	unsigned int i;
> >> +
> >>  	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
> >>  
> >> +	for (i = 0; i < (1UL << order); ++i, ++new_page) {
> >> +		struct folio *new_folio = (struct folio *)new_page;
> >> +
> >> +		/*
> >> +		 * new_page could have been part of previous higher order folio
> >> +		 * which encodes the order, in page + 1, in the flags bits. We
> >> +		 * blindly clear bits which could have set my order field here,
> >> +		 * including page head.
> >> +		 */
> >> +		new_page->flags.f &= ~0xffUL;	/* Clear possible order, page head */
> >> +
> >> +#ifdef NR_PAGES_IN_LARGE_FOLIO
> >> +		/*
> >> +		 * This pointer math looks odd, but new_page could have been
> >> +		 * part of a previous higher order folio, which sets _nr_pages
> >> +		 * in page + 1 (new_page). Therefore, we use pointer casting to
> >> +		 * correctly locate the _nr_pages bits within new_page which
> >> +		 * could have modified by previous higher order folio.
> >> +		 */
> >> +		((struct folio *)(new_page - 1))->_nr_pages = 0;
> >> +#endif
> > 
> > This seems too weird, why is it in the loop?  There is only one
> > _nr_pages per folio.
> 
> I suppose we could be getting say an order-9 folio that was previously used
> as two order-8 folios? And each of them had their _nr_pages in their head

Yes, this is a good example. At this point we have idea what previous
allocation(s) order(s) were - we could have multiple places in the loop
where _nr_pages is populated, thus we have to clear this everywhere. 

> and we can't know that at this point so we have to reset everything?
> 

Yes, see above, correct. We have no visablity to previous state of the
pages so the only option is to reset everything.

> AFAIU this would not be a problem if the clearing of the previous state was
> done upon freeing, as e.g. v4 did, but I think you also argued it meant
> processing the pages when freeing and then again at reallocation, so it's
> now like this instead?

Yes, if we cleanup the previous folio state upon freeing, then this
problem goes away but the we back passing in the order as argument to
->folio_free(). 

> 
> Or maybe you mean that stray _nr_pages in some tail page from previous
> lifetimes can't affect the current lifetime in a wrong way for something
> looking at said page? I don't know immediately.
> 
> > This is mostly zeroing some memory in the tail pages? Why?
> > 
> > Why can't this use the normal helpers, like memmap_init_compound()?
> > 
> >  struct folio *new_folio = page
> > 
> >  /* First 4 tail pages are part of struct folio */
> >  for (i = 4; i < (1UL << order); i++) {
> >      prep_compound_tail(..)
> >  }
> > 
> >  prep_comound_head(page, order)
> >  new_folio->_nr_pages = 0
> > 
> > ??

I've beat this to death with Alistair, normal helpers do not work here.

An order zero allocation could have _nr_pages set in its page,
new_folio->_nr_pages is page + 1 memory.

Matt

> > 
> > Jason
>