Re: [PATCH v4 1/7] mm/zone_device: Add order argument to folio_free callback

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

On Mon, Jan 12, 2026 at 12:50:01PM -0400, Jason Gunthorpe wrote:
> On Mon, Jan 12, 2026 at 11:31:04AM -0500, Zi Yan wrote:
> > > folio_free()
> > >
> > > 1) Allocator finds free memory
> > > 2) zone_device_page_init() allocates the memory and makes refcount=1
> > > 3) __folio_put() knows the recount 0.
> > > 4) free_zone_device_folio() calls folio_free(), but it doesn't
> > >    actually need to undo prep_compound_page() because *NOTHING* can
> > >    use the page pointer at this point.
> > > 5) Driver puts the memory back into the allocator and now #1 can
> > >    happen. It knows how much memory to put back because folio->order
> > >    is valid from #2
> > > 6) #1 happens again, then #2 happens again and the folio is in the
> > >    right state for use. The successor #2 fully undoes the work of the
> > >    predecessor #2.
> > 
> > But how can a successor #2 undo the work if the second #1 only allocates
> > half of the original folio? For example, an order-9 at PFN 0 is
> > allocated and freed, then an order-8 at PFN 0 is allocated and another
> > order-8 at PFN 256 is allocated. How can two #2s undo the same order-9
> > without corrupting each other’s data?
> 
> What do you mean? The fundamental rule is you can't read the folio or
> the order outside folio_free once it's refcount reaches 0.
> 
> So the successor #2 will write updated heads and order to the order 8
> pages at PFN 0 and the ones starting at PFN 256 will remain with
> garbage.
> 
> This is OK because nothing is allowed to read them as their refcount
> is 0.
> 
> If later PFN256 is allocated then it will get updated head and order
> at the same time it's refcount becomes 1.
> 
> There is corruption and they don't corrupt each other's data.
> 
> > > If the allocator is using the struct page memory then step #5 should
> > > also clean up the struct page with the allocator data before returning
> > > it to the allocator.
> > 
> > Do you mean ->folio_free() callback should undo prep_compound_page()
> > instead?
> 
> I wouldn't say undo, I was very careful to say it needs to get the
> struct page memory into a state that the allocator algorithm expects,
> whatever that means.
> 

Hi Jason,

A lot of back and forth with Zi — if I’m understanding correctly, your
suggestion is to just call free_zone_device_folio_prepare() [1] in
->folio_free() if required by the driver. This is the function that puts
struct page into a state my allocator expects. That works just fine for
me.

Matt

[1] https://patchwork.freedesktop.org/patch/697877/?series=159120&rev=4

> Jason