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

On 12 Jan 2026, at 11:50, Jason Gunthorpe wrote:

On Mon, Jan 12, 2026 at 11:31:04AM -0500, Zi Yan wrote:

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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.

There is no such a rule. In core MM, folio_split(), which splits a high
order folio to low order ones, freezes the folio (turning refcount to 0)
and manipulates the folio order and all tail pages compound_head to
restructure the folio. Your fundamental rule breaks this.
Allowing compound information to stay after a folio is freed means
you cannot tell whether a folio is under split or freed.

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.

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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.

Jason

Best Regards,
Yan, Zi