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

Zi Yan
2026-01-12T18:56:12Z

On 12 Jan 2026, at 13:25, Jason Gunthorpe wrote:

> On Mon, Jan 12, 2026 at 12:46:57PM -0500, Zi Yan wrote:
>> On 12 Jan 2026, at 11:50, 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.
>>
>> 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.
>
> That's different, I am talking about reaching 0 because it has been
> freed, meaning there are no external pointers to it.
>
> Further, when a page is frozen page_ref_freeze() takes in the number
> of references the caller has ownership over and it doesn't succeed if
> there are stray references elsewhere.
>
> This is very important because the entire operating model of split
> only works if it has exclusive locks over all the valid pointers into
> that page.
>
> Spurious refcount failures concurrent with split cannot be allowed.
>
> I don't see how pointing at __folio_freeze_and_split_unmapped() can
> justify this series.
>

But from anyone looking at the folio state, refcount == 0, compound_head
is set, they cannot tell the difference.

If what you said is true, why is free_pages_prepare() needed? No one
should touch these free pages. Why bother resetting these states.

>> 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.
>
> You can't refcount a folio out of nothing. It has to come from a
> memory location that already is holding a refcount, and then you can
> incr it.

Right. There is also no guarantee that all code is correct and follows
this.

My point here is that calling prep_compound_page() on a compound page
does not follow core MM’s conventions.

Best Regards,
Yan, Zi