Re: [PATCH v2 1/3] mm/page_alloc: Optimize free_contig_range()

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 3/16/26 17:02, Zi Yan wrote:
> On 16 Mar 2026, at 11:21, Vlastimil Babka wrote:
> 
>>> +/*
>>> + * free_pages_prepare() has already been called for page(s) being freed.
>>> + * TODO: Perform per-subpage free_pages_prepare() checks for order > 0 pages
>>> + * (HWPoison, PageNetpp, bad free page).
>>> + */
>>
>> I'm confused, and reading the v1 thread didn't help either. Where would the
>> subpages to check come from? AFAICS we start from order-0 pages always.
>> __free_contig_range calls free_pages_prepare on every page with order 0
>> unconditionally, so we check every page as an order-0 page. If we then free
>> the bunch of individually checked pages as a high-order page, there's no
>> reason to check those subpages again, no? Am I missing something?
> 
> There are two kinds of order > 0 pages, compound and not compound.
> free_pages_prepare() checks all tail pages of a compound order > 0 pages too.
> For non compound ones, free_pages_prepare() only has free_page_is_bad()
> check on tail ones.
> 
> So my guess is that the TODO is to check all subpages on a non compound
> order > 0 one in the same manner. This is based on the assumption that

OK but:

1) Why put that TODO specifically on FPI_PREPARED definition, which is for
the case we skip the prepare/check?
2) Why add it in this series which AFAICS doesn't handle non-compound
order>0 anywhere.
3) We'd better work on eliminating the non-compound order>0 usages
altogether, rather than work on support them better.

> all non compound order > 0 page users use split_page() after the allocation,
> treat each page individually, and free them back altogether. But I am not
> sure if this is true for all users allocating non compound order > 0 pages.

Maybe as part of the elimination (point 3 above) we should combine the
allocation+split so it's never the first without the second anymore.

> And free_pages_prepare_bulk() might be a better name for such functions.
> 
> The above confusion is also a reason I asked Ryan to try adding a unsplit_page()
> function to fuse back non compound order > 0 pages and free the fused one
> as we are currently doing. But that looks like a pain to implment. Maybe an

Yeah not sure it's worth it either.

> alternative to this FPI_PREPARED is to add FPI_FREE_BULK and loop through all
> subpages if FPI_FREE_BULK is set with
> __free_pages_prepare(page + i, 0, fpi_flags & ~FPI_FREE_BULK) in
> __free_pages_ok().

Hmm, maybe...

> 
> Best Regards,
> Yan, Zi