Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

[Baolin Wang]

On 2024/4/4 18:03, Vlastimil Babka wrote:
> On 4/3/24 3:47 PM, Baolin Wang wrote:
> > Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
> > on the per-cpu lists") extends the PCP allocator to store THP pages, and
> > it determines whether to cache THP pags in PCP by comparing with pageblock_order.
> > But the pageblock_order is not always equal to THP order, it might also
> > be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
> >
> > Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
> > THP for PCP can fix this issue
> >
> > Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>
> IIUC this happens with CONFIG_HUGETLB_PAGE disabled because HUGETLBFS is
> disabled? But THPs are still enabled? I think there might be more of THP

Right, and seems the Powerpc arch will set pageblock_order via 
set_pageblock_order() when the huge page sizes are variable (not sure if 
this is always equal to THP order).

Moreover, it still does not make sense to use pageblock_order to 
indicate a THP page, especially when we already have HPAGE_PMD_ORDER to 
represent THP.

> working suboptimally in that case with pageblock_order being larger
> (MAX_PAGE_ORDER).
>
> In other words, should be rather make pageblock_order itself defined as
>
>   min_t(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
>
> in case with !CONFIG_HUGETLB_PAGE but THP enabled.

Yes, this makes sense to me (I wonder why this wasn't done before?). I 
can create a seperate patch to do this, what do you think? Thanks.