linux-kernel - Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating non-CMA THP-sized page

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Date: Mon, 17 Jun 2024 22:43:47 +1200
From: Barry Song <21cnbao@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: yangge1116 <yangge1116@....com>, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, liuzixing@...on.cn, 
	Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating
 non-CMA THP-sized page

On Thu, Jun 6, 2024 at 3:07 PM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2024/6/4 20:36, yangge1116 wrote:
> >
> >
> > 在 2024/6/4 下午8:01, Baolin Wang 写道:
> >> Cc Johannes, Zi and Vlastimil.
> >>
> >> On 2024/6/4 17:14, yangge1116@....com wrote:
> >>> From: yangge <yangge1116@....com>
> >>>
> >>> Since commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for
> >>> THP-sized allocations") no longer differentiates the migration type
> >>> of pages in THP-sized PCP list, it's possible to get a CMA page from
> >>> the list, in some cases, it's not acceptable, for example, allocating
> >>> a non-CMA page with PF_MEMALLOC_PIN flag returns a CMA page.
> >>>
> >>> The patch forbids allocating non-CMA THP-sized page from THP-sized
> >>> PCP list to avoid the issue above.
> >>>
> >>> Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for
> >>> THP-sized allocations")
> >>> Signed-off-by: yangge <yangge1116@....com>
> >>> ---
> >>>   mm/page_alloc.c | 10 ++++++++++
> >>>   1 file changed, 10 insertions(+)
> >>>
> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >>> index 2e22ce5..0bdf471 100644
> >>> --- a/mm/page_alloc.c
> >>> +++ b/mm/page_alloc.c
> >>> @@ -2987,10 +2987,20 @@ struct page *rmqueue(struct zone
> >>> *preferred_zone,
> >>>       WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
> >>>       if (likely(pcp_allowed_order(order))) {
> >>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >>> +        if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
> >>> +                        order != HPAGE_PMD_ORDER) {
> >>
> >> Seems you will also miss the non-CMA THP from the PCP, so I wonder if
> >> we can add a migratetype comparison in __rmqueue_pcplist(), and if
> >> it's not suitable, then fallback to buddy?
> >
> > Yes, we may miss some non-CMA THPs in the PCP. But, if add a migratetype
> > comparison in __rmqueue_pcplist(), we may need to compare many times
> > because of pcp batch.
>
> I mean we can only compare once, focusing on CMA pages.
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3734fe7e67c0..960a3b5744d8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2973,6 +2973,11 @@ struct page *__rmqueue_pcplist(struct zone *zone,
> unsigned int order,
>                  }
>
>                  page = list_first_entry(list, struct page, pcp_list);
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +               if (order == HPAGE_PMD_ORDER &&
> !is_migrate_movable(migratetype) &&
> +                   is_migrate_cma(get_pageblock_migratetype(page)))
> +                       return NULL;
> +#endif

This doesn't seem ideal either. It's possible that the PCP still has many
non-CMA folios, but due to bad luck, the first entry is "always" CMA.
In this case,
allocations with is_migrate_movable(migratetype) == false will always lose the
chance to use the PCP.   It also appears to incur a PCP spin lock/unlock.

I don't see an ideal solution unless we bring back the CMA PCP :-)

>                  list_del(&page->pcp_list);
>                  pcp->count -= 1 << order;
>          } while (check_new_pages(page, order));
>