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

[Barry Song <21cnbao@gmail.com>]

On Mon, Jun 17, 2024 at 7:36 PM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2024/6/17 18:43, Barry Song wrote:
> > On Thu, Jun 6, 2024 at 3:07 PM Baolin Wang
> > <baolin.wang@linux.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@126.com wrote:
> >>>>> From: yangge <yangge1116@126.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@126.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.
>
> Yes, just some ideas to to mitigate the issue...
>
> >
> > I don't see an ideal solution unless we bring back the CMA PCP :-)
>
> Tend to agree, and adding a CMA PCP seems the overhead can be acceptable?

yes. probably. Hi Ge,

Could we printk the size before and after adding 1 to NR_PCP_LISTS?
Does it increase one cacheline?

struct per_cpu_pages {
spinlock_t lock; /* Protects lists field */
int count; /* number of pages in the list */
int high; /* high watermark, emptying needed */
int high_min; /* min high watermark */
int high_max; /* max high watermark */
int batch; /* chunk size for buddy add/remove */
u8 flags; /* protected by pcp->lock */
u8 alloc_factor; /* batch scaling factor during allocate */
#ifdef CONFIG_NUMA
u8 expire; /* When 0, remote pagesets are drained */
#endif
short free_count; /* consecutive free count */

/* Lists of pages, one per migrate type stored on the pcp-lists */
struct list_head lists[NR_PCP_LISTS];
} ____cacheline_aligned_in_smp;