Re: [PATCH] mm/page_alloc: add one PCP list for THP

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

+Mel, the original author of commit 5d0a661d808f
('mm/page_alloc: use only one PCP list..."


On Wed, Jun 19, 2024 at 9:48 PM <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 that non-movable
> allocation requests may get a CMA page from the list, in some cases,
> it's not acceptable.
>
> If a large number of CMA memory are configured in system (for
> example, the CMA memory accounts for 50% of the system memory),
> starting a virtual machine with device passthrough will get stuck.
> During starting the virtual machine, it will call
> pin_user_pages_remote(..., FOLL_LONGTERM, ...) to pin memory. Normally
> if a page is present and in CMA area, pin_user_pages_remote() will
> migrate the page from CMA area to non-CMA area because of
> FOLL_LONGTERM flag. But if non-movable allocation requests return
> CMA memory, migrate_longterm_unpinnable_pages() will migrate a CMA
> page to another CMA page, which will fail to pass the check in
> check_and_migrate_movable_pages() and cause migration endless.
> Call trace:
> pin_user_pages_remote
> --__gup_longterm_locked // endless loops in this function
> ----_get_user_pages_locked
> ----check_and_migrate_movable_pages
> ------migrate_longterm_unpinnable_pages
> --------alloc_migration_target
>

Please also describe its potential negative impact to cma_alloc().

> To fix the problem above, we add one PCP list for THP, which will
> not introduce a new cacheline. THP will have 2 PCP lists, one PCP

not introduce a new cacheline for struct per_cpu_pages.

> list is used by MOVABLE allocation, and the other PCP list is used
> by UNMOVABLE allocation. MOVABLE allocation contains GPF_MOVABLE,
> and UNMOVABLE allocation contains GFP_UNMOVABLE and GFP_RECLAIMABLE.
>
> Link: https://lore.kernel.org/all/1717492460-19457-1-git-send-email-yangge1116@126.com/

no this tag.

> Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations")

Cc: <stable@vger.kernel.org> ?

> Signed-off-by: yangge <yangge1116@126.com>
> ---
>  include/linux/mmzone.h | 9 ++++-----
>  mm/page_alloc.c        | 9 +++++++--
>  2 files changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index b7546dd..cb7f265 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -656,13 +656,12 @@ enum zone_watermarks {
>  };
>
>  /*
> - * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. One additional list
> - * for THP which will usually be GFP_MOVABLE. Even if it is another type,
> - * it should not contribute to serious fragmentation causing THP allocation
> - * failures.
> + * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. Two additional lists
> + * are added for THP. One PCP list is used by GPF_MOVABLE, and the other PCP list
> + * is used by GFP_UNMOVABLE and GFP_RECLAIMABLE.
>   */
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -#define NR_PCP_THP 1
> +#define NR_PCP_THP 2
>  #else
>  #define NR_PCP_THP 0
>  #endif
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8f416a0..0ecbde3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -504,10 +504,15 @@ static void bad_page(struct page *page, const char *reason)
>
>  static inline unsigned int order_to_pindex(int migratetype, int order)
>  {
> +       bool __maybe_unused movable;
> +
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>         if (order > PAGE_ALLOC_COSTLY_ORDER) {
>                 VM_BUG_ON(order != HPAGE_PMD_ORDER);
> -               return NR_LOWORDER_PCP_LISTS;
> +
> +               movable = migratetype == MIGRATE_MOVABLE;
> +
> +               return NR_LOWORDER_PCP_LISTS + movable;
>         }
>  #else
>         VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
> @@ -521,7 +526,7 @@ static inline int pindex_to_order(unsigned int pindex)
>         int order = pindex / MIGRATE_PCPTYPES;
>
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -       if (pindex == NR_LOWORDER_PCP_LISTS)
> +       if (order > PAGE_ALLOC_COSTLY_ORDER)

pindex >= NR_LOWORDER_PCP_LISTS

>                 order = HPAGE_PMD_ORDER;
>  #else
>         VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
> --
> 2.7.4
>

Thanks
Barry