Re: [PATCH v4 1/2] mm: migration: fix the FOLL_GET failure on following huge page

[Alistair Popple]

On Monday, 15 August 2022 11:59:08 AM AEST Haiyue Wang wrote:
> Not all huge page APIs support FOLL_GET option, so the __NR_move_pages
> will fail to get the page node information for huge page.

I think you should be explicit in the commit message about which functions do 
not support FOLL_GET as it's not obvious what support needs to be added before 
this fix can be reverted.

Thanks.

 - Alistair

> This is an temporary solution to mitigate the racing fix.
> 
> After supporting follow huge page by FOLL_GET is done, this fix can be
> reverted safely.
> 
> Fixes: 4cd614841c06 ("mm: migration: fix possible do_pages_stat_array racing 
with memory offline")
> Signed-off-by: Haiyue Wang <haiyue.wang@xxxxxxxxx>
> ---
>  mm/migrate.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6a1597c92261..581dfaad9257 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1848,6 +1848,7 @@ static void do_pages_stat_array(struct mm_struct *mm, 
unsigned long nr_pages,
>  
>  	for (i = 0; i < nr_pages; i++) {
>  		unsigned long addr = (unsigned long)(*pages);
> +		unsigned int foll_flags = FOLL_DUMP;
>  		struct vm_area_struct *vma;
>  		struct page *page;
>  		int err = -EFAULT;
> @@ -1856,8 +1857,12 @@ static void do_pages_stat_array(struct mm_struct *mm, 
unsigned long nr_pages,
>  		if (!vma)
>  			goto set_status;
>  
> +		/* Not all huge page follow APIs support 'FOLL_GET' */
> +		if (!is_vm_hugetlb_page(vma))
> +			foll_flags |= FOLL_GET;
> +
>  		/* FOLL_DUMP to ignore special (like zero) pages */
> -		page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
> +		page = follow_page(vma, addr, foll_flags);
>  
>  		err = PTR_ERR(page);
>  		if (IS_ERR(page))
> @@ -1865,7 +1870,8 @@ static void do_pages_stat_array(struct mm_struct *mm, 
unsigned long nr_pages,
>  
>  		if (page && !is_zone_device_page(page)) {
>  			err = page_to_nid(page);
> -			put_page(page);
> +			if (foll_flags & FOLL_GET)
> +				put_page(page);
>  		} else {
>  			err = -ENOENT;
>  		}
>