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Subject: Re: [PATCH] mm: gup: remove FOLL_SPLIT
To:     Yang Shi <shy828301@gmail.com>, <akpm@linux-foundation.org>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
References: <20210329193828.179993-1-shy828301@gmail.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <624d1172-cdf9-ce82-aa0b-6646f3bee757@nvidia.com>
Date:   Tue, 30 Mar 2021 00:08:34 -0700
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On 3/29/21 12:38 PM, Yang Shi wrote:
> Since commit 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT")
> and commit ba925fa35057 ("s390/gmap: improve THP splitting") FOLL_SPLIT
> has not been used anymore.  Remove the dead code.
> 
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>   include/linux/mm.h |  1 -
>   mm/gup.c           | 28 ++--------------------------
>   2 files changed, 2 insertions(+), 27 deletions(-)
> 

Looks nice.

As long as I'm running git grep here, there is one more search hit that should also
be fixed up, as part of a "remove FOLL_SPLIT" patch:

git grep -nw FOLL_SPLIT
Documentation/vm/transhuge.rst:57:follow_page, the FOLL_SPLIT bit can be specified as a parameter to

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8ba434287387..3568836841f9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2780,7 +2780,6 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
>   #define FOLL_NOWAIT	0x20	/* if a disk transfer is needed, start the IO
>   				 * and return without waiting upon it */
>   #define FOLL_POPULATE	0x40	/* fault in page */
> -#define FOLL_SPLIT	0x80	/* don't return transhuge pages, split them */
>   #define FOLL_HWPOISON	0x100	/* check page is hwpoisoned */
>   #define FOLL_NUMA	0x200	/* force NUMA hinting page fault */
>   #define FOLL_MIGRATION	0x400	/* wait for page to replace migration entry */
> diff --git a/mm/gup.c b/mm/gup.c
> index e40579624f10..f3d45a8f18ae 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -435,18 +435,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>   		}
>   	}
>   
> -	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
> -		get_page(page);
> -		pte_unmap_unlock(ptep, ptl);
> -		lock_page(page);
> -		ret = split_huge_page(page);
> -		unlock_page(page);
> -		put_page(page);
> -		if (ret)
> -			return ERR_PTR(ret);
> -		goto retry;
> -	}
> -
>   	/* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
>   	if (unlikely(!try_grab_page(page, flags))) {
>   		page = ERR_PTR(-ENOMEM);
> @@ -591,7 +579,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>   		spin_unlock(ptl);
>   		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
>   	}
> -	if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
> +	if (flags & FOLL_SPLIT_PMD) {
>   		int ret;
>   		page = pmd_page(*pmd);
>   		if (is_huge_zero_page(page)) {
> @@ -600,19 +588,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>   			split_huge_pmd(vma, pmd, address);
>   			if (pmd_trans_unstable(pmd))
>   				ret = -EBUSY;
> -		} else if (flags & FOLL_SPLIT) {
> -			if (unlikely(!try_get_page(page))) {
> -				spin_unlock(ptl);
> -				return ERR_PTR(-ENOMEM);
> -			}
> -			spin_unlock(ptl);
> -			lock_page(page);
> -			ret = split_huge_page(page);
> -			unlock_page(page);
> -			put_page(page);
> -			if (pmd_none(*pmd))
> -				return no_page_table(vma, flags);
> -		} else {  /* flags & FOLL_SPLIT_PMD */
> +		} else {
>   			spin_unlock(ptl);
>   			split_huge_pmd(vma, pmd, address);
>   			ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
>