linux-kernel - RE: [PATCH 1/2] mm/gup: Remove unnecessary check in memfd_pin

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Message-ID: <IA0PR11MB7185E3BD6CC4DB1CF7F49351F8822@IA0PR11MB7185.namprd11.prod.outlook.com>
Date: Thu, 1 May 2025 21:23:08 +0000
From: "Kasireddy, Vivek" <vivek.kasireddy@...el.com>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>, "linux-mm@...ck.org"
	<linux-mm@...ck.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Andrew
 Morton" <akpm@...ux-foundation.org>
Subject: RE: [PATCH 1/2] mm/gup: Remove unnecessary check in
 memfd_pin_folios()

Hi Vishal,

> Subject: [PATCH 1/2] mm/gup: Remove unnecessary check in
> memfd_pin_folios()
> 
> Commit 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning
> memfd folios")
> checks if filemap_get_folios_contig() returned duplicate folios to
> prevent multiple attempts at pinning the same folio.
> 
> Commit 8ab1b1602396 ("mm: fix filemap_get_folios_contig returning batches
> of identical folios")
> ensures that filemap_get_folios_contig() returns a batch of distinct folios.
> 
> We can remove the duplicate folio check to simplify the code and save
> 58 bytes of text.
> 
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>
> ---
>  mm/gup.c | 15 +--------------
>  1 file changed, 1 insertion(+), 14 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index f32168339390..1fb8f3b9a493 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3589,7 +3589,7 @@ long memfd_pin_folios(struct file *memfd, loff_t
> start, loff_t end,
>  {
>  	unsigned int flags, nr_folios, nr_found;
>  	unsigned int i, pgshift = PAGE_SHIFT;
> -	pgoff_t start_idx, end_idx, next_idx;
> +	pgoff_t start_idx, end_idx;
>  	struct folio *folio = NULL;
>  	struct folio_batch fbatch;
>  	struct hstate *h;
> @@ -3639,19 +3639,7 @@ long memfd_pin_folios(struct file *memfd, loff_t
> start, loff_t end,
>  				folio = NULL;
>  			}
> 
> -			next_idx = 0;
>  			for (i = 0; i < nr_found; i++) {
> -				/*
> -				 * As there can be multiple entries for a
> -				 * given folio in the batch returned by
> -				 * filemap_get_folios_contig(), the below
> -				 * check is to ensure that we pin and return a
> -				 * unique set of folios between start and end.
> -				 */
> -				if (next_idx &&
> -				    next_idx != folio_index(fbatch.folios[i]))
> -					continue;
At that time, I believed that filemap_get_folios_contig() returning duplicate
folios was by design and not a result of a bug and hence added this workaround,
which is no longer needed in light of your fix.
Anyway, LGTM.

Acked-by: Vivek Kasireddy <vivek.kasireddy@...el.com>

Thanks,
Vivek

> -
>  				folio = page_folio(&fbatch.folios[i]->page);
> 
>  				if (try_grab_folio(folio, 1, FOLL_PIN)) {
> @@ -3664,7 +3652,6 @@ long memfd_pin_folios(struct file *memfd, loff_t
> start, loff_t end,
>  					*offset = offset_in_folio(folio, start);
> 
>  				folios[nr_folios] = folio;
> -				next_idx = folio_next_index(folio);
>  				if (++nr_folios == max_folios)
>  					break;
>  			}
> --
> 2.49.0
>