linux-kernel - Re: [PATCH 2/3] mm: memfd_luo: use memfd_alloc_file() instead of shmem_file

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Message-ID: <CA+CK2bBA8=NHx0KtjsrOdFRCdQAo7ooDaw4OBj1HWr4doZh0PA@mail.gmail.com>
Date: Thu, 22 Jan 2026 13:01:54 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Hugh Dickins <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] mm: memfd_luo: use memfd_alloc_file() instead of shmem_file_setup()

On Thu, Jan 22, 2026 at 10:19 AM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> From: "Pratyush Yadav (Google)" <pratyush@...nel.org>
>
> When restoring a memfd, the file is created using shmem_file_setup().
> While memfd creation also calls this function to get the file, it also
> does other things:
>
>   1. The O_LARGEFILE flag is set on the file. If this is not done,
>   writes on the memfd exceeding 2 GiB fail.
>
>   2. FMODE_LSEEK, FMODE_PREAD, and FMODE_PWRITE are set on the file.
>   This makes sure the file is seekable and can be used with pread() and
>   pwrite().
>
>   3. Initializes the security field for the inode and makes sure that
>   inode creation is permitted by the security module.
>
> Currently, none of those things are done. This means writes above 2 GiB
> fail, pread(), and pwrite() fail, and so on. lseek() happens to work
> because file_init_path() sets it because shmem defines fop->llseek.
>
> Fix this by using memfd_alloc_file() to get the file to make sure the
> initialization sequence for normal and preserved memfd is the same.
>
> Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
> Signed-off-by: Pratyush Yadav (Google) <pratyush@...nel.org>
> ---
>  mm/memfd_luo.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
> index 4f6ba63b4310..01a72e4d3ef6 100644
> --- a/mm/memfd_luo.c
> +++ b/mm/memfd_luo.c
> @@ -78,6 +78,7 @@
>  #include <linux/liveupdate.h>
>  #include <linux/shmem_fs.h>
>  #include <linux/vmalloc.h>
> +#include <linux/memfd.h>
>  #include "internal.h"
>
>  static int memfd_luo_preserve_folios(struct file *file,
> @@ -443,8 +444,7 @@ static int memfd_luo_retrieve(struct liveupdate_file_op_args *args)
>         if (!ser)
>                 return -EINVAL;
>
> -       file = shmem_file_setup("", 0, VM_NORESERVE);
> -
> +       file = memfd_alloc_file("", 0);


Reviewed-by: Pasha Tatashin <pasha.tatashin@...een.com>