linux-kernel - Re: [PATCH] fuse: change fuse_wr_pages() to avoid signedness error from min()

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Message-ID: <CAJnrk1Zm7+-ha-Oyfamm0D1nEtzmYqP6cDF_mc7JftqWmENewg@mail.gmail.com>
Date: Fri, 19 Dec 2025 11:24:23 +0800
From: Joanne Koong <joannelkoong@...il.com>
To: david.laight.linux@...il.com
Cc: Bernd Schubert <bschubert@....com>, Miklos Szeredi <mszeredi@...hat.com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] fuse: change fuse_wr_pages() to avoid signedness error
 from min()

On Wed, Dec 17, 2025 at 12:22 AM <david.laight.linux@...il.com> wrote:
>
> From: David Laight <david.laight.linux@...il.com>
>
> On 32bit builds the 'number of pages required' calculation is signed
> and min() complains because max_pages is unsigned.
> Change the calcualtion that determines the number of pages by adding the
> 'offset in page' to 'len' rather than subtracting the end and start pages.
> Although the 64bit value is still signed, the compiler knows it isn't
> negative so min() doesn't complain.
> The generated code is also slightly better.
>
> Forcing the calculation to 32 bits (eg len + (size_t)(pos & ...))
> generates much better code and is probably safe because len should
> be limited to 'INT_MAX - PAGE_SIZE).
>
> Fixes: 0f5bb0cfb0b4 ("fs: use min() or umin() instead of min_t()")
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202512160948.O7QqxHj2-lkp@intel.com/
> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
>  fs/fuse/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 4f71eb5a9bac..98edb6a2255d 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1323,7 +1323,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_io_args *ia,
>  static inline unsigned int fuse_wr_pages(loff_t pos, size_t len,
>                                      unsigned int max_pages)
>  {
> -       return min(((pos + len - 1) >> PAGE_SHIFT) - (pos >> PAGE_SHIFT) + 1,
> +       return min(((len + (pos & (PAGE_SIZE - 1)) - 1) >> PAGE_SHIFT) + 1,
>                    max_pages);

I find this logic a bit confusing to read still, what about something like:

unsigned int nr_pages = DIV_ROUND_UP(offset_in_page(pos) + len, PAGE_SIZE);
return min(nr_pages, max_pages);

instead? I think the compiler will automatically optimize the
DIV_ROUND_UP to use a bit shift.

Thanks,
Joanne
>  }
>
> --
> 2.39.5
>
>