linux-kernel - Re: [PATCH v2 1/2] procfs: Add 'size' to /proc/<pid>/fdinfo/

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Message-ID: <Yrrrz7MxMu8OoEPU@bfoster>
Date:   Tue, 28 Jun 2022 07:53:51 -0400
From:   Brian Foster <bfoster@...hat.com>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     ckoenig.leichtzumerken@...il.com, christian.koenig@....com,
        viro@...iv.linux.org.uk, hch@...radead.org,
        stephen.s.brennan@...cle.com, David.Laight@...lab.com,
        ilkos@...gle.com, tjmercier@...gle.com, surenb@...gle.com,
        kernel-team@...roid.com, Jonathan Corbet <corbet@....net>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Christoph Anton Mitterer <mail@...istoph.anton.mitterer.name>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Mike Rapoport <rppt@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v2 1/2] procfs: Add 'size' to /proc/<pid>/fdinfo/

On Thu, Jun 23, 2022 at 03:06:06PM -0700, Kalesh Singh wrote:
> To be able to account the amount of memory a process is keeping pinned
> by open file descriptors add a 'size' field to fdinfo output.
> 
> dmabufs fds already expose a 'size' field for this reason, remove this
> and make it a common field for all fds. This allows tracking of
> other types of memory (e.g. memfd and ashmem in Android).
> 
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> Reviewed-by: Christian König <christian.koenig@....com>
> ---
> 
> Changes in v2:
>   - Add Christian's Reviewed-by
> 
> Changes from rfc:
>   - Split adding 'size' and 'path' into a separate patches, per Christian
>   - Split fdinfo seq_printf into separate lines, per Christian
>   - Fix indentation (use tabs) in documentaion, per Randy
> 
>  Documentation/filesystems/proc.rst | 12 ++++++++++--
>  drivers/dma-buf/dma-buf.c          |  1 -
>  fs/proc/fd.c                       |  9 +++++----
>  3 files changed, 15 insertions(+), 7 deletions(-)
> 
...
> diff --git a/fs/proc/fd.c b/fs/proc/fd.c
> index 913bef0d2a36..464bc3f55759 100644
> --- a/fs/proc/fd.c
> +++ b/fs/proc/fd.c
> @@ -54,10 +54,11 @@ static int seq_show(struct seq_file *m, void *v)
>  	if (ret)
>  		return ret;
>  
> -	seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\nino:\t%lu\n",
> -		   (long long)file->f_pos, f_flags,
> -		   real_mount(file->f_path.mnt)->mnt_id,
> -		   file_inode(file)->i_ino);
> +	seq_printf(m, "pos:\t%lli\n", (long long)file->f_pos);
> +	seq_printf(m, "flags:\t0%o\n", f_flags);
> +	seq_printf(m, "mnt_id:\t%i\n", real_mount(file->f_path.mnt)->mnt_id);
> +	seq_printf(m, "ino:\t%lu\n", file_inode(file)->i_ino);
> +	seq_printf(m, "size:\t%lli\n", (long long)file_inode(file)->i_size);

Hi Kalesh,

Any reason not to use i_size_read() here?

Also not sure if it matters that much for your use case, but something
worth noting at least with shmem is that one can do something like:

# cat /proc/meminfo | grep Shmem:
Shmem:               764 kB
# xfs_io -fc "falloc -k 0 10m" ./file
# ls -alh file 
-rw-------. 1 root root 0 Jun 28 07:22 file
# stat file 
  File: file
  Size: 0               Blocks: 20480      IO Block: 4096   regular empty file
# cat /proc/meminfo | grep Shmem:
Shmem:             11004 kB

... where the resulting memory usage isn't reflected in i_size (but is
is in i_blocks/bytes).

Brian

>  
>  	/* show_fd_locks() never deferences files so a stale value is safe */
>  	show_fd_locks(m, file, files);
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog
>