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Date: Mon, 17 Aug 2015 16:43:13 -0700 (PDT)
From: Hugh Dickins <hughd@google.com>
X-X-Sender: hugh@eggly.anvils
To: Yu Zhao <yuzhao@google.com>
cc: Andrew Morton <akpm@linux-foundation.org>, Hugh Dickins <hughd@google.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] shmem: recalculate file inode when fstat
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On Fri, 10 Jul 2015, Yu Zhao wrote:

> Shmem uses shmem_recalc_inode to update i_blocks when it allocates
> page, undoes range or swaps. But mm can drop clean page without
> notifying shmem. This makes fstat sometimes return out-of-date
> block size.
> 
> The problem can be partially solved when we add
> inode_operations->getattr which calls shmem_recalc_inode to update
> i_blocks for fstat.
> 
> shmem_recalc_inode also updates counter used by statfs and
> vm_committed_as. For them the situation is not changed. They still
> suffer from the discrepancy after dropping clean page and before
> the function is called by aforementioned triggers.

"partially" indeed.

Thanks, your patch is sensible in itself, though nobody cared before;
and I hope nobody is fooled by your improvement into thinking that
shmem_recalc_inode() does a very good job.

I looked once again into how to improve the situation, but yet again
had to retreat for now: as I found before, mapping->a_ops->freepage
almost does what's needed, but is frustratingly not quite usable.

So let's go with your patch: I'll add my signoff and send on to
akpm for 4.3.

Hugh

> 
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  mm/shmem.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 4caf8ed..37e7933 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -542,6 +542,21 @@ void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
>  }
>  EXPORT_SYMBOL_GPL(shmem_truncate_range);
>  
> +static int shmem_getattr(struct vfsmount *mnt, struct dentry *dentry,
> +			 struct kstat *stat)
> +{
> +	struct inode *inode = dentry->d_inode;
> +	struct shmem_inode_info *info = SHMEM_I(inode);
> +
> +	spin_lock(&info->lock);
> +	shmem_recalc_inode(inode);
> +	spin_unlock(&info->lock);
> +
> +	generic_fillattr(inode, stat);
> +
> +	return 0;
> +}
> +
>  static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
>  {
>  	struct inode *inode = d_inode(dentry);
> @@ -3122,6 +3137,7 @@ static const struct file_operations shmem_file_operations = {
>  };
>  
>  static const struct inode_operations shmem_inode_operations = {
> +	.getattr	= shmem_getattr,
>  	.setattr	= shmem_setattr,
>  #ifdef CONFIG_TMPFS_XATTR
>  	.setxattr	= shmem_setxattr,
> -- 
> 2.4.3.573.g4eafbef