Re: [PATCH] shmem: recalculate file inode when fstat

Hugh Dickins
2015-08-17T23:44:36Z

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 This makes fstat sometimes return out-of-date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