On Aug 27, 2018, at 6:56 AM, Amir Goldstein <amir73il@gmail.com> wrote:
> 
> Implement stacked fadvise to fix syscalls readahead(2) and fadvise64(2)
> on an overlayfs file.

I was just looking into the existence of the "new" fadvise() method in
the VFS being able to communicate application hints directly to the
filesystem to see if it could be used to address the word size issue in
https://bugzilla.kernel.org/show_bug.cgi?id=205957 without adding a new
syscall, and came across this patch and the 4/6 patch that adds the
vfs_fadvise() function itself (copied below for clarity).

It seems to me that this implementation is broken?  Only vfs_fadvise()
is called from the fadvise64() syscall, and it will call f_op->fadvise()
if the filesystem provides this method.  Only overlayfs provides the
.fadvise method today.  However, it looks that ovl_fadvise() calls back
into vfs_fadvise() again, in a seemingly endless loop?

It seems like generic_fadvise() should be EXPORT_SYMBOL() so that any
filesystem that implements its own .fadvise method can do its own thing,
and then call generic_fadvise() to handle the remaining MM-specific work.

Thoughts?

> +int vfs_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
> +{
> +	if (file->f_op->fadvise)
> +		return file->f_op->fadvise(file, offset, len, advice);
> +
> +	return generic_fadvise(file, offset, len, advice);
> +}
> +EXPORT_SYMBOL(vfs_fadvise);
> 
> +int ovl_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
> +{
> +	struct fd real;
> +	int ret;
> +
> +	ret = ovl_real_fdget(file, &real);
> +	if (ret)
> +		return ret;
> +
> +	/* XXX: do we need mounter credentials? */
> +	ret = vfs_fadvise(real.file, offset, len, advice);
> +
> +	fdput(real);
> +
> +	return ret;
> +}

Cheers, Andreas