Re: [PATCH 4.9 0/1] selinux: drop super_block backpointer from superblock_security_struct

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Aug 11, 2022 at 01:04:35PM +0200, Alexander Grund wrote:
> On 11.08.22 12:21, Greg KH wrote:
> > On Thu, Aug 11, 2022 at 11:46:09AM +0200, Alexander Grund wrote:
> >> I mean this patch removes a superflous pointer of the superblock struct
> >> making the kernel use less memory.
> > 
> > Also, how much measurable memory does this save?  You did not quantify
> > it.
> 
> It saves one pointer, i.e. usually 8 Byte, per superblock when using SELinux.
> 
> I don't know how many of those superblocks will be allocated in total on usual systems
> as I'm not familiar with the details of the filesystems.
> However following one callchain leads to
> 
> > /*
> > * Common helper for pseudo-filesystems (sockfs, pipefs, bdev - stuff that
> > * will never be mountable)
> > */
> > struct dentry *mount_pseudo_xattr(struct file_system_type *fs_type, char *name,
> 
> So it seems one superblock even for each pseudo-fs of which there can be many.
> 
> A quick experiment [1] on my phone shows about 300 superblocks allocated which means
> that the patch saves a bit over 2kB of memory.
> So not that much on usual systems but could be much for some embedded systems.

Again, we are not adding "slim the kernel down by an infinitesimal %"
patches to older stable kernels, especially ones that only have a few
more months to live.  Let's stick with real bugfixes please, that's what
matters here.

thanks,

greg k-h