Re: Tmpfs size accounting misses memory allocations

[Hugh Dickins <hughd@google.com>]

On Sat, 25 Apr 2020, Topi Miettinen wrote:
> Hi,
> 
> It seems that tmpfs does not count memory which is allocated for short
> symlinks or xattrs towards size= limit.

Yes, you are right. And that is why tmpfs does not (so far) support
user xattrs, because the unprivileged user could take up too much
memory that way.

> I guess the fix would be to change
> shmem_sb_info->{used_blocks,max_blocks} to use bytes as units (instead of
> blocks) and then add accounting and checks to shmem_symlink() and
> shmem_initxattrs(). Would a patch for that be acceptable?

Thank you for offering, but I don't think a patch for exactly that would
be acceptable. Because "size=" has just been another way of expressing
"nr_blocks=" ever since it was added in 2.4.4, and tmpfs has never
counted the kernel metadata towards its data blocks limit.

You could certainly argue that it should have done from the start; but
in order to keep the accounting suitably simple (pages rather than bytes)
it never did. And I believe there are many users who expect a tmpfs of a
certain size to be able to accommodate data of that size, who would not
care to have to change their scripts or apps to meet a lower limitation.

> 
> Another issue is that inodes aren't counted towards size= limit either, but
> perhaps that's intentional because there's nr_inodes= mount option for
> exactly that.

Yes, tmpfs lets the nr_inodes limit be used to constrain the kernel
metadata (and tmpfs has a peculiarity, that it actually counts hard
links out of nr_inodes, in order to limit the memory spent on dentries).

I doubt the nr_inodes limit is depended upon so critically as the
nr_blocks, and I think we might extend it (say, consider each 1 of
nr_inodes to grant approximately 1kB of unswappable lowmem metadata)
to enable limited user xattrs: a patch along those lines might well
be acceptable.

> 
> If these are not bugs but intentional features, I think the manual page
> tmpfs(5) should be clearer in this respect.

Nobody has asked for that before, it seems to have met expectations as is.
But a sentence could be added to the manpage: do you have wording in mind?

Thanks,
Hugh