Re: Tmpfs size accounting misses memory allocations

[Topi Miettinen <toiwoton@gmail.com>]

On 1.5.2020 6.14, Hugh Dickins wrote:
> On Tue, 28 Apr 2020, Topi Miettinen wrote:
>> On 28.4.2020 4.34, Hugh Dickins wrote:
>>> 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.
>>
>> I'm interested in restricting the amount of memory allocated to tmpfs mounts
>> in the system rather than granting more. I've seen a system lock up because
>> tmpfs mounts consumed the entire memory. Possible contributing factors could
>> be use of LVM and encryption for the swap.
> 
> Yes, it is too easy to get into a terrible state that way.  With OOM
> killer doing no good at all, because it's busy killing processes, which
> does nothing to free the memory held by tmpfs files.  I've never found
> a good answer to that in general, though marking files as suitable for
> truncation on OOM has been useful in special cases.

It seems that similar state can be reached also when an unprivileged 
process allocates and maps lots of SysV shm (if the limit isn't set, 
which seems to be the case at least for Debian).

-Topi