Re: [PATCH] fs: inode: Reduce volatile inode wraparound risk when ino_t is 64 bit

[Chris Down <chris@chrisdown.name>]

Matthew Wilcox writes:
>On Fri, Dec 20, 2019 at 07:35:38PM +0200, Amir Goldstein wrote:
>> On Fri, Dec 20, 2019 at 6:46 PM Matthew Wilcox <willy@infradead.org> wrote:
>> >
>> > On Fri, Dec 20, 2019 at 03:41:11PM +0200, Amir Goldstein wrote:
>> > > Suggestion:
>> > > 1. Extend the kmem_cache API to let the ctor() know if it is
>> > > initializing an object
>> > >     for the first time (new page) or recycling an object.
>> >
>> > Uh, what?  The ctor is _only_ called when new pages are allocated.
>> > Part of the contract with the slab user is that objects are returned to
>> > the slab in an initialised state.
>>
>> Right. I mixed up the ctor() with alloc_inode().
>> So is there anything stopping us from reusing an existing non-zero
>> value of  i_ino in shmem_get_inode()? for recycling shmem ino
>> numbers?
>
>I think that would be an excellent solution to the problem!  At least,
>I can't think of any problems with it.

Thanks for the suggestions and feedback, Amir and Matthew :-)

The slab i_ino recycling approach works somewhat, but is unfortunately neutered 
quite a lot by the fact that slab recycling is per-memcg. That is, replacing 
with recycle_or_get_next_ino(old_ino)[0] for shmfs and a few other trivial 
callsites only leads to about 10% slab reuse, which doesn't really stem the 
bleeding of 32-bit inums on an affected workload:

     # tail -5000 /sys/kernel/debug/tracing/trace | grep -o 'recycle_or_get_next_ino:.*' | sort | uniq -c
         4454 recycle_or_get_next_ino: not recycled
          546 recycle_or_get_next_ino: recycled

Roman (who I've just added to cc) tells me that currently we only have 
per-memcg slab reuse instead of global when using CONFIG_MEMCG. This 
contributes fairly significantly here since there are multiple tasks across 
multiple cgroups which are contributing to the get_next_ino() thrash.

I think this is a good start, but we need something of a different magnitude in 
order to actually solve this problem with the current slab infrastructure. How 
about something like the following?

1. Add get_next_ino_full, which uses whatever the full width of ino_t is
2. Use get_next_ino_full in tmpfs (et al.)
3. Add a mount option to tmpfs (et al.), say `32bit-inums`, which people can 
    pass if they want the 32-bit inode numbers back. This would still allow 
    people who want to make this tradeoff to use xino.
4. (If you like) Also add a CONFIG option to disable this at compile time.

I'd appreciate your thoughts on that approach or others you have ideas about. 
Thanks! :-)

0:

unsigned int recycle_or_get_next_ino(ino_t old_ino)
{
	/*
	 * get_next_ino returns unsigned int. If this fires then i_ino must be
	 * >32 bits and have been changed later, so the caller shouldn't be
	 * recycling inode numbers
	 */
	WARN_ONCE(old_ino >> (sizeof(unsigned int) * 8),
		  "Recyclable i_ino uses more bits than unsigned int: %llu",
		  (u64)old_ino);

	if (old_ino) {
		if (prandom_u32() % 100 == 0)
			trace_printk("recycled\n");
		return old_ino;
	} else {
		if (prandom_u32() % 100 == 0)
			trace_printk("not recycled\n");
		return get_next_ino();
	}
}