Re: [PATCH v2 06/11] memcg: kmem controller infrastructure

[Glauber Costa <glommer@parallels.com>]

>> + * memcg_kmem_new_page: verify if a new kmem allocation is allowed.
>> + * @gfp: the gfp allocation flags.
>> + * @handle: a pointer to the memcg this was charged against.
>> + * @order: allocation order.
>> + *
>> + * returns true if the memcg where the current task belongs can hold this
>> + * allocation.
>> + *
>> + * We return true automatically if this allocation is not to be accounted to
>> + * any memcg.
>> + */
>> +static __always_inline bool
>> +memcg_kmem_new_page(gfp_t gfp, void *handle, int order)
>> +{
>> +	if (!memcg_kmem_on)
>> +		return true;
>> +	if (!(gfp & __GFP_KMEMCG) || (gfp & __GFP_NOFAIL))
> 
> OK, I see the point behind __GFP_NOFAIL but it would deserve a comment
> or a mention in the changelog.

documentation can't hurt!

Just added.

> [...]
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 54e93de..e9824c1 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
> [...]
>> +EXPORT_SYMBOL(__memcg_kmem_new_page);
> 
> Why is this exported?
> 

It shouldn't be. Removed.

>> +
>> +void __memcg_kmem_commit_page(struct page *page, void *handle, int order)
>> +{
>> +	struct page_cgroup *pc;
>> +	struct mem_cgroup *memcg = handle;
>> +
>> +	if (!memcg)
>> +		return;
>> +
>> +	WARN_ON(mem_cgroup_is_root(memcg));
>> +	/* The page allocation must have failed. Revert */
>> +	if (!page) {
>> +		size_t size = PAGE_SIZE << order;
>> +
>> +		memcg_uncharge_kmem(memcg, size);
>> +		mem_cgroup_put(memcg);
>> +		return;
>> +	}
>> +
>> +	pc = lookup_page_cgroup(page);
>> +	lock_page_cgroup(pc);
>> +	pc->mem_cgroup = memcg;
>> +	SetPageCgroupUsed(pc);
> 
> Don't we need a write barrier before assigning memcg? Same as
> __mem_cgroup_commit_charge. This tests the Used bit always from within
> lock_page_cgroup so it should be safe but I am not 100% sure about the
> rest of the code.
> 
Well, I don't see the reason, precisely because we'll always grab it
from within the locked region. That should ensure all the necessary
serialization.

>> +#ifdef CONFIG_MEMCG_KMEM
>> +int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp, s64 delta)
>> +{
>> +	struct res_counter *fail_res;
>> +	struct mem_cgroup *_memcg;
>> +	int ret;
>> +	bool may_oom;
>> +	bool nofail = false;
>> +
>> +	may_oom = (gfp & __GFP_WAIT) && (gfp & __GFP_FS) &&
>> +	    !(gfp & __GFP_NORETRY);
> 
> This deserves a comment.
> 
can't hurt!! =)

>> +
>> +	ret = 0;
>> +
>> +	if (!memcg)
>> +		return ret;
>> +
>> +	_memcg = memcg;
>> +	ret = __mem_cgroup_try_charge(NULL, gfp, delta / PAGE_SIZE,
>> +	    &_memcg, may_oom);
> 
> This is really dangerous because atomic allocation which seem to be
> possible could result in deadlocks because of the reclaim. 

Can you elaborate on how this would happen?

> Also, as I
> have mentioned in the other email in this thread. Why should we reclaim
> just because of kernel allocation when we are not reclaiming any of it
> because shrink_slab is ignored in the memcg reclaim.


Don't get too distracted by the fact that shrink_slab is ignored. It is
temporary, and while this being ignored now leads to suboptimal
behavior, it will 1st, only affect its users, and 2nd, not be disastrous.

I see it this as more or less on pair with the soft limit reclaim
problem we had. It is not ideal, but it already provided functionality

>> +
>> +	if (ret == -EINTR)  {
>> +		nofail = true;
>> +		/*
>> +		 * __mem_cgroup_try_charge() chosed to bypass to root due to
>> +		 * OOM kill or fatal signal.  Since our only options are to
>> +		 * either fail the allocation or charge it to this cgroup, do
>> +		 * it as a temporary condition. But we can't fail. From a
>> +		 * kmem/slab perspective, the cache has already been selected,
>> +		 * by mem_cgroup_get_kmem_cache(), so it is too late to change
>> +		 * our minds
>> +		 */
>> +		res_counter_charge_nofail(&memcg->res, delta, &fail_res);
>> +		if (do_swap_account)
>> +			res_counter_charge_nofail(&memcg->memsw, delta,
>> +						  &fail_res);
> 
> Hmmm, this is kind of ugly but I guess unvoidable with the current
> implementation. Oh well...
> 

Oh well...

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