Re: [PATCH v4] memcg: add charging of already allocated slab objects

[Vlastimil Babka <vbabka@suse.cz>]

On 9/6/24 19:19, Yosry Ahmed wrote:
> [..]
>> I felt it could be improved more, so ended up with this. Thoughts?
>>
>> /**
>>  * kmem_cache_charge - memcg charge an already allocated slab memory
>>  * @objp: address of the slab object to memcg charge
>>  * @gfpflags: describe the allocation context
>>  *
>>  * kmem_cache_charge allows charging a slab object to the current memcg,
>>  * primarily in cases where charging at allocation time might not be possible
>>  * because the target memcg is not known (i.e. softirq context)
>>  *
>>  * The objp should be pointer returned by the slab allocator functions like
>>  * kmalloc (with __GFP_ACCOUNT in flags) or kmem_cache_alloc. The memcg charge
> 
> Aren't allocations done with kmalloc(__GFP_ACCOUNT) already accounted?
> Why would we need to call kmem_cache_charge() for those?

AFAIU current_obj_cgroup() returns NULL because we're in the interrupt
context and no remote memcg context has been set. Thus the charging is
skipped. The patch commit log describes such scenario for network receive.
But in case of kmalloc() the allocation must have been still attempted with
__GFP_ACCOUNT so a kmalloc-cg cache is used even if the charging fails.

If there's another usage for kmem_cache_charge() where the memcg is
available but we don't want to charge immediately on purpose (such as the
Linus' idea for struct file), we might need to find another way to tell
kmalloc() to use the kmalloc-cg cache but not charge immediately...

> I am assuming what you are referring to is kmalloc() allocations that
> are not fulfilled from KMALLOC_NORMAL caches, but I am not sure how to
> capture this here.
> 
>>  * behavior can be controlled through gfpflags parameter, which affects how the
>>  * necessary internal metadata can be allocated. Including __GFP_NOFAIL denotes
>>  * that overcharging is requested instead of failure, but is not applied for the
>>  * internal metadata allocation.
>>  *
>>  * There are several cases where it will return true even if the charging was
>>  * not done:
>>  * More specifically:
>>  *
>>  * 1. For !CONFIG_MEMCG or cgroup_disable=memory systems.
>>  * 2. Already charged slab objects.
>>  * 3. For slab objects from KMALLOC_NORMAL caches - allocated by kmalloc()
>>  *    without __GFP_ACCOUNT
>>  * 4. Allocating internal metadata has failed
>>  *
>>  * Return: true if charge was successful otherwise false.
>>  */
>>
>> >> > +
>> >> > +       /* Ignore KMALLOC_NORMAL cache to avoid circular dependency. */
>> >>
>> >> Is it possible to point to the commit that has the explanation here?
>> >> The one you pointed me to before? Otherwise it's not really obvious
>> >> where the circular dependency comes from (at least to me).
>> >>
>> >
>> > Not sure about the commit reference. We can add more text here.
>> > Vlastimil, how much detail do you prefer?
>>
>> What about:
>>
>>         /*
>>          * Ignore KMALLOC_NORMAL cache to avoid possible circular dependency
>>          * of slab_obj_exts being allocated from the same slab and thus the slab
>>          * becoming effectively unfreeable.
>>          */
>>
>>
>> > thanks,
>> > Shakeel
>>