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

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Message-ID: <CAJD7tkZ+PYqvq6oUHtrtq1JE670A+kUBcOAbtRVudp1JBPkCwA@mail.gmail.com>
Date: Fri, 6 Sep 2024 10:19:16 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	David Rientjes <rientjes@...gle.com>, Hyeonggon Yoo <42.hyeyoo@...il.com>, 
	Eric Dumazet <edumazet@...gle.com>, "David S . Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>, 
	cgroups@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v4] memcg: add charging of already allocated slab objects

[..]
> 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?

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
>