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Date:   Mon, 18 Oct 2021 18:22:28 -0700
From:   Rustam Kovhaev <rkovhaev@gmail.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     cl@linux.com, penberg@kernel.org, rientjes@google.com,
        iamjoonsoo.kim@lge.com, akpm@linux-foundation.org,
        djwong@kernel.org, david@fromorbit.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        gregkh@linuxfoundation.org, viro@zeniv.linux.org.uk,
        dvyukov@google.com
Subject: Re: [PATCH] slob: add size header to all allocations
Message-ID: <YW4d1FppmaqAuPHd@nuc10>
References: <20211015005729.GD24333@magnolia>
 <20211018033841.3027515-1-rkovhaev@gmail.com>
 <20758764-8139-ab0b-a782-dc63559b43ba@suse.cz>
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On Mon, Oct 18, 2021 at 11:22:46AM +0200, Vlastimil Babka wrote:
> On 10/18/21 05:38, Rustam Kovhaev wrote:
> > Let's prepend all  allocations of (PAGE_SIZE - align_offset) and less
> > with the size header. This way kmem_cache_alloc() memory can be freed
> > with kfree() and the other way around, as long as they are less than
> > (PAGE_SIZE - align_offset).
> 
> This size limitation seems like an unnecessary gotcha. Couldn't we make
> these large allocations in slob_alloc_node() (that use slob_new_pages()
> directly) similar enough to large kmalloc() ones, so that kfree() can
> recognize them and free properly? AFAICS it might mean just adding
> __GFP_COMP to make sure there's a compound order stored, as these already
> don't seem to set PageSlab.

Thanks for the pointers, I'll send a new version.

> > The main reason for this change is to simplify SLOB a little bit, make
> > it a bit easier to debug whenever something goes wrong.
> 
> I would say the main reason is to simplify the slab API and guarantee that
> both kmem_cache_alloc() and kmalloc() can be freed by kfree().
> 
> We should also update the comments at top of slob.c to reflect the change.
> And Documentation/core-api/memory-allocation.rst (the last paragraph).

OK, thank you!

> > meminfo right after the system boot, without the patch:
> > Slab:              35500 kB
> > 
> > the same, with the patch:
> > Slab:              36396 kB
> 
> 2.5% increase, hopefully acceptable.
> 
> Thanks!