linux-kernel - Re: [PATCH] slob: add size header to all allocations

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Date:   Mon, 18 Oct 2021 18:22:28 -0700
From:   Rustam Kovhaev <rkovhaev@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        djwong@...nel.org, david@...morbit.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk,
        dvyukov@...gle.com
Subject: Re: [PATCH] slob: add size header to all allocations

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!