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Date: Wed, 27 May 2026 16:07:24 +0200
From: Christoph Hellwig <hch@lst.de>
To: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: Jesper Dangaard Brouer <hawk@kernel.org>,
	Christoph Hellwig <hch@lst.de>, Vlastimil Babka <vbabka@kernel.org>,
	Harry Yoo <harry@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hao Li <hao.li@linux.dev>, Christoph Lameter <cl@gentwo.org>,
	David Rientjes <rientjes@google.com>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	linux-arm-msm@vger.kernel.org, dri-devel@lists.freedesktop.org,
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	linux-mm@kvack.org, io-uring@vger.kernel.org,
	kasan-dev@googlegroups.com, bpf@vger.kernel.org,
	netdev@vger.kernel.org, Matt Fleming <mfleming@cloudflare.com>,
	kernel-team <kernel-team@cloudflare.com>
Subject: Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk
Message-ID: <20260527140724.GA13256@lst.de>
References: <20260527070239.2252948-1-hch@lst.de> <20260527070239.2252948-2-hch@lst.de> <e4dcfbc8-2666-452c-90b2-25c4b2c50c9f@kernel.org> <777c7229-4285-4c7c-9340-dfaebd2ab291@intel.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Wed, May 27, 2026 at 03:56:38PM +0200, Alexander Lobakin wrote:
> >> -    n -= kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
> >> -                   GFP_ATOMIC | __GFP_ZERO | __GFP_NOWARN,
> >> -                   n - nc->skb_count, &skbs[nc->skb_count]);
> >> +    if (kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
> >> +                  GFP_ATOMIC | __GFP_ZERO | __GFP_NOWARN,
> >> +                  n - nc->skb_count, &skbs[nc->skb_count]))
> >> +        n = nc->skb_count;
> 
> kmem_cache_alloc_bulk() allocates `n - nc->skb_count`, but here you
> assign `nc->skb_count` to n.
> Ah wait,
> 
> n -= n - nc->skb_count
> n = n - (n - nc->skb_count)
> n = n - n + nc->skb_count
> n = nc->skb_count
> 
> Correct :D

Exactly the steps I went through when writing this patch :)