From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8318E1B4F1F; Wed, 3 Jun 2026 09:17:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780478271; cv=none; b=bzpJHoCSVyOQlDObniTeEE/npX3GiLR2UpJRT18HatqEnLe0TexNdhbNAVywx4IZrCYgFDMTo+Pwp9v1d5bs/+JBThszLbAsEyEHZYsQ2Q8WUgMKzvOWhBtHWU/1KyJWu754Yrct9quPz0paggX8JE8TNn76GtPeHbx5Ky/M+Og= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780478271; c=relaxed/simple; bh=D9Yc2SINKmQ2h2akpVdRq5/TomrG4EGKZcHVrLMBdtU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=XLuyz6wxQQV/LHNy8GpnIKajYgf0zxhQUhRlKvZVVIerkPaOttrorCED4GRI2pJ2lFQsDgyt/fi05FLc6gbgLzh74VTC5uf2q8dk7S4sVWG3/mQAEvtxpxQuH5c7M65junergJRW7DCnXDNX98HQjzbKBTM03kCgIqqaQY0Ff8I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NkYh6RYz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="NkYh6RYz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 491851F00893; Wed, 3 Jun 2026 09:17:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780478270; bh=M0pUtaR4+MO5aeujygdu2I6w5ZAQoI82CxqwwUxfoiE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=NkYh6RYztD/v9Cd5qUUF+73iEm/9P8Dxg515rIIt8cpNkQXn0uWsbSjzZ94uOOdJT 8lr2VDF/pFtEj8vN8v2caT9WCT2G9Z4Rq6UiHuCW/5CM0K/fYFpJlmzxzzvELk0/uq BVOLqgNF9knDhwESnGP5ZFMste8UryYsCYqyJ7VFjS0OmKQkf1uy9VheypRjnl2pvZ xlHfC6AbaKXzeHOGbNyuhpkqOZUsbCol0WdydURkXHIocM5fXBl244JSyT3vk5npBp gRrddaqENxSHYNaMAO7aZvprpMklJyWR7Frr7S1uw1kJfHZkXs5zXbnpDTfZihHg04 6J50JPcqEnSKA== Message-ID: <5e6948b3-d235-4b61-aed7-e8b4d0f5b452@kernel.org> Date: Wed, 3 Jun 2026 11:17:44 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk Content-Language: en-US To: rob.clark@oss.qualcomm.com Cc: Christoph Hellwig , Harry Yoo , Andrew Morton , Mark Brown , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Jesper Dangaard Brouer , linux-arm-msm@vger.kernel.org, dri-devel@lists.freedesktop.org, freedreno@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, io-uring@vger.kernel.org, kasan-dev@googlegroups.com, bpf@vger.kernel.org, netdev@vger.kernel.org, Alexander Lobakin , Boris Brezillon References: <20260528093437.2519248-1-hch@lst.de> <20260528093437.2519248-2-hch@lst.de> <5f7f90d8-cb32-4ffb-8f1c-0722aafbe869@kernel.org> <20260529135045.GA10647@lst.de> <5f3ba603-a6ad-4cf2-9a54-aebc10273c59@kernel.org> <58cc76e7-2348-443d-a989-2a06e61178af@kernel.org> <20260601113831.GA25535@lst.de> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 6/1/26 16:39, Rob Clark wrote: > On Mon, Jun 1, 2026 at 6:32 AM Rob Clark wrote: >> >> On Mon, Jun 1, 2026 at 5:50 AM Vlastimil Babka (SUSE) wrote: >> > >> > On 6/1/26 13:38, Christoph Hellwig wrote: >> > > On Mon, Jun 01, 2026 at 10:16:30AM +0200, Vlastimil Babka (SUSE) wrote: >> > >> > kmem_cache_alloc_bulk() returning 0 was considered a success in that case. >> > >> > >> > >> > Either fixing kmem_cache_alloc_bulk() (and the comment) or fixing the >> > >> > user sounds fine to me. >> > >> >> > >> Would it be wrong if we just returned true for size of 0? Would something >> > >> else break? >> > > >> > > I don't think it is wrong per se, but it feels like the wrong kind of >> > > API. I.e. I don't think the MSM caller actually wants this, as they'd >> > > also do a zero-sized kvmalloc. >> > >> > If p->count is 0 then indeed there's a zero-sized kvmalloc so p->pages == >> > ZERO_SIZE_PTR but then nothing breaks because nothing tries to dereference it? >> > >> > msm_iommu_pagetable_prealloc_cleanup() has a "if (p->count > 0)" branch so >> > it seems it's considered possible. But then the rest of the functions also >> > seems working fine, i.e. kmem_cache_free_bulk() of zero size does nothing, >> > kvfree() of ZERO_SIZE_PTR does nothing. >> > >> > It seems to me kmem_cache_alloc_bulk() returning true for size == 0 fits >> > naturally in this world and is less likely to result in a gotcha? >> >> I think I was probably expecting kvmalloc(0) => NULL ... but it >> happened to work out before >> >> Adding an "if (!p->count) return 0;" at the top of >> msm_iommu_pagetable_prealloc_allocate() seems like the thing to do.. >> if you want, I can send that patch (but traveling this week... so >> let's see what I can do) > > Aaaaaand.. sending patch from hotel wifi doesn't seem to be a thing > that works.. but I've tested the following w/ deqp-vk cts, and works > as expected Thanks, I will amend the commit in slab tree with this as the easiest way to go foward. Just a quick question though: > ---------------- > > diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c > index 7d449e5202c5..ef744d154bbe 100644 > --- a/drivers/gpu/drm/msm/msm_iommu.c > +++ b/drivers/gpu/drm/msm/msm_iommu.c > @@ -332,13 +332,16 @@ msm_iommu_pagetable_prealloc_allocate(struct > msm_mmu *mmu, struct msm_mmu_preall > struct kmem_cache *pt_cache = get_pt_cache(mmu); > int ret; > > + if (!p->count) > + return 0; We know p->pages is NULL in this case, right? Because it was allocated by vm_bind_job_create() using kzalloc(). And the job can't be reused with a leftover value? (msm_iommu_pagetable_prealloc_cleanup doesn't set p->pages to zero). Or should we set p->pages to NULL here. > + > p->pages = kvmalloc_objs(*p->pages, p->count); > if (!p->pages) > return -ENOMEM; > > ret = kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, p->count, p->pages); > if (ret != p->count) { > - kfree(p->pages); > + kvfree(p->pages); > p->pages = NULL; > p->count = ret; > return -ENOMEM;