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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 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 X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 360531C0002 X-Stat-Signature: 9y7knzmx84piwmrngogumoob6pwcyjd4 X-Rspam-User: X-HE-Tag: 1780478271-957904 X-HE-Meta: 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 DKcG68Hs nTr2+Z2AXc3XYq5qTT/eOO25gCbyz0SJrL/QPAw2rFetD5js/01//7M5wDQeVXslaeodgrR/ajH7DfMoqRP4HuDVSK/KsIN1pDZ4XwW5YrAiph0kFuyiKxOzOygTAwfSdhF/84zp/4dtC61YCbXZVbBdJ/RWh2ZJqTjHmb+i+T6Rgd6jpuQRUjvuETmrBNtgqquI2fU+X7UT2T3Um9IYZ6SzYl1TJ8pCibuCiZTIt0QjGvw88MAqsbeMRT3TiLvPNVtSjMuSqEaxpsVyojZmvQUbQ+lhdUmZprsFfi9NJnLm5cFy/iiyQ5xRJxnsJpUtTo8Mu3O7FP58RVnl/gWqF896Tk57t91Sutv+QvCHMfBr/VfM5k4EqN3SiDN+rYHfAQbYk Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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;