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Wed, 28 Oct 2026 03:47:01 -0700 (PDT) Received: from [192.168.1.22] ([83.231.69.9]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3ecd39a9sm28645811f8f.46.2026.10.28.03.47.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 Oct 2026 03:47:00 -0700 (PDT) Message-ID: Date: Wed, 28 Oct 2026 11:46:59 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm/vmalloc: drop VM_DMA_COHERENT from remap_vmalloc_range_partial To: Ye Liu , Andrew Morton , Uladzislau Rezki Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ye Liu References: <20261026-vmalloc_remap-v1-0-b7a907814e90@linux.dev> <20261026-vmalloc_remap-v1-1-b7a907814e90@linux.dev> Content-Language: en-US From: "Jose A. Perez de Azpillaga" In-Reply-To: <20261026-vmalloc_remap-v1-1-b7a907814e90@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/26/26 7:09 AM, Ye Liu wrote: > From: Ye Liu > > No caller maps a VM_DMA_COHERENT area through this path: such areas > come from dma_alloc_coherent() internals and are freed via vunmap(), > never through remap_vmalloc_range[_partial](); all 13 callers pass > vmalloc_user()/vmalloc()-backed addresses. > > The flag was added in commit fe9041c245196 ("vmalloc: lift the arm > flag for coherent mappings to common code") as a mechanical > translation of ARM's VM_ARM_DMA_CONSISTENT. It is also unsafe to keep > since commit 5ea8ec74c57c0 ("mm/vmalloc: free unused pages on > vrealloc() shrink"), which frees tail pages of non-VM_USERMAP areas > (VM_DMA_COHERENT is not in that skip-list): a shrunk-then-mapped area > would hit vmalloc_to_page()==NULL and vm_insert_page() would > dereference it unchecked. this isn't quite true, is it? the dma remap helpers vmap() without VM_MAP_PUT_PAGES, so nr_pages stays 0 and the shrink never frees anything there. > > Drop it so DMA-coherent areas can never enter the path. > > Signed-off-by: Ye Liu > --- > mm/vmalloc.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > index db669103dc66..a888eca016a8 100644 > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -4894,7 +4894,7 @@ int remap_vmalloc_range_partial(struct vm_area_struct *vma, unsigned long uaddr, > if (!area) > return -EINVAL; > > - if (!(area->flags & (VM_USERMAP | VM_DMA_COHERENT))) > + if (!(area->flags & VM_USERMAP)) > return -EINVAL; > > if (check_add_overflow(size, off, &end_index) || > the change itself looks fine to me. -- cheers, jose a. p-a