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[2003:cb:c738:3100:8133:26cf:7877:94aa]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a8e383654sm15703698f8f.30.2025.01.14.09.23.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Jan 2025 09:23:14 -0800 (PST) Message-ID: <3db5a5e8-62f3-4915-9fae-49a7f88952c9@redhat.com> Date: Tue, 14 Jan 2025 18:23:12 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] /dev/zero: make private mapping full anonymous mapping To: Yang Shi , Lorenzo Stoakes Cc: arnd@arndb.de, gregkh@linuxfoundation.org, Liam.Howlett@oracle.com, vbabka@suse.cz, jannh@google.com, willy@infradead.org, liushixin2@huawei.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250113223033.4054534-1-yang@os.amperecomputing.com> <08e7f845-6b86-4fdc-830d-23f15ad90475@redhat.com> <8e88bce5-3be3-4e84-a327-841c91906a79@lucifer.local> <75dbebfa-d13c-436e-a9ac-87f98b140cc6@redhat.com> <6b87e597-e3e6-4e46-a4c2-ca358d52e557@os.amperecomputing.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 14.01.25 18:01, Yang Shi wrote: > > > > On 1/14/25 7:06 AM, David Hildenbrand wrote: >> On 14.01.25 15:52, Lorenzo Stoakes wrote: >>> On Tue, Jan 14, 2025 at 02:01:32PM +0100, David Hildenbrand wrote: >>>> On 13.01.25 23:30, Yang Shi wrote: >>>>> When creating private mapping for /dev/zero, the driver makes it an >>>>> anonymous mapping by calling set_vma_anonymous().  But it just sets >>>>> vm_ops to NULL, vm_file is still valid and vm_pgoff is also file >>>>> offset. >>>>> >>>>> This is a special case and the VMA doesn't look like either >>>>> anonymous VMA >>>>> or file VMA.  It confused other kernel subsystem, for example, >>>>> khugepaged [1]. >>>>> >>>>> It seems pointless to keep such special case.  Making private >>>>> /dev/zero> >>>> mapping a full anonymous mapping doesn't change the semantic of >>>>> /dev/zero either. >>>>> >>>>> The user visible effect is the mapping entry shown in >>>>> /proc//smaps >>>>> and /proc//maps. >>>>> >>>>> Before the change: >>>>> ffffb7190000-ffffb7590000 rw-p 00001000 00:06 >>>>> 8                          /dev/zero >>>>> >>>>> After the change: >>>>> ffffb6130000-ffffb6530000 rw-p 00000000 00:00 0 >>>>> >>>> >>>> Hm, not sure about this. It's actually quite consistent to have that >>>> output >>>> in smaps the way it is. You mapped a file at an offset, and it >>>> behaves like >>>> an anonymous mapping apart from that. >>>> >>>> Not sure if the buggy khugepaged thing is a good indicator to >>>> warrant this >>>> change. > > I admit this may be a concern, but I doubt who really care about it... > There is an example in the man page [1] about /proc/self/map_files/. I assume that will also change here. It's always hard to tell who that could affect, but I'm not convinced this is worth it to find it out :) >>> >>> Yeah, this is a user-facing fundamental change that hides information >>> and >>> defies expectation so I mean - it's a no go really isn't it? >>> >>> I'd rather we _not_ make this anon though, because isn't life confusing >>> enough David? I thought it was bad enough with 'anon, file and lol >>> shmem' >>> but 'lol lol also /dev/zero' is enough to make me want to frolick in the >>> fields... >> >> I recall there are users that rely on this memory to get the shared >> zeropage on reads etc (in comparison to shmem!), so I better not ... >> mess with this *at all* :) > > The behavior won't be changed. Yes, I know. And that's good ;) [1] https://man7.org/linux/man-pages/man5/proc_pid_map_files.5.html -- Cheers, David / dhildenb