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[2003:cb:c738:3100:8133:26cf:7877:94aa]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-436e9e03f8bsm180695835e9.23.2025.01.14.09.24.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Jan 2025 09:24:15 -0800 (PST) Message-ID: <52632fda-a0e5-4b53-974b-4030b926d9ad@redhat.com> Date: Tue, 14 Jan 2025 18:24:14 +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> <7c1087c5-570d-4380-850b-ac26f01d325a@redhat.com> <8ec50b96-8c75-4381-81f2-8140421d186d@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 18:20, Yang Shi wrote: > > > > On 1/14/25 9:02 AM, David Hildenbrand wrote: >> On 14.01.25 16:06, 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. >>>> >>>> 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* :) >> >> Heh, and I recall reading something about odd behavior of /dev/zero >> and some interesting history [1]. >> >> " >> Unlike /dev/null, /dev/zero may be used as a source, not only as a >> sink for data. All write operations to /dev/zero succeed with no other >> effects. However, /dev/null is more commonly used for this purpose. >> >> When /dev/zero is memory-mapped, e.g., with mmap, to the virtual >> address space, it is equivalent to using anonymous memory; i.e. memory >> not connected to any file. >> " >> >> "equivalent to using anonymous memory" is interesting. > > For private mapping. Shared mapping is equivalent to shmem. "shared anonymous memory", yes. -- Cheers, David / dhildenb