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[2003:cb:c738:3100:8133:26cf:7877:94aa]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-436e9e37d74sm182835915e9.29.2025.01.14.09.46.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Jan 2025 09:46:06 -0800 (PST) Message-ID: Date: Tue, 14 Jan 2025 18:46:05 +0100 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> <3db5a5e8-62f3-4915-9fae-49a7f88952c9@redhat.com> <1cb09c4b-8756-4d3a-b5d2-ebdd6046d947@os.amperecomputing.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 867F2180009 X-Stat-Signature: zy8zq1zrhq74oe8rmwc8uc9pk5ipxc45 X-Rspam-User: X-HE-Tag: 1736876771-328096 X-HE-Meta: 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 JubpNlFD iibQT3bVEUTIizLTGO7v3R9tpUw68YwQqtJ4nEwkKdKS5z6SO2EAWsdWWct+16ifWKFyDiNG8JaTZVFiLOv4NoWLc1rAY5SLiJX4tqqiG96o2Wb0/LNFiiY9OERi/d7rRuZGUicRW6ikhCdB70+uKAjUB4C4U27xgT8KSQfsKuAAsaKX2WoFGoGaI4ZouJwPJgCiMFprJLEhZrjbOFltIC55/l5z5NOPixDGbdLFoeGCBJ9RF0VWE++hHJVddmZZgzwnMOQ9Th1HruNTWN0rMtQtdQB7tztPfURlad6SexBrZlDG5Hyj6kfT7geALCR8a0uUpoqXF/2PH63lVVrzVumF/DK/JiM9nvdawrVeVEQtKlNYK8cvp7Lh1T8X9zUdULEbHVCnnLIeP4T69bF2BsRURn/P6hsiLQnZmEIstlD3nd84= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 14.01.25 18:38, Yang Shi wrote: > > > > On 1/14/25 9:23 AM, David Hildenbrand wrote: >> 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. > > IIUC, that example is specific to "anonymous shared memory" created by > shared mapping of /dev/zero. Note that MAP_PRIVATE of /dev/zero will also make it appear in the same way right now (I just tried). The example is about MAP_FILE in general, not just MAP_SHARED IIUC. -- Cheers, David / dhildenb