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 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 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. > > > Also, /dev/zero was there before MAP_ANONYMOUS was invented according > to [1], which is quite interesting. Interesting... Didn't know this before. > > ... so this is anonymous memory as "real" as it can get :) Let's make /dev/zero as real as anonymous memory :) > > > [1] https://en.wikipedia.org/wiki//dev/zero >