5 ms·
The phaseout of the mmap() file operation in Linux
- Denvercoder9 1y agoTo be clear, this is about the internal implementation in the kernel, the mmap() system call is not going anywhere.
- K0IN 1y agothank you that was the first thing I had to check.
- porridgeraisin 1y ago"We do NOT break userspace"
- aa-jv 1y agoI'm relieved, but also somewhat befuddled that someone would write such a shocking headline. It immediately had me reaching for the lkml archives to find out whats really going on.
- denotational 1y agoIn its defence, the headline says "file operation" rather than "syscall", which makes it slightly less egregious: it's referring to `mmap` as a member of `struct file_operations`.
- dooglius 1y agoThe mmap syscall operates on files so it's still very easily misinterpreted
- deleted 1y ago[deleted]
- commandersaki 1y agoWhich worked as intended; I first had a shock, did a double take, and realised there was nuance in file operation, read a little bit of the article and confirmed my suspicion it didn't have anything to do with the syscall.
- arp242 1y agommap is POSIX, so it's not going anywhere and you can rely on it until POSIX systems are phased out or the heat death of the universe, whichever comes sooner.
- ajross 1y agoIndeed. But even so, it's mildly shocking, as struct file_operations has been one of the deepest (and most promiscuously) integrated and most conservative bits of the kernel API. This stuff dates back decades and almost never changes. And there are a lot of raw file drivers[1] still there from eras most people have forgotten about. This is a big, big reorg even for Linux. [1] To be fair, most of which probably don't support mapping.
- kragen 1y agoYes, that's true. However, it's a kernel-internal API, and those have never been considered stable, unlike the system call ABIs, which are mostly sacrosanct. Except for, like, uselib(). This is because pretty much all the code that calls the kernel-internal APIs is in a monorepo, so you can fix it all when you make the change. Also, it's not that the core kernel is ceasing to provide a facility that drivers depended on; rather, it's ceasing to depend on a facility that drivers provided. But doing so involves adding this new mmap_prepare() thing, which is making the kernel depend on a new facility that drivers now must provide, I guess?
- doubletwoyou 1y agoThat comment about how /dev/zero used to be used to allocate anonymous pages, anyone have more info? All I could find was a wikipedia article [0] [0] https://en.m.wikipedia.org/wiki//dev/zero https://en.m.wikipedia.org/wiki//dev/zero
- jeffbee 1y agoThat's it, there isn't any more to know. When the ancient unixes first began to support anonymous maps, they were just hacked into existing code with "zero" as the file, because the only through-line in the unix family history is that the API must be as hideous as it needs to be to accommodate lazy system authors.
- ars 1y agoThere is more to know, does the code special case this? Does it use the file name? Major minor number? Or does it actually read zero's from it and place them in memory?
- convolvatron 1y agoit wouldn't be too hard to look at the source for mmap and zero, but since the topic of this article is the removal of the mmap 'virtual function' in the file, that would have been a pretty good place to throw a zero-page allocation
- mort96 1y agoWait, by "allocating anonymous pages" we just mean memory allocation from the kernel that's used for implementing e.g malloc, right? Did memory mapped files come before memory allocation so that it "made sense" to implement memory allocation as "mapping in /dev/zero"? Or was some amount heap memory always just mapped into the process in early UNIX so that the need to map more pages only appeared as programs started to demand more heap memory than whatever the standard amount was?
- pdw 1y ago
- DonHopkins 1y agoI hope the new API is flexible enough to support my proposed /dev/seven, an infinite source of ^G. https://news.ycombinator.com/item?id=17532285 https://news.ycombinator.com/item?id=17532285 >DonHopkins on July 14, 2018 | parent | context | favorite | on: The everything-is-a-file principle – Linus Torvald... >I always wanted /dev/zero, which is used to mmap zeros into memory, to be more general and use the device minor number to define which byte gets mapped, so you could mknod /dev/seven with a minor number of 7, to provide an infinite source of beeps!
- mort96 1y agoI want a /dev/yes. I'm tired of typing 'yes | apt install' etc. I want to type '</dev/yes apt install'. Just an infinite stream of "y\n".
- danudey 1y agoCan we also have /dev/bseven, which is the same thing but as a block device? Convenient if you ever want to be able to acquire ^G at larger scale, or randomly seek through your infinite ^G.
- matja 1y agoCUSE on Linux can do that: https://libfuse.github.io/doxygen/cuse_8c.html https://libfuse.github.io/doxygen/cuse_8c.html
- drob518 1y agoYea, that’s a clickbait headline. A lot less concerning once you read the first paragraphs.