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diff --git a/Documentation/Kernel/TmpFS.md b/Documentation/Kernel/TmpFS.md new file mode 100644 index 0000000000..7ba607eb60 --- /dev/null +++ b/Documentation/Kernel/TmpFS.md @@ -0,0 +1,70 @@ +# `TmpFS` filesystem and its purposes + +`TmpFS` is a RAM-backed filesystem. It is used to hold files and directories in the `/tmp` directory and +device nodes in the `/dev` directory. + +## What are the `TmpFS` filesystem characteristics? + +`TmpFS` is a pure RAM-backed filesystem, which means all files and directories +actually live in memory, each in its own `TmpFS` instance in the kernel. + +The `TmpFS` in its current design is very conservative about allocating virtual memory ranges +for itself, and instead it uses the `AnonymousVMObject` object to hold physical pages containing +data for its inodes. When doing actual IO, the `TmpFS` code temporarily allocates a small virtual memory +`Memory::Region` to perform the task, which works quite well although it puts a strain on the virtual memory +mapping code. The current design also ensures that fabricated huge files can be easily created in the filesystem +with very small overhead until actual IO is performed. + +### The `/tmp` directory and its purposes + +Currently, the `/tmp` directory is the **place** for facilitating the inter-process +communication layer, with many Unix sockets nodes being present in the directory. + +Many test suites in the project leverage the `/tmp` for placing their test files +when trying to check the correctness of many system-related functionality. +Other programs rely on `/tmp` for placing their temporary files to properly function. + +### Why does the `TmpFS` work well for the `/dev` directory? + +To understand why `TmpFS` works reliably when mounted on `/dev`, we must understand +first what we did in the past and how `TmpFS` solves many of the issues with the previous design. + +At first, we didn't have any special filesystem mounted in `/dev` as the image build +script generated all the required device nodes in `/dev`. This was quite sufficient in +the early days of the project, where hardware support was extremely limited and of course +hotplugging any kind of hardware was not even a consideration. + +As the project grew larger and more hardware support was introduced, it became obvious +that this "solution" was not future-proof. For example, if one user has two SATA drives +connected to his computer, and another user has just one old IDE drive being used, +then how should we support both cases? The answer was that each user could simply invoke +the `mknod` utility to create device nodes. This solution meant that user interaction as well +as a deep understanding of kernel internals was required to achieve a proper setup. + +When it became apparent that another solution was needed, the `DevFS` filesystem was +invented. The idea was plain simple - the `DevFS` is a read-only filesystem that only +lists all present char and block devices. Permissions were hardcoded at known value, +and modifying the filesystem (including adding subdirectories) was strictly prohibited. +This solution was efficient in the sense of ensuring minimal user interaction for using +device nodes in `/dev`. The shortcomings were strictly immutable filesystem layout and hardcoded +permissions. Also, the filesystem implementation was specific to `/dev`, because no other +mount in the system used this special filesystem, which meant it needed special test cases, etc. + +The `DevFS` solution was short-lived, and was quickly replaced by the `DevTmpFS` solution. +That new shiny filesystem was again specific to `/dev`, but it solved many of the issues +`DevFS` suffered from - no more hardcoded permissions and now the design has flexible filesystem +layout in its mindset. +This was achieved by implementing from scratch a filesystem that resembles the `TmpFS` +filesystem, but was different in one major aspect - only device nodes and directories are allowed +to be in `/dev`. This strict requirement has been mandated to ensure the user doesn't +accidentally put unrelated files in `/dev`. When the `DevTmpFS` was invented, it clearly +needed userspace cooperation to create device nodes in `/dev`, so `SystemServer` was modified +to create those during boot. The process of how `SystemServer` does that is not discussed +in this document, but ultimately evolved to be flexible enough to work quite well. + +Everything worked quite well, but there was still a prominent problem with `DevTmpFS` - +it was an entire filesystem solution just for `/dev` and nobody else used it. +Testing the filesystem was quite clunky and truthfully lacking from the beginning until its removal. +To solve this problem, it was decided to stop using it, and instead just use `TmpFS`. +To ensure the current behavior of disallowing regular files in `/dev`, a new mount flag called +`MS_NOREGULAR` was invented, so it could be mounted with it. |