This post is part 5 of the "From the diary of AArch64 porter" series:
- From the diary of AArch64 porter — autoconf
- From the diary of AArch64 porter — rpm packaging
- From the diary of AArch64 porter — testsuites
- From the diary of AArch64 porter — POSIX.1 functionality
- From the diary of AArch64 porter — PAGE_SIZE
- From the diary of AArch64 porter — vfp precision
- From the diary of AArch64 porter — system calls
- From the diary of AArch64 porter — parallel builds
- From the diary of AArch64 porter — firefighting
- From the diary of AArch64 porter — drive-by coding
- From the diary of AArch64 porter — manylinux2014
- From the diary of AArch64 porter — handling big patches
- From the diary of AArch64 porter — Arm CPU features table
During fixing software to make it build and run on AArch64 sooner or later you can meet magical constant named PAGE_SIZE. And in most situations it will be used in a wrong way.
What it does is simple — tell how big page size is. But it does not work that way on AArch64 architecture as we have different values possible: 4K, 16K (may not be supported in all cpus) and 64K with latter being used in Fedora and other distributions. There were some packages which we built at time of 4K kernel being used and then wondered why things fail under 64K kernel…
But how to handle it as a userspace software developer? Simplest solution is to use sysconf(_SC_PAGESIZE) function call (same as getpagesize()). But remember to not hardcode anything based on what you get as a result. Otherwise your application can misbehave when run on kernel with other PAGE_SIZE size.
The good part is that if someone uses PAGE_SIZE constant in code then it will just do not compile on AArch64 as it is not present in system headers. From what I checked sys/user.h header has it defined on some platforms and does not on other so it can not be assumed as available.
UPDATE: added 16K page size which may not be supported in some cpus.