As usual, I'm going to start with:
Step 0: What problem are you trying to solve by doing this?
Working on a custom driver that tracks I/Os on block level. For some use case, I need to write metadata of tracked I/Os from driver in shutdown sequence (more likely in reboot handler). In reboot handler, performing I/O on disk is tricky (no userspace, no VFS. no sysfs).
Although, I am able to write one sector, by creating custom bio and giving manually sector to disk. Still, looking some correct API like write_IO_on_disk(buffer, dev, offset, len)
The problem is that the kernel shouldn't be scribbling on files itself, and
that's *extremely* discouraged. The gory details of why basically boil down
to "the VFS assumes that all filesytem scribbling is called from a user process
context", which causes all sorts of problems if you're in a non-user process
context, or a non-process context entirely.
I understand it. Ideally, best practices, I should be not do that, but here is custom scenario, in which in reboot handler no VFS APIs will be accessible.
(There's also deep philosophical issues if you're over-writing blocks of a user
created file, because users have a very reasonable expectation that if they
write a given piece of data into a file system, they get the exact same data
back when they read it...)
But kernel developer should have an interface which takes only buffer, and write on a particular sector, even though it discouraged, but having that facility would not harm.
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Jitendra
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