[LU-14922] mmap read of writable mapping updates mtime

• Type: Bug
• Resolution: Unresolved
• Priority: Minor
• Severity: 3
• Assignee: Patrick Farrell
• Reporter: Patrick Farrell

Description

Generating a read fault (ie, reading) from a memory mapped file mapped with PROT_WRITE updates mtime.  This is incorrect (compare with behavior of other POSIX file systems).

This seems to be related to how we handle the difference between a writable mapping and a mkwrite operation - There is an oddity where ll_fault_io_init is called and sets ft_writable by directly checking the vm_flags, but mkwrite also sets ft_writable directly.

This is definitely strange and should be cleaned up, but it's not clear to me if it's the issue - a read of a writable mapping doesn't cause a mkwrite, it's still just a page fault.

But it could be we're not handling this correctly - this is unclear to me.

In any case, the specific bug is that reading a writable mmap updates mtime, which it should not.