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

• Status: Open
• Project: Lustre
• Type: Bug
• Priority: Minor
• Reporter: Patrick Farrell
• Assignee: Patrick Farrell
• Resolution: Unresolved
• Severity: 3

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.

Comments

[Patrick Farrell]

qian_wc, bobijam did one of you fix this issue in another patch?  I think I remember that but I can't find it right now.

[Qian Yingjin]

I did not fix this issue IIRC.

After dig the code, the problem is caused by:

osc_io_fault_start() ->
        if (fio->ft_writable)
		osc_page_touch_at(env, ios->cis_obj,
				  fio->ft_index, fio->ft_nob);

osc_page_touch_at()->
     attr->cat_mtime = attr->cat_ctime = ktime_get_real_seconds();
	valid = CAT_MTIME | CAT_CTIME;