On 07/28/2015 03:49 PM, Eric B Munson wrote:
On Tue, 28 Jul 2015, Michal Hocko wrote:
[...]
The only
remaining question I have is should we have 2 new mlockall flags so that
the caller can explicitly set VM_LOCKONFAULT in the mm->def_flags vs
locking all current VMAs on fault. I ask because if the user wants to
lock all current VMAs the old way, but all future VMAs on fault they
have to call mlockall() twice:
mlockall(MCL_CURRENT);
mlockall(MCL_CURRENT | MCL_FUTURE | MCL_ONFAULT);
This has the side effect of converting all the current VMAs to
VM_LOCKONFAULT, but because they were all made present and locked in the
first call, this should not matter in most cases.
Shouldn't the user be able to do this?
mlockall(MCL_CURRENT)
mlockall(MCL_FUTURE | MCL_ONFAULT);
Note that the second call shouldn't change (i.e. munlock) existing vma's
just because MCL_CURRENT is not present. The current implementation
doesn't do that thanks to the following in do_mlockall():
if (flags == MCL_FUTURE)
goto out;
before current vma's are processed and MCL_CURRENT is checked. This is
probably so that do_mlockall() can also handle the munlockall() syscall.
So we should be careful not to break this, but otherwise there are no
limitations by not having two MCL_ONFAULT flags. Having to do invoke
syscalls instead of one is not an issue as this shouldn't be frequent
syscall.
The catch is that,
like mmap(MAP_LOCKED), mlockall() does not communicate if mm_populate()
fails. This has been true of mlockall() from the beginning so I don't
know if it needs more than an entry in the man page to clarify (which I
will add when I add documentation for MCL_ONFAULT).
Good point.
In a much less
likely corner case, it is not possible in the current setup to request
all current VMAs be VM_LOCKONFAULT and all future be VM_LOCKED.
So again this should work:
mlockall(MCL_CURRENT | MCL_ONFAULT)
mlockall(MCL_FUTURE);
But the order matters here, as current implementation of do_mlockall()
will clear VM_LOCKED from def_flags if MCL_FUTURE is not passed. So
*it's different* from how it handles MCL_CURRENT (as explained above).
And not documented in manpage. Oh crap, this API is a closet full of
skeletons. Maybe it was an unnoticed regression and we can restore some
sanity?
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