On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson emunson@akamai.com wrote:
On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote:
On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson emunson@akamai.com wrote:
On Mon, 24 Aug 2015, Vlastimil Babka wrote:
On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote:
On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka vbabka@suse.cz wrote: >On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote: >>> >>> >>>I am in the middle of implementing lock on fault this way, but I cannot >>>see how we will hanlde mremap of a lock on fault region. Say we have >>>the following: >>> >>> addr = mmap(len, MAP_ANONYMOUS, ...); >>> mlock(addr, len, MLOCK_ONFAULT); >>> ... >>> mremap(addr, len, 2 * len, ...) >>> >>>There is no way for mremap to know that the area being remapped was lock >>>on fault so it will be locked and prefaulted by remap. How can we avoid >>>this without tracking per vma if it was locked with lock or lock on >>>fault? >> >> >>remap can count filled ptes and prefault only completely populated areas. > > >Does (and should) mremap really prefault non-present pages? Shouldn't it >just prepare the page tables and that's it?
As I see mremap prefaults pages when it extends mlocked area.
Also quote from manpage : If the memory segment specified by old_address and old_size is locked : (using mlock(2) or similar), then this lock is maintained when the segment is : resized and/or relocated. As a consequence, the amount of memory locked : by the process may change.
Oh, right... Well that looks like a convincing argument for having a sticky VM_LOCKONFAULT after all. Having mremap guess by scanning existing pte's would slow it down, and be unreliable (was the area completely populated because MLOCK_ONFAULT was not used or because the process aulted it already? Was it not populated because MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to populate it all?).
Given this, I am going to stop working in v8 and leave the vma flag in place.
The only sane alternative is to populate always for mremap() of VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not be enough for Eric's usecase, but it's somewhat ugly.
I don't think that this is the right solution, I would be really surprised as a user if an area I locked with MLOCK_ONFAULT was then fully locked and prepopulated after mremap().
If mremap is the only problem then we can add opposite flag for it:
"MREMAP_NOPOPULATE"
- do not populate new segment of locked areas
- do not copy normal areas if possible (anonymous/special must be copied)
addr = mmap(len, MAP_ANONYMOUS, ...); mlock(addr, len, MLOCK_ONFAULT); ... addr2 = mremap(addr, len, 2 * len, MREMAP_NOPOPULATE); ...
But with this, the user must remember what areas are locked with MLOCK_LOCKONFAULT and which are locked the with prepopulate so the correct mremap flags can be used.
Yep. Shouldn't be hard. You anyway have to do some changes in user-space.
Sorry if I wasn't clear enough in my last reply, I think forcing userspace to track this is the wrong choice. The VM system is responsible for tracking these attributes and should continue to be.
Much simpler for users-pace solution is a mm-wide flag which turns all further mlocks and MAP_LOCKED into lock-on-fault. Something like mlockall(MCL_NOPOPULATE_LOCKED).
This set certainly adds the foundation for such a change if you think it would be useful. That particular behavior was not part of my inital use case though.