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.