On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote: > On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka <vbabka at 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?). 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. >> >>> There might be a problem after failed populate: remap will handle them >>> as lock on fault. In this case we can fill ptes with swap-like non-present >>> entries to remember that fact and count them as should-be-locked pages. >> >> >> I don't think we should strive to have mremap try to fix the inherent >> unreliability of mmap (MAP_POPULATE)? > > I don't think so. MAP_POPULATE works only when mmap happens. > Flag MREMAP_POPULATE might be a good idea. Just for symmetry. Maybe, but please do it as a separate series.
