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@xxxxxxx> 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(). > >> > >>>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. > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
Attachment:
signature.asc
Description: Digital signature
