On Mon, Aug 24, 2015 at 6:09 PM, Eric B Munson <emunson@xxxxxxxxxx> 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@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(). 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); ... > >> >> >> >>>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> -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
