On Mon, Aug 24, 2015 at 6:55 PM, Eric B Munson <emunson@xxxxxxxxxx> wrote: > On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote: > >> 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); >> ... >> > > 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. 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). -- 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>