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.
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