On Mon, 24 Aug 2015, Konstantin Khlebnikov wrote: > 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. > Sorry if I wasn't clear enough in my last reply, I think forcing userspace to track this is the wrong choice. The VM system is responsible for tracking these attributes and should continue to be. > > 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). This set certainly adds the foundation for such a change if you think it would be useful. That particular behavior was not part of my inital use case though.
Attachment:
signature.asc
Description: Digital signature
_______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
