* Christian Borntraeger (borntraeger@xxxxxxxxxx) wrote: > On 11/18/2015 02:31 PM, Vlastimil Babka wrote: > > [CC += linux-api@xxxxxxxxxxxxxxx] > > > > Since this is a kernel-user-space API change, please CC linux-api@. The kernel > > source file Documentation/SubmitChecklist notes that all Linux kernel patches > > that change userspace interfaces should be CCed to linux-api@xxxxxxxxxxxxxxx, so > > that the various parties who are interested in API changes are informed. For > > further information, see https://www.kernel.org/doc/man-pages/linux-api-ml.html > > > > On 11/12/2015 04:18 PM, Jason J. Herne wrote: > >> MADV_NOHUGEPAGE processing is too restrictive. kvm already disables > >> hugepage but hugepage_madvise() takes the error path when we ask to turn > >> on the MADV_NOHUGEPAGE bit and the bit is already on. This causes Qemu's > >> new postcopy migration feature to fail on s390 because its first action is > >> to madvise the guest address space as NOHUGEPAGE. This patch modifies the > >> code so that the operation succeeds without error now. > >> > >> Signed-off-by: Jason J. Herne <jjherne@xxxxxxxxxxxxxxxxxx> > >> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> > > > > Looks like the manpage should be fine, as it wasn't very specific wrt these > > madvise flags. The only thing that potentially applies is: > > > > "EINVAL advice is not a valid." > > > > which itself looks like it needs fixing. Valid what, value? As in completely > > unknown flags, or flags not valid for the given vma? > > > > Anyway, I agree that it doesn't make sense to fail madvise when the given flag > > is already set. On the other hand, I don't think the userspace app should fail > > just because of madvise failing? It should in general be an advice that the > > kernel is also strictly speaking free to ignore as it shouldn't affect > > correctnes, just performance. Yeah, there are exceptions today like > > MADV_DONTNEED, but that shouldn't apply to hugepages? > > So I think Qemu needs fixing too. > > yes, I agree. David, Juan. I think The postcopy code should not fail if the madvise. > Can you fix that? Yes, I can change that. > Also what happens if the kernel is build > > without CONFIG_TRANSPARENT_HUGEPAGE? Then madvise also returns EINVAL, > > Does it? To me it looks more like we would trigger a kernel bug. Yes, it does return EINVAL; it's a shame - there's no way to distinguish between a kernel that doesn't have hugepage and a screwup in the addresses passed; everything gets squashed into EINVAL. Dave > mm/madvise.c: > case MADV_HUGEPAGE: > case MADV_NOHUGEPAGE: > error = hugepage_madvise(vma, &new_flags, behavior); <----- > if (error) > goto out; > break; > } > > > include/linux/huge_mm.h: > static inline int hugepage_madvise(struct vm_area_struct *vma, > unsigned long *vm_flags, int advice) > { > BUG(); > return 0; > } > > If we just remove the BUG() statement the code would actually be correct > in simply ignoring an MADVISE it cannot handle. If you agree, I can > spin a patch. > > > > > > how does Qemu handle that? > > The normal qemu startup ignores the return value of the madvise. Only the > recent post migration changes want to disable huge pages for userfaultd. > And this code checks the return value. And yes, we should change that > in QEMU. > > > > > > > >> --- > >> mm/huge_memory.c | 4 ++-- > >> 1 file changed, 2 insertions(+), 2 deletions(-) > >> > >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c > >> index c29ddeb..62fe06b 100644 > >> --- a/mm/huge_memory.c > >> +++ b/mm/huge_memory.c > >> @@ -2009,7 +2009,7 @@ int hugepage_madvise(struct vm_area_struct *vma, > >> /* > >> * Be somewhat over-protective like KSM for now! > >> */ > >> - if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP)) > >> + if (*vm_flags & VM_NO_THP) > >> return -EINVAL; > >> *vm_flags &= ~VM_NOHUGEPAGE; > >> *vm_flags |= VM_HUGEPAGE; > >> @@ -2025,7 +2025,7 @@ int hugepage_madvise(struct vm_area_struct *vma, > >> /* > >> * Be somewhat over-protective like KSM for now! > >> */ > >> - if (*vm_flags & (VM_NOHUGEPAGE | VM_NO_THP)) > >> + if (*vm_flags & VM_NO_THP) > >> return -EINVAL; > >> *vm_flags &= ~VM_HUGEPAGE; > >> *vm_flags |= VM_NOHUGEPAGE; > >> > > > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- 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>