Re: [Qemu-devel] [PATCH] mm: Loosen MADV_NOHUGEPAGE to enable Qemu postcopy on s390

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 11/18/2015 02:31 PM, Vlastimil Babka wrote:
> [CC += linux-api@vger.kernel.org]
> 
> 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@vger.kernel.org, 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@linux.vnet.ibm.com>
>> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> 
> 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? 

 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.

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;
>>
>