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

["Jason J. Herne" <jjherne@linux.vnet.ibm.com>]

On 11/19/2015 08:10 AM, Dr. David Alan Gilbert wrote:
> * Christian Borntraeger (borntraeger@de.ibm.com) wrote:
>> On 11/18/2015 02:31 PM, Vlastimil Babka wrote:
>>> [CC += linux-api@vger.kernel.org]
...
> Can you tell me if the following works for you:
>
>
>  From 545809a18fa768eccdaafe9bd842490c3390b00c Mon Sep 17 00:00:00 2001
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> Date: Thu, 19 Nov 2015 12:05:36 +0000
> Subject: [PATCH] Assume madvise for (no)hugepage works
>
> madvise() returns EINVAL in the case of many failures, but also
> returns it in cases where the host kernel doesn't have THP enabled.
> Postcopy only really cares that THP is off before it detects faults,
> and turns it back on afterwards; so we're going to have
> to assume that if the madvise fails then the host just doesn't do
> THP and we can carry on with the postcopy.
>
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>   migration/postcopy-ram.c | 10 ++--------
>   1 file changed, 2 insertions(+), 8 deletions(-)
>
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index 22d6b18..3946aa9 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -241,10 +241,7 @@ static int cleanup_range(const char *block_name, void *host_addr,
>        * We turned off hugepage for the precopy stage with postcopy enabled
>        * we can turn it back on now.
>        */
> -    if (qemu_madvise(host_addr, length, QEMU_MADV_HUGEPAGE)) {
> -        error_report("%s HUGEPAGE: %s", __func__, strerror(errno));
> -        return -1;
> -    }
> +    qemu_madvise(host_addr, length, QEMU_MADV_HUGEPAGE);
>
>       /*
>        * We can also turn off userfault now since we should have all the
> @@ -345,10 +342,7 @@ static int nhp_range(const char *block_name, void *host_addr,
>        * do delete areas of the page, even if THP thinks a hugepage would
>        * be a good idea, so force hugepages off.
>        */
> -    if (qemu_madvise(host_addr, length, QEMU_MADV_NOHUGEPAGE)) {
> -        error_report("%s: NOHUGEPAGE: %s", __func__, strerror(errno));
> -        return -1;
> -    }
> +    qemu_madvise(host_addr, length, QEMU_MADV_NOHUGEPAGE);
>
>       return 0;
>   }
>

I tried this without my madvise kernel patch, and was able to get by the 
problem as expected.

We still need the kernel patch set "Allow gmap fault to retry​" as 
posted to linux-mm to get
userfaultfd support playing nicely with s390 async page faults. But that 
is a separate problem
entirely.

Tested-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>

-- 
-- Jason J. Herne (jjherne@linux.vnet.ibm.com)