Re: [PATCH 3.10 060/268] xen/gntdev: Use VM_MIXEDMAP instead of VM_IO to avoid NUMA balancing
From: Hugh Dickins
Date: Mon Jun 19 2017 - 22:57:42 EST
On Mon, 19 Jun 2017, Willy Tarreau wrote:
> From: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> commit 30faaafdfa0c754c91bac60f216c9f34a2bfdf7e upstream.
> Commit 9c17d96500f7 ("xen/gntdev: Grant maps should not be subject to
> NUMA balancing") set VM_IO flag to prevent grant maps from being
> subjected to NUMA balancing.
> It was discovered recently that this flag causes get_user_pages() to
> always fail with -EFAULT.
> (which can happen if guest's vdisk has direct-io-safe option).
> To avoid this let's use VM_MIXEDMAP flag instead --- it prevents
> NUMA balancing just as VM_IO does and has no effect on
This is only valid if kernel/sched/fair.c is checking for VM_MIXEDMAP,
and that came in v4.1's 8e76d4eecf7a ("sched, numa: do not hint for
NUMA balancing on VM_MIXEDMAP mappings"), which I don't see in your
tree nor in this series (please double check, I may have missed it).
It would be good to have that one in too, and it was marked for
stable; but maybe it didn't apply, because of depending on another
commit adding the is_vm_hugetlb_page(vma) check there? Which I
expect would also be good to have, but I haven't looked it up.
Maybe drop this one for this round, and gather up its dependencies
for the next round.
Ben's 3.16 tree appeared to be in the same position,
I didn't look at the EOL 3.18.
(I've not yet checked through backports of the "larger stack guard gap"
- thank you all for those - will do so, but won't get through them
tonight - I must look into DaveJ's trinity VM_BUG_ON now.)
> Reported-by: Olaf Hering <olaf@xxxxxxxxx>
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Tested-by: Olaf Hering <olaf@xxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Signed-off-by: Willy Tarreau <w@xxxxxx>
> drivers/xen/gntdev.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 3d8e609..6c6d253 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -770,7 +770,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
> vma->vm_ops = &gntdev_vmops;
> - vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP | VM_IO;
> + vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP | VM_MIXEDMAP;
> if (use_ptemod)
> vma->vm_flags |= VM_DONTCOPY;