lkml.org 
[lkml]   [2018]   [Jun]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH/RFC] mm: do not drop unused pages when userfaultd is running
From
Date


On 06/28/2018 02:39 PM, Christian Borntraeger wrote:
> KVM guests on s390 can notify the host of unused pages. This can result
> in pte_unused callbacks to be true for KVM guest memory.
>
> If a page is unused (checked with pte_unused) we might drop this page
> instead of paging it. This can have side-effects on userfaultd, when the
> page in question was already migrated:
>
> The next access of that page will trigger a fault and a user fault
> instead of faulting in a new and empty zero page. As QEMU does not
> expect a userfault on an already migrated page this migration will fail.
>
> The most straightforward solution is to ignore the pte_unused hint if a
> userfault context is active for this VMA.
>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
> mm/rmap.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 6db729dc4c50..3f3a72aa99f2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1481,7 +1481,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> set_pte_at(mm, address, pvmw.pte, pteval);
> }
>
> - } else if (pte_unused(pteval)) {
> + } else if (pte_unused(pteval) && !vma->vm_userfaultfd_ctx.ctx) {

FWIW, this needs a fix for !CONFIG_USERFAULTFD.
Still: more opinions on the patch itself?

> /*
> * The guest indicated that the page content is of no
> * interest anymore. Simply discard the pte, vmscan
>

\
 
 \ /
  Last update: 2018-06-29 08:52    [W:0.072 / U:0.460 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site