On 07/02/2018 11:06 PM, Andrew Morton wrote:
> On Mon, 2 Jul 2018 09:50:49 +0200 Christian Borntraeger <borntraeger@xxxxxxxxxx> 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.
>>
>> ...
>>
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -64,6 +64,7 @@
>> #include <linux/backing-dev.h>
>> #include <linux/page_idle.h>
>> #include <linux/memremap.h>
>> +#include <linux/userfaultfd_k.h>
>>
>> #include <asm/tlbflush.h>
>>
>> @@ -1481,7 +1482,7 @@ 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) && !userfaultfd_armed(vma)) {
>> /*
>> * The guest indicated that the page content is of no
>> * interest anymore. Simply discard the pte, vmscan
>
> A reader of this code will wonder why we're checking
> userfaultfd_armed(). So the writer of this code should add a comment
> which explains this to them ;) Please.
>
Something like: /*
* The guest indicated that the page content is of no
* interest anymore. Simply discard the pte, vmscan
* will take care of the rest.
* A future reference will then fault in a new zero
* page. When userfaultfd is active, we must not drop
* this page though, as its main user (postcopy
* migration) will not expect userfaults on already
* copied pages.
*/
?
