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Message-ID: <17c1f86e-e6bf-4be0-88cd-c4afecb02310@redhat.com>
Date: Fri, 22 Mar 2024 09:13:02 +0100
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Janosch Frank <frankja@...ux.ibm.com>,
Claudio Imbrenda <imbrenda@...ux.ibm.com>, Heiko Carstens
<hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Alexander Gordeev <agordeev@...ux.ibm.com>,
Sven Schnelle <svens@...ux.ibm.com>,
Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
linux-s390@...r.kernel.org
Subject: Re: [PATCH v1 1/2] mm/userfaultfd: don't place zeropages when
zeropages are disallowed
On 21.03.24 23:46, Peter Xu wrote:
> On Thu, Mar 21, 2024 at 11:29:45PM +0100, David Hildenbrand wrote:
>> On 21.03.24 23:20, Peter Xu wrote:
>>> On Thu, Mar 21, 2024 at 10:59:53PM +0100, David Hildenbrand wrote:
>>>> s390x must disable shared zeropages for processes running VMs, because
>>>> the VMs could end up making use of "storage keys" or protected
>>>> virtualization, which are incompatible with shared zeropages.
>>>>
>>>> Yet, with userfaultfd it is possible to insert shared zeropages into
>>>> such processes. Let's fallback to simply allocating a fresh zeroed
>>>> anonymous folio and insert that instead.
>>>>
>>>> mm_forbids_zeropage() was introduced in commit 593befa6ab74 ("mm: introduce
>>>> mm_forbids_zeropage function"), briefly before userfaultfd went
>>>> upstream.
>>>>
>>>> Note that we don't want to fail the UFFDIO_ZEROPAGE request like we do
>>>> for hugetlb, it would be rather unexpected. Further, we also
>>>> cannot really indicated "not supported" to user space ahead of time: it
>>>> could be that the MM disallows zeropages after userfaultfd was already
>>>> registered.
>>>>
>>>> Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>
>>> Reviewed-by: Peter Xu <peterx@...hat.com>
>>>
>>> Still, a few comments below.
>>>
>>>> ---
>>>> mm/userfaultfd.c | 35 +++++++++++++++++++++++++++++++++++
>>>> 1 file changed, 35 insertions(+)
>>>>
>>>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>>>> index 712160cd41eca..1d1061ccd1dea 100644
>>>> --- a/mm/userfaultfd.c
>>>> +++ b/mm/userfaultfd.c
>>>> @@ -316,6 +316,38 @@ static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
>>>> goto out;
>>>> }
>>>> +static int mfill_atomic_pte_zeroed_folio(pmd_t *dst_pmd,
>>>> + struct vm_area_struct *dst_vma, unsigned long dst_addr)
>>>> +{
>>>> + struct folio *folio;
>>>> + int ret;
>>>
>>> nitpick: we can set -ENOMEM here, then
>>>
>>>> +
>>>> + folio = vma_alloc_zeroed_movable_folio(dst_vma, dst_addr);
>>>> + if (!folio)
>>>> + return -ENOMEM;
>>>
>>> return ret;
>>>
>>>> +
>>>> + ret = -ENOMEM;
>>>
>>> drop.
>>
>> Sure!
>>
>>>
>>>> + if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
>>>> + goto out_put;
>>>> +
>>>> + /*
>>>> + * The memory barrier inside __folio_mark_uptodate makes sure that
>>>> + * preceding stores to the page contents become visible before
>>>> + * the set_pte_at() write.
>>>> + */
>>>
>>> This comment doesn't apply. We can drop it.
>>>
>>
>> I thought the same until I spotted that comment (where uffd originally
>> copied this from I strongly assume) in do_anonymous_page().
>>
>> "Preceding stores" here are: zeroing out the memory.
>
> Ah.. that's okay then.
>
> Considering that userfault used to be pretty cautious on such ordering, as
> its specialty to involve many user updates on the page, would you mind we
> mention those details out?
>
> /*
> * __folio_mark_uptodate contains the memory barrier to make sure
> * the page updates to the zero page will be visible before
> * installing the pgtable entries. See do_anonymous_page().
> */
>
> Or anything better than my wordings.
Sure, I'd slightly reword it. The following on top:
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 1d1061ccd1dea..9d385696fb891 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -320,20 +320,19 @@ static int mfill_atomic_pte_zeroed_folio(pmd_t *dst_pmd,
struct vm_area_struct *dst_vma, unsigned long dst_addr)
{
struct folio *folio;
- int ret;
+ int ret = -ENOMEM;
folio = vma_alloc_zeroed_movable_folio(dst_vma, dst_addr);
if (!folio)
- return -ENOMEM;
+ return ret;
- ret = -ENOMEM;
if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
goto out_put;
/*
* The memory barrier inside __folio_mark_uptodate makes sure that
- * preceding stores to the page contents become visible before
- * the set_pte_at() write.
+ * zeroing out the folio become visible before mapping the page
+ * using set_pte_at(). See do_anonymous_page().
*/
__folio_mark_uptodate(folio);
Thanks!
--
Cheers,
David / dhildenb
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