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From:   Zi Yan <ziy@nvidia.com>
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Christoph Lameter <cl@linux.com>,
        Matthew Wilcox <willy@infradead.org>,
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        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        Vishal Moola <vishal.moola@gmail.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        Michal Hocko <mhocko@suse.com>,
        "\"Huang, Ying\"" <ying.huang@intel.com>,
        Nhat Pham <nphamcs@gmail.com>,
        Yosry Ahmed <yosryahmed@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v3 10/12] mempolicy: alloc_pages_mpol() for NUMA policy
 without vma
Date:   Mon, 23 Oct 2023 17:13:40 -0400
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On 23 Oct 2023, at 17:10, Hugh Dickins wrote:

> On Mon, 23 Oct 2023, Zi Yan wrote:
>> On 19 Oct 2023, at 16:39, Hugh Dickins wrote:
>>> @@ -1170,6 +1170,15 @@ static struct folio *new_folio(struct folio *s=
rc, unsigned long start)
>>>  			break;
>>>  	}
>>>
>>> +	/*
>>> +	 * __get_vma_policy() now expects a genuine non-NULL vma. Return NU=
LL
>>> +	 * when the page can no longer be located in a vma: that is not ide=
al
>>> +	 * (migrate_pages() will give up early, presuming ENOMEM), but good=

>>> +	 * enough to avoid a crash by syzkaller or concurrent holepunch.
>>> +	 */
>>> +	if (!vma)
>>> +		return NULL;
>>> +
>>
>> How often would this happen? I just want to point out that ENOMEM can =
cause
>> src THPs or large folios to be split by migrate_pages().
>
> The only case I know of it happening was when a file was mapped, then t=
hat
> file truncated (cutting out the source page) before migrate_pages(&page=
list)
> reached it - rather a syzbotty thing to do, not of great reallife conce=
rn.
>
> I won't assert that's the only way: I've a ghost of a memory of another=
 way,
> that I can't quite resurface, from a long-ago version of queue_pages_ra=
nge().
>
> But in the end just didn't care enough about it, because this is really=
 just
> to save a bisection point from crashing - the possibility goes away in =
the
> 11/12 commit which follows this one, which takes VMA out of it altogeth=
er.

Got it. Thanks for the explanation. I should have finished the whole seri=
es.

--
Best Regards,
Yan, Zi

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