Re: [PATCH v3 10/12] mempolicy: alloc_pages_mpol() for NUMA policy without vma

[Zi Yan <ziy@nvidia.com>]

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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 *src, unsigned long start)
>>>  			break;
>>>  	}
>>>
>>> +	/*
>>> +	 * __get_vma_policy() now expects a genuine non-NULL vma. Return NULL
>>> +	 * when the page can no longer be located in a vma: that is not ideal
>>> +	 * (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 that
> file truncated (cutting out the source page) before migrate_pages(&pagelist)
> reached it - rather a syzbotty thing to do, not of great reallife concern.
>
> 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_range().
>
> 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 altogether.

Got it. Thanks for the explanation. I should have finished the whole series.

--
Best Regards,
Yan, Zi

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