Re: [RFC PATCH 7/8] hugetlb: add update_and_free_page_no_sleep for irq context

Mike Kravetz
2021-03-23T18:52:45Z

On 3/22/21 11:10 AM, Roman Gushchin wrote:
> On Mon, Mar 22, 2021 at 10:42:23AM -0700, Mike Kravetz wrote:
>> Cc: Roman, Christoph
>>
>> On 3/22/21 1:41 AM, Peter Zijlstra wrote:
>>> On Fri, Mar 19, 2021 at 03:42:08PM -0700, Mike Kravetz wrote:
>>>> The locks acquired in free_huge_page are irq safe. However, in certain
>>>> circumstances the routine update_and_free_page could sleep. Since
>>>> free_huge_page can be called from any context, it can not sleep.
>>>>
>>>> Use a waitqueue to defer freeing of pages if the operation may sleep. A
>>>> new routine update_and_free_page_no_sleep provides this functionality
>>>> and is only called from free_huge_page.
>>>>
>>>> Note that any 'pages' sent to the workqueue for deferred freeing have
>>>> already been removed from the hugetlb subsystem. What is actually
>>>> deferred is returning those base pages to the low level allocator.
>>>
>>> So maybe I'm stupid, but why do you need that work in hugetlb? Afaict it
>>> should be in cma_release().
>>
>> My thinking (which could be totally wrong) is that cma_release makes no
>> claims about calling context. From the code, it is pretty clear that it
>> can only be called from task context with no locks held. Although,
>> there could be code incorrectly calling it today hugetlb does. Since
>> hugetlb is the only code with this new requirement, it should do the
>> work.
>>
>> Wait!!! That made me remember something.
>> Roman had code to create a non-blocking version of cma_release().
>> https://lore.kernel.org/linux-mm/20201022225308.2927890-1-guro@fb.com/
>>
>> There were no objections, and Christoph even thought there may be
>> problems with callers of dma_free_contiguous.
>>
>> Perhaps, we should just move forward with Roman's patches to create
>> cma_release_nowait() and avoid this workqueue stuff?
>
> Sounds good to me. If it's the preferred path, I can rebase and resend
> those patches (they been carried for some time by Zi Yan for his 1GB THP work,
> but they are completely independent).

Thanks Roman,

Yes, this is the preferred path. If there is a non blocking version of
cma_release, then it makes fixup of hugetlb put_page path much easier.

If you would prefer, I can rebase your patches and send with this series.
--
Mike Kravetz