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

[Mike Kravetz]

On 3/24/21 1:43 AM, Michal Hocko wrote:
> On Tue 23-03-21 11:51:04, Mike Kravetz wrote:
>> 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@xxxxxx/
>>>>
>>>> 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.
> 
> I do not object to the plan I just want to point out that the sparse
> vmemmap for hugetlb pages will need to recognize sleep/nosleep variants
> of the freeing path as well to handle its vmemmap repopulate games.
> 

Yes,

I also commented elsewhere that we will likely want to do the
drop/reacquire lock for each page in the looping page free routines when
adding the vmemmap freeing support.

Unless someone thinks otherwise, I still think it is better to first fix
the hugetlb put_page/free_huge_page path with this series.  Then move on
to the free vmemmap series.
-- 
Mike Kravetz