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

[Michal Hocko]

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.

-- 
Michal Hocko
SUSE Labs