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

[Peter Zijlstra]

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?

Ha!, that basically does as I suggested. Using that page is unfortunate
in that it will destroy the contig range for allocations until the work
happens, but I'm not sure I see a nice alternative.