Re: [PATCH 1/8] mm: cma: introduce cma_release_nowait()

[Michal Hocko]

On Thu 25-03-21 10:56:38, David Hildenbrand wrote:
> On 25.03.21 01:28, Mike Kravetz wrote:
> > From: Roman Gushchin <guro@xxxxxx>
> > 
> > cma_release() has to lock the cma_lock mutex to clear the cma bitmap.
> > It makes it a blocking function, which complicates its usage from
> > non-blocking contexts. For instance, hugetlbfs code is temporarily
> > dropping the hugetlb_lock spinlock to call cma_release().
> > 
> > This patch introduces a non-blocking cma_release_nowait(), which
> > postpones the cma bitmap clearance. It's done later from a work
> > context. The first page in the cma allocation is used to store
> > the work struct. Because CMA allocations and de-allocations are
> > usually not that frequent, a single global workqueue is used.
> > 
> > To make sure that subsequent cma_alloc() call will pass, cma_alloc()
> > flushes the cma_release_wq workqueue. To avoid a performance
> > regression in the case when only cma_release() is used, gate it
> > by a per-cma area flag, which is set by the first call
> > of cma_release_nowait().
> > 
> > Signed-off-by: Roman Gushchin <guro@xxxxxx>
> > [mike.kravetz@xxxxxxxxxx: rebased to v5.12-rc3-mmotm-2021-03-17-22-24]
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > ---
> 
> 
> 1. Is there a real reason this is a mutex and not a spin lock? It seems to
> only protect the bitmap. Are bitmaps that huge that we spend a significant
> amount of time in there?

Good question. Looking at the code it doesn't seem that there is any
blockable operation or any heavy lifting done under the lock.
7ee793a62fa8 ("cma: Remove potential deadlock situation") has introduced
the lock and there was a simple bitmat protection back then. I suspect
the patch just followed the cma_mutex lead and used the same type of the
lock. cma_mutex used to protect alloc_contig_range which is sleepable.

This all suggests that there is no real reason to use a sleepable lock
at all and we do not need all this heavy lifting.

Thanks!
-- 
Michal Hocko
SUSE Labs