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

[Minchan Kim <minchan@kernel.org>]

On Thu, Mar 25, 2021 at 06:15:11PM +0100, David Hildenbrand wrote:
> On 25.03.21 17:56, Mike Kravetz wrote:
> > On 3/25/21 3:22 AM, Michal Hocko wrote:
> > > On Thu 25-03-21 10:56:38, David Hildenbrand wrote:
> > > > On 25.03.21 01:28, Mike Kravetz wrote:
> > > > > From: Roman Gushchin <guro@fb.com>
> > > > > 
> > > > > 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@fb.com>
> > > > > [mike.kravetz@oracle.com: rebased to v5.12-rc3-mmotm-2021-03-17-22-24]
> > > > > Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> > > > > ---
> > > > 
> > > > 
> > > > 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.
> > > 
> > 
> > When Roman first proposed these patches, I brought up the same issue:
> > 
> > https://lore.kernel.org/linux-mm/20201022023352.GC300658@carbon.dhcp.thefacebook.com/
> > 
> > Previously, Roman proposed replacing the mutex with a spinlock but
> > Joonsoo was opposed.
> > 
> > Adding Joonsoo on Cc:
> > 
> 
> There has to be a good reason not to. And if there is a good reason,
> lockless clearing might be one feasible alternative.

I also don't think nowait variant is good idea. If the scanning of
bitmap is *really* significant, it might be signal that we need to
introduce different technique or data structure not bitmap rather
than a new API variant.