Re: [PATCH] mm/cma.c: remove redundant cma_mutex lock

[Vlastimil Babka]

On 10/20/20 1:27 PM, David Hildenbrand wrote:
> On 20.10.20 12:22, Lecopzer Chen wrote:
> > The cma_mutex which protects alloc_contig_range() was first appeared in
> > commit 7ee793a62fa8c ("cma: Remove potential deadlock situation"),
> > at that time, there is no guarantee the behavior of concurrency inside
> > alloc_contig_range().
> >
> > After the commit 2c7452a075d4db2dc
> > ("mm/page_isolation.c: make start_isolate_page_range() fail if already isolated")
> >   > However, two subsystems (CMA and gigantic
> >   > huge pages for example) could attempt operations on the same range.  If
> >   > this happens, one thread may 'undo' the work another thread is doing.
> >   > This can result in pageblocks being incorrectly left marked as
> >   > MIGRATE_ISOLATE and therefore not available for page allocation.
> > The concurrency inside alloc_contig_range() was clarified.
> >
> > Now we can find that hugepage and virtio call alloc_contig_range() without
> > any lock, thus cma_mutex is "redundant" in cma_alloc() now.
> >
> > Signed-off-by: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>
> > ---
> >  mm/cma.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/mm/cma.c b/mm/cma.c
> > index 7f415d7cda9f..3692a34e2353 100644
> > --- a/mm/cma.c
> > +++ b/mm/cma.c
> > @@ -38,7 +38,6 @@
> >  
> >  struct cma cma_areas[MAX_CMA_AREAS];
> >  unsigned cma_area_count;
> > -static DEFINE_MUTEX(cma_mutex);
> >  
> >  phys_addr_t cma_get_base(const struct cma *cma)
> >  {
> > @@ -454,10 +453,9 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
> >  		mutex_unlock(&cma->lock);
> >  
> >  		pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
> > -		mutex_lock(&cma_mutex);
> >  		ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA,
> >  				     GFP_KERNEL | (no_warn ? __GFP_NOWARN : 0));
> > -		mutex_unlock(&cma_mutex);
> > +
> >  		if (ret == 0) {
> >  			page = pfn_to_page(pfn);
> >  			break;
>
> I guess this is fine. In case there is a race we return with -EBUSY -
> which is suboptimal (as it could just be a temporary issue if the other
> user backs off), but should be good enough for now.

Agreed.

> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>