RE: [External] Re: [PATCH v2 1/8] mm/cma: change cma mutex to irq safe spinlock

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Muchun Song [mailto:songmuchun@bytedance.com]
> Sent: Tuesday, March 30, 2021 9:09 PM
> To: Michal Hocko <mhocko@suse.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>; Linux Memory Management List
> <linux-mm@kvack.org>; LKML <linux-kernel@vger.kernel.org>; Roman Gushchin
> <guro@fb.com>; Shakeel Butt <shakeelb@google.com>; Oscar Salvador
> <osalvador@suse.de>; David Hildenbrand <david@redhat.com>; David Rientjes
> <rientjes@google.com>; linmiaohe <linmiaohe@huawei.com>; Peter Zijlstra
> <peterz@infradead.org>; Matthew Wilcox <willy@infradead.org>; HORIGUCHI NAOYA
> <naoya.horiguchi@nec.com>; Aneesh Kumar K . V <aneesh.kumar@linux.ibm.com>;
> Waiman Long <longman@redhat.com>; Peter Xu <peterx@redhat.com>; Mina Almasry
> <almasrymina@google.com>; Hillf Danton <hdanton@sina.com>; Joonsoo Kim
> <iamjoonsoo.kim@lge.com>; Song Bao Hua (Barry Song)
> <song.bao.hua@hisilicon.com>; Will Deacon <will@kernel.org>; Andrew Morton
> <akpm@linux-foundation.org>
> Subject: Re: [External] Re: [PATCH v2 1/8] mm/cma: change cma mutex to irq safe
> spinlock
> 
> On Tue, Mar 30, 2021 at 4:01 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Mon 29-03-21 16:23:55, Mike Kravetz wrote:
> > > Ideally, cma_release could be called from any context.  However,
> > > that is not possible because a mutex is used to protect the per-area bitmap.
> > > Change the bitmap to an irq safe spinlock.
> >
> > I would phrase the changelog slightly differerent "
> > cma_release is currently a sleepable operatation because the bitmap
> > manipulation is protected by cma->lock mutex. Hugetlb code which
> > relies on cma_release for CMA backed (giga) hugetlb pages, however,
> > needs to be irq safe.
> >
> > The lock doesn't protect any sleepable operation so it can be changed
> > to a (irq aware) spin lock. The bitmap processing should be quite fast
> > in typical case but if cma sizes grow to TB then we will likely need
> > to replace the lock by a more optimized bitmap implementation.
> > "
> >
> > it seems that you are overusing irqsave variants even from context
> > which are never called from the IRQ context so they do not need storing flags.
> >
> > [...]
> > > @@ -391,8 +391,9 @@ static void cma_debug_show_areas(struct cma *cma)
> > >       unsigned long start = 0;
> > >       unsigned long nr_part, nr_total = 0;
> > >       unsigned long nbits = cma_bitmap_maxno(cma);
> > > +     unsigned long flags;
> > >
> > > -     mutex_lock(&cma->lock);
> > > +     spin_lock_irqsave(&cma->lock, flags);
> >
> > spin_lock_irq should be sufficient. This is only called from the
> > allocation context and that is never called from IRQ context.
> 
> This makes me think more. I think that spin_lock should be sufficient. Right?
> 

It seems Mike's point is that cma_release might be called from both
irq context and process context.

If it is running in process context, we need the irq-disable to lock
the irq context which might jump to call cma_release at the same time.

We have never seen cma_release has been really called in irq context
by now, anyway.

> 
> >
> > >       pr_info("number of available pages: ");
> > >       for (;;) {
> > >               next_zero_bit = find_next_zero_bit(cma->bitmap, nbits,
> > > start); @@ -407,7 +408,7 @@ static void cma_debug_show_areas(struct cma
> *cma)
> > >               start = next_zero_bit + nr_zero;
> > >       }
> > >       pr_cont("=> %lu free of %lu total pages\n", nr_total, cma->count);
> > > -     mutex_unlock(&cma->lock);
> > > +     spin_unlock_irqrestore(&cma->lock, flags);
> > >  }
> > >  #else
> > >  static inline void cma_debug_show_areas(struct cma *cma) { } @@
> > > -430,6 +431,7 @@ struct page *cma_alloc(struct cma *cma, size_t count,
> unsigned int align,
> > >       unsigned long pfn = -1;
> > >       unsigned long start = 0;
> > >       unsigned long bitmap_maxno, bitmap_no, bitmap_count;
> > > +     unsigned long flags;
> > >       size_t i;
> > >       struct page *page = NULL;
> > >       int ret = -ENOMEM;
> > > @@ -454,12 +456,12 @@ struct page *cma_alloc(struct cma *cma, size_t count,
> unsigned int align,
> > >               goto out;
> > >
> > >       for (;;) {
> > > -             mutex_lock(&cma->lock);
> > > +             spin_lock_irqsave(&cma->lock, flags);
> > >               bitmap_no = bitmap_find_next_zero_area_off(cma->bitmap,
> > >                               bitmap_maxno, start, bitmap_count, mask,
> > >                               offset);
> > >               if (bitmap_no >= bitmap_maxno) {
> > > -                     mutex_unlock(&cma->lock);
> > > +                     spin_unlock_irqrestore(&cma->lock, flags);
> > >                       break;
> > >               }
> > >               bitmap_set(cma->bitmap, bitmap_no, bitmap_count);
> >
> > same here.
> >
> > > @@ -468,7 +470,7 @@ struct page *cma_alloc(struct cma *cma, size_t count,
> unsigned int align,
> > >                * our exclusive use. If the migration fails we will take the
> > >                * lock again and unmark it.
> > >                */
> > > -             mutex_unlock(&cma->lock);
> > > +             spin_unlock_irqrestore(&cma->lock, flags);
> > >
> > >               pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
> > >               ret = alloc_contig_range(pfn, pfn + count,
> > > MIGRATE_CMA, diff --git a/mm/cma.h b/mm/cma.h index
> > > 68ffad4e430d..2c775877eae2 100644
> > > --- a/mm/cma.h
> > > +++ b/mm/cma.h
> > > @@ -15,7 +15,7 @@ struct cma {
> > >       unsigned long   count;
> > >       unsigned long   *bitmap;
> > >       unsigned int order_per_bit; /* Order of pages represented by one bit
> */
> > > -     struct mutex    lock;
> > > +     spinlock_t      lock;
> > >  #ifdef CONFIG_CMA_DEBUGFS
> > >       struct hlist_head mem_head;
> > >       spinlock_t mem_head_lock;
> > > diff --git a/mm/cma_debug.c b/mm/cma_debug.c index
> > > d5bf8aa34fdc..6379cfbfd568 100644
> > > --- a/mm/cma_debug.c
> > > +++ b/mm/cma_debug.c
> > > @@ -35,11 +35,12 @@ static int cma_used_get(void *data, u64 *val)  {
> > >       struct cma *cma = data;
> > >       unsigned long used;
> > > +     unsigned long flags;
> > >
> > > -     mutex_lock(&cma->lock);
> > > +     spin_lock_irqsave(&cma->lock, flags);
> > >       /* pages counter is smaller than sizeof(int) */
> > >       used = bitmap_weight(cma->bitmap, (int)cma_bitmap_maxno(cma));
> > > -     mutex_unlock(&cma->lock);
> > > +     spin_unlock_irqrestore(&cma->lock, flags);
> > >       *val = (u64)used << cma->order_per_bit;
> >
> > same here
> >
> > >
> > >       return 0;
> > > @@ -52,8 +53,9 @@ static int cma_maxchunk_get(void *data, u64 *val)
> > >       unsigned long maxchunk = 0;
> > >       unsigned long start, end = 0;
> > >       unsigned long bitmap_maxno = cma_bitmap_maxno(cma);
> > > +     unsigned long flags;
> > >
> > > -     mutex_lock(&cma->lock);
> > > +     spin_lock_irqsave(&cma->lock, flags);
> > >       for (;;) {
> > >               start = find_next_zero_bit(cma->bitmap, bitmap_maxno, end);
> > >               if (start >= bitmap_maxno)
> > > @@ -61,7 +63,7 @@ static int cma_maxchunk_get(void *data, u64 *val)
> > >               end = find_next_bit(cma->bitmap, bitmap_maxno, start);
> > >               maxchunk = max(end - start, maxchunk);
> > >       }
> > > -     mutex_unlock(&cma->lock);
> > > +     spin_unlock_irqrestore(&cma->lock, flags);
> > >       *val = (u64)maxchunk << cma->order_per_bit;
> > >
> > >       return 0;
> >
> > and here.
> > --
> > Michal Hocko
> > SUSE Labs