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Message-ID: <CAMZfGtUv4O_+W5rHt0P4Xbw=WXJ-ZwHYMrg=iJa2CEkfxb91gA@mail.gmail.com>
Date: Tue, 30 Mar 2021 16:08:36 +0800
From: Muchun Song <songmuchun@...edance.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>,
Linux Memory Management List <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
Roman Gushchin <guro@...com>,
Shakeel Butt <shakeelb@...gle.com>,
Oscar Salvador <osalvador@...e.de>,
David Hildenbrand <david@...hat.com>,
David Rientjes <rientjes@...gle.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Peter Zijlstra <peterz@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
HORIGUCHI NAOYA <naoya.horiguchi@....com>,
"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
Waiman Long <longman@...hat.com>, Peter Xu <peterx@...hat.com>,
Mina Almasry <almasrymina@...gle.com>,
Hillf Danton <hdanton@...a.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Barry Song <song.bao.hua@...ilicon.com>,
Will Deacon <will@...nel.org>,
Andrew Morton <akpm@...ux-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@...e.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?
>
> > 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
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