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Date: Wed, 31 Mar 2021 09:52:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
Shakeel Butt <shakeelb@...gle.com>,
Oscar Salvador <osalvador@...e.de>,
Muchun Song <songmuchun@...edance.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: [PATCH v3 1/8] mm/cma: change cma mutex to irq safe spinlock
On 31.03.21 05:41, Mike Kravetz wrote:
> 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.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
> mm/cma.c | 18 +++++++++---------
> mm/cma.h | 2 +-
> mm/cma_debug.c | 8 ++++----
> 3 files changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/mm/cma.c b/mm/cma.c
> index b2393b892d3b..2380f2571eb5 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -24,7 +24,6 @@
> #include <linux/memblock.h>
> #include <linux/err.h>
> #include <linux/mm.h>
> -#include <linux/mutex.h>
> #include <linux/sizes.h>
> #include <linux/slab.h>
> #include <linux/log2.h>
> @@ -83,13 +82,14 @@ static void cma_clear_bitmap(struct cma *cma, unsigned long pfn,
> unsigned int count)
> {
> unsigned long bitmap_no, bitmap_count;
> + unsigned long flags;
>
> bitmap_no = (pfn - cma->base_pfn) >> cma->order_per_bit;
> bitmap_count = cma_bitmap_pages_to_bits(cma, count);
>
> - mutex_lock(&cma->lock);
> + spin_lock_irqsave(&cma->lock, flags);
> bitmap_clear(cma->bitmap, bitmap_no, bitmap_count);
> - mutex_unlock(&cma->lock);
> + spin_unlock_irqrestore(&cma->lock, flags);
> }
>
> static void __init cma_activate_area(struct cma *cma)
> @@ -118,7 +118,7 @@ static void __init cma_activate_area(struct cma *cma)
> pfn += pageblock_nr_pages)
> init_cma_reserved_pageblock(pfn_to_page(pfn));
>
> - mutex_init(&cma->lock);
> + spin_lock_init(&cma->lock);
>
> #ifdef CONFIG_CMA_DEBUGFS
> INIT_HLIST_HEAD(&cma->mem_head);
> @@ -392,7 +392,7 @@ static void cma_debug_show_areas(struct cma *cma)
> unsigned long nr_part, nr_total = 0;
> unsigned long nbits = cma_bitmap_maxno(cma);
>
> - mutex_lock(&cma->lock);
> + spin_lock_irq(&cma->lock);
> pr_info("number of available pages: ");
> for (;;) {
> next_zero_bit = find_next_zero_bit(cma->bitmap, nbits, start);
> @@ -407,7 +407,7 @@ 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_irq(&cma->lock);
> }
> #else
> static inline void cma_debug_show_areas(struct cma *cma) { }
> @@ -454,12 +454,12 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
> goto out;
>
> for (;;) {
> - mutex_lock(&cma->lock);
> + spin_lock_irq(&cma->lock);
> 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_irq(&cma->lock);
> break;
> }
> bitmap_set(cma->bitmap, bitmap_no, bitmap_count);
> @@ -468,7 +468,7 @@ 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_irq(&cma->lock);
>
> 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..2e7704955f4f 100644
> --- a/mm/cma_debug.c
> +++ b/mm/cma_debug.c
> @@ -36,10 +36,10 @@ static int cma_used_get(void *data, u64 *val)
> struct cma *cma = data;
> unsigned long used;
>
> - mutex_lock(&cma->lock);
> + spin_lock_irq(&cma->lock);
> /* pages counter is smaller than sizeof(int) */
> used = bitmap_weight(cma->bitmap, (int)cma_bitmap_maxno(cma));
> - mutex_unlock(&cma->lock);
> + spin_unlock_irq(&cma->lock);
> *val = (u64)used << cma->order_per_bit;
>
> return 0;
> @@ -53,7 +53,7 @@ static int cma_maxchunk_get(void *data, u64 *val)
> unsigned long start, end = 0;
> unsigned long bitmap_maxno = cma_bitmap_maxno(cma);
>
> - mutex_lock(&cma->lock);
> + spin_lock_irq(&cma->lock);
> for (;;) {
> start = find_next_zero_bit(cma->bitmap, bitmap_maxno, end);
> if (start >= bitmap_maxno)
> @@ -61,7 +61,7 @@ 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_irq(&cma->lock);
> *val = (u64)maxchunk << cma->order_per_bit;
>
> return 0;
>
Reviewed-by: David Hildenbrand <david@...hat.com>
--
Thanks,
David / dhildenb
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