[PATCH v1 1/2] mm: cma: introduce cma_release_nowait()
From: Roman Gushchin
Date: Thu Oct 22 2020 - 18:53:27 EST
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@xxxxxx>
---
include/linux/cma.h | 2 +
mm/cma.c | 93 +++++++++++++++++++++++++++++++++++++++++++++
mm/cma.h | 5 +++
3 files changed, 100 insertions(+)
diff --git a/include/linux/cma.h b/include/linux/cma.h
index 217999c8a762..497eca478c2f 100644
--- a/include/linux/cma.h
+++ b/include/linux/cma.h
@@ -47,6 +47,8 @@ extern int cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
extern struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
bool no_warn);
extern bool cma_release(struct cma *cma, const struct page *pages, unsigned int count);
+extern bool cma_release_nowait(struct cma *cma, const struct page *pages,
+ unsigned int count);
extern int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data);
#endif
diff --git a/mm/cma.c b/mm/cma.c
index 7f415d7cda9f..9fcfddcf1a6c 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -36,10 +36,19 @@
#include "cma.h"
+struct cma_clear_bitmap_work {
+ struct work_struct work;
+ struct cma *cma;
+ unsigned long pfn;
+ unsigned int count;
+};
+
struct cma cma_areas[MAX_CMA_AREAS];
unsigned cma_area_count;
static DEFINE_MUTEX(cma_mutex);
+struct workqueue_struct *cma_release_wq;
+
phys_addr_t cma_get_base(const struct cma *cma)
{
return PFN_PHYS(cma->base_pfn);
@@ -148,6 +157,10 @@ static int __init cma_init_reserved_areas(void)
for (i = 0; i < cma_area_count; i++)
cma_activate_area(&cma_areas[i]);
+ cma_release_wq = create_workqueue("cma_release");
+ if (!cma_release_wq)
+ return -ENOMEM;
+
return 0;
}
core_initcall(cma_init_reserved_areas);
@@ -205,6 +218,7 @@ int __init cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
cma->base_pfn = PFN_DOWN(base);
cma->count = size >> PAGE_SHIFT;
+ cma->flags = 0;
cma->order_per_bit = order_per_bit;
*res_cma = cma;
cma_area_count++;
@@ -437,6 +451,14 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
return NULL;
for (;;) {
+ /*
+ * If the CMA bitmap is cleared asynchronously after
+ * cma_release_nowait(), cma release workqueue has to be
+ * flushed here in order to make the allocation succeed.
+ */
+ if (test_bit(CMA_DELAYED_RELEASE, &cma->flags))
+ flush_workqueue(cma_release_wq);
+
mutex_lock(&cma->lock);
bitmap_no = bitmap_find_next_zero_area_off(cma->bitmap,
bitmap_maxno, start, bitmap_count, mask,
@@ -528,6 +550,77 @@ bool cma_release(struct cma *cma, const struct page *pages, unsigned int count)
return true;
}
+static void cma_clear_bitmap_fn(struct work_struct *work)
+{
+ struct cma_clear_bitmap_work *w;
+
+ w = container_of(work, struct cma_clear_bitmap_work, work);
+
+ cma_clear_bitmap(w->cma, w->pfn, w->count);
+
+ __free_page(pfn_to_page(w->pfn));
+}
+
+/**
+ * cma_release_nowait() - release allocated pages without blocking
+ * @cma: Contiguous memory region for which the allocation is performed.
+ * @pages: Allocated pages.
+ * @count: Number of allocated pages.
+ *
+ * Similar to cma_release(), this function releases memory allocated
+ * by cma_alloc(), but unlike cma_release() is non-blocking and can be
+ * called from an atomic context.
+ * It returns false when provided pages do not belong to contiguous area
+ * and true otherwise.
+ */
+bool cma_release_nowait(struct cma *cma, const struct page *pages,
+ unsigned int count)
+{
+ struct cma_clear_bitmap_work *work;
+ unsigned long pfn;
+
+ if (!cma || !pages)
+ return false;
+
+ pr_debug("%s(page %p)\n", __func__, (void *)pages);
+
+ pfn = page_to_pfn(pages);
+
+ if (pfn < cma->base_pfn || pfn >= cma->base_pfn + cma->count)
+ return false;
+
+ VM_BUG_ON(pfn + count > cma->base_pfn + cma->count);
+
+ /*
+ * Set CMA_DELAYED_RELEASE flag: subsequent cma_alloc()'s
+ * will wait for the async part of cma_release_nowait() to
+ * finish.
+ */
+ if (unlikely(!test_bit(CMA_DELAYED_RELEASE, &cma->flags)))
+ set_bit(CMA_DELAYED_RELEASE, &cma->flags);
+
+ /*
+ * To make cma_release_nowait() non-blocking, cma bitmap is cleared
+ * from a work context (see cma_clear_bitmap_fn()). The first page
+ * in the cma allocation is used to store the work structure,
+ * so it's released after the cma bitmap clearance. Other pages
+ * are released immediately as previously.
+ */
+ if (count > 1)
+ free_contig_range(pfn + 1, count - 1);
+
+ work = (struct cma_clear_bitmap_work *)page_to_virt(pages);
+ INIT_WORK(&work->work, cma_clear_bitmap_fn);
+ work->cma = cma;
+ work->pfn = pfn;
+ work->count = count;
+ queue_work(cma_release_wq, &work->work);
+
+ trace_cma_release(pfn, pages, count);
+
+ return true;
+}
+
int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data)
{
int i;
diff --git a/mm/cma.h b/mm/cma.h
index 42ae082cb067..e9293871d122 100644
--- a/mm/cma.h
+++ b/mm/cma.h
@@ -7,6 +7,7 @@
struct cma {
unsigned long base_pfn;
unsigned long count;
+ unsigned long flags;
unsigned long *bitmap;
unsigned int order_per_bit; /* Order of pages represented by one bit */
struct mutex lock;
@@ -18,6 +19,10 @@ struct cma {
char name[CMA_MAX_NAME];
};
+enum cma_flags {
+ CMA_DELAYED_RELEASE, /* cma bitmap is cleared asynchronously */
+};
+
extern struct cma cma_areas[MAX_CMA_AREAS];
extern unsigned cma_area_count;
--
2.26.2