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Subject[PATCH v4] mm: cma: indefinitely retry allocations in cma_alloc
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CMA allocations will fail if 'pinned' pages are in a CMA area, since we
cannot migrate pinned pages. The _refcount of a struct page being greater
than _mapcount for that page can cause pinning for anonymous pages. This
is because try_to_unmap(), which (1) is called in the CMA allocation path,
and (2) decrements both _refcount and _mapcount for a page, will stop
unmapping a page from VMAs once the _mapcount for a page reaches 0. This
implies that after try_to_unmap() has finished successfully for a page
where _recount > _mapcount, that _refcount will be greater than 0. Later
in the CMA allocation path in migrate_page_move_mapping(), we will have one
more reference count than intended for anonymous pages, meaning the
allocation will fail for that page.

If a process ends up causing _refcount > _mapcount for a page (by either
incrementing _recount or decrementing _mapcount), such that the process is
context switched out after modifying one refcount but before modifying the
other, the page will be temporarily pinned.

One example of where _refcount can be greater than _mapcount is inside of
zap_pte_range(), which is called for all the entries of a PMD when a
process is exiting, to unmap the process's memory. Inside of
zap_pte_range(), after unammping a page with page_remove_rmap(), we have
that _recount > _mapcount. _refcount can only be decremented after a TLB
flush is performed for the page - this doesn't occur until enough pages
have been batched together for flushing. The flush can either occur inside
of zap_pte_range() (during the same invocation or a later one), or if there
aren't enough pages collected by the time we unmap all of the pages in a
process, the flush will occur in tlb_finish_mmu() in exit_mmap(). After
the flush has occurred, tlb_batch_pages_flush() will decrement the
references on the flushed pages.

Another such example like the above is inside of copy_one_pte(), which is
called during a fork. For PTEs for which pte_present(pte) == true,
copy_one_pte() will increment the _refcount field followed by the
_mapcount field of a page.

So, inside of cma_alloc(), add the option of letting users pass in
__GFP_NOFAIL to indicate that we should retry CMA allocations indefinitely,
in the event that alloc_contig_range() returns -EBUSY after having scanned
a whole CMA-region bitmap.

Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>
Co-developed-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
---
arch/powerpc/kvm/book3s_hv_builtin.c | 2 +-
drivers/dma-buf/heaps/cma_heap.c | 2 +-
drivers/s390/char/vmcp.c | 2 +-
drivers/staging/android/ion/ion_cma_heap.c | 2 +-
include/linux/cma.h | 2 +-
kernel/dma/contiguous.c | 4 ++--
mm/cma.c | 36 +++++++++++++++++++++++++-----
mm/cma_debug.c | 2 +-
mm/hugetlb.c | 4 ++--
9 files changed, 40 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_builtin.c b/arch/powerpc/kvm/book3s_hv_builtin.c
index 073617c..21c3f6a 100644
--- a/arch/powerpc/kvm/book3s_hv_builtin.c
+++ b/arch/powerpc/kvm/book3s_hv_builtin.c
@@ -74,7 +74,7 @@ struct page *kvm_alloc_hpt_cma(unsigned long nr_pages)
VM_BUG_ON(order_base_2(nr_pages) < KVM_CMA_CHUNK_ORDER - PAGE_SHIFT);

return cma_alloc(kvm_cma, nr_pages, order_base_2(HPT_ALIGN_PAGES),
- false);
+ 0);
}
EXPORT_SYMBOL_GPL(kvm_alloc_hpt_cma);

diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/cma_heap.c
index 626cf7f..7657359 100644
--- a/drivers/dma-buf/heaps/cma_heap.c
+++ b/drivers/dma-buf/heaps/cma_heap.c
@@ -66,7 +66,7 @@ static int cma_heap_allocate(struct dma_heap *heap,
helper_buffer->heap = heap;
helper_buffer->size = len;

- cma_pages = cma_alloc(cma_heap->cma, nr_pages, align, false);
+ cma_pages = cma_alloc(cma_heap->cma, nr_pages, align, 0);
if (!cma_pages)
goto free_buf;

diff --git a/drivers/s390/char/vmcp.c b/drivers/s390/char/vmcp.c
index 9e06628..11c4e3b 100644
--- a/drivers/s390/char/vmcp.c
+++ b/drivers/s390/char/vmcp.c
@@ -70,7 +70,7 @@ static void vmcp_response_alloc(struct vmcp_session *session)
* anymore the system won't work anyway.
*/
if (order > 2)
- page = cma_alloc(vmcp_cma, nr_pages, 0, false);
+ page = cma_alloc(vmcp_cma, nr_pages, 0, 0);
if (page) {
session->response = (char *)page_to_phys(page);
session->cma_alloc = 1;
diff --git a/drivers/staging/android/ion/ion_cma_heap.c b/drivers/staging/android/ion/ion_cma_heap.c
index bf65e67..128d3a5 100644
--- a/drivers/staging/android/ion/ion_cma_heap.c
+++ b/drivers/staging/android/ion/ion_cma_heap.c
@@ -39,7 +39,7 @@ static int ion_cma_allocate(struct ion_heap *heap, struct ion_buffer *buffer,
if (align > CONFIG_CMA_ALIGNMENT)
align = CONFIG_CMA_ALIGNMENT;

- pages = cma_alloc(cma_heap->cma, nr_pages, align, false);
+ pages = cma_alloc(cma_heap->cma, nr_pages, align, 0);
if (!pages)
return -ENOMEM;

diff --git a/include/linux/cma.h b/include/linux/cma.h
index 6ff79fe..2bd8544 100644
--- a/include/linux/cma.h
+++ b/include/linux/cma.h
@@ -43,7 +43,7 @@ extern int cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
const char *name,
struct cma **res_cma);
extern struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
- bool no_warn);
+ gfp_t gfp_mask);
extern bool cma_release(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);
diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
index cff7e60..55c62b2 100644
--- a/kernel/dma/contiguous.c
+++ b/kernel/dma/contiguous.c
@@ -196,7 +196,7 @@ struct page *dma_alloc_from_contiguous(struct device *dev, size_t count,
if (align > CONFIG_CMA_ALIGNMENT)
align = CONFIG_CMA_ALIGNMENT;

- return cma_alloc(dev_get_cma_area(dev), count, align, no_warn);
+ return cma_alloc(dev_get_cma_area(dev), count, align, no_warn ? __GFP_NOWARN : 0);
}

/**
@@ -219,7 +219,7 @@ static struct page *cma_alloc_aligned(struct cma *cma, size_t size, gfp_t gfp)
{
unsigned int align = min(get_order(size), CONFIG_CMA_ALIGNMENT);

- return cma_alloc(cma, size >> PAGE_SHIFT, align, gfp & __GFP_NOWARN);
+ return cma_alloc(cma, size >> PAGE_SHIFT, align, gfp);
}

/**
diff --git a/mm/cma.c b/mm/cma.c
index 7f415d7..5d63331 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -32,6 +32,8 @@
#include <linux/highmem.h>
#include <linux/io.h>
#include <linux/kmemleak.h>
+#include <linux/sched.h>
+#include <linux/jiffies.h>
#include <trace/events/cma.h>

#include "cma.h"
@@ -403,13 +405,15 @@ static inline void cma_debug_show_areas(struct cma *cma) { }
* @cma: Contiguous memory region for which the allocation is performed.
* @count: Requested number of pages.
* @align: Requested alignment of pages (in PAGE_SIZE order).
- * @no_warn: Avoid printing message about failed allocation
+ * @gfp_mask: If __GFP_NOWARN is passed, suppress messages about failed
+ * allocations. If __GFP_NOFAIL is passed, try doing the CMA
+ * allocation indefinitely until the allocation succeeds.
*
* This function allocates part of contiguous memory on specific
* contiguous memory area.
*/
struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
- bool no_warn)
+ gfp_t gfp_mask)
{
unsigned long mask, offset;
unsigned long pfn = -1;
@@ -442,8 +446,28 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
bitmap_maxno, start, bitmap_count, mask,
offset);
if (bitmap_no >= bitmap_maxno) {
- mutex_unlock(&cma->lock);
- break;
+ if (ret == -EBUSY && gfp_mask & __GFP_NOFAIL) {
+ mutex_unlock(&cma->lock);
+
+ /*
+ * Page may be momentarily pinned by some other
+ * process which has been scheduled out, e.g.
+ * in exit path, during unmap call, or process
+ * fork and so cannot be freed there. Sleep
+ * for 100 ms and retry the allocation.
+ */
+ start = 0;
+ ret = -ENOMEM;
+ schedule_timeout_killable(msecs_to_jiffies(100));
+ continue;
+ } else {
+ /*
+ * ret == -ENOMEM - all bits in cma->bitmap are
+ * set, so we break accordingly.
+ */
+ mutex_unlock(&cma->lock);
+ break;
+ }
}
bitmap_set(cma->bitmap, bitmap_no, bitmap_count);
/*
@@ -456,7 +480,7 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
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));
+ GFP_KERNEL | (gfp_mask & __GFP_NOWARN));
mutex_unlock(&cma_mutex);
if (ret == 0) {
page = pfn_to_page(pfn);
@@ -485,7 +509,7 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
page_kasan_tag_reset(page + i);
}

- if (ret && !no_warn) {
+ if (ret && !(gfp_mask & __GFP_NOWARN)) {
pr_err("%s: alloc failed, req-size: %zu pages, ret: %d\n",
__func__, count, ret);
cma_debug_show_areas(cma);
diff --git a/mm/cma_debug.c b/mm/cma_debug.c
index d5bf8aa..76aea84 100644
--- a/mm/cma_debug.c
+++ b/mm/cma_debug.c
@@ -137,7 +137,7 @@ static int cma_alloc_mem(struct cma *cma, int count)
if (!mem)
return -ENOMEM;

- p = cma_alloc(cma, count, 0, false);
+ p = cma_alloc(cma, count, 0, 0);
if (!p) {
kfree(mem);
return -ENOMEM;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 67fc6383..97bdba9 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1260,7 +1260,7 @@ static struct page *alloc_gigantic_page(struct hstate *h, gfp_t gfp_mask,

if (hugetlb_cma[nid]) {
page = cma_alloc(hugetlb_cma[nid], nr_pages,
- huge_page_order(h), true);
+ huge_page_order(h), __GFP_NOWARN);
if (page)
return page;
}
@@ -1271,7 +1271,7 @@ static struct page *alloc_gigantic_page(struct hstate *h, gfp_t gfp_mask,
continue;

page = cma_alloc(hugetlb_cma[node], nr_pages,
- huge_page_order(h), true);
+ huge_page_order(h), __GFP_NOWARN);
if (page)
return page;
}
--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project
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