linux-kernel - [PATCH v2] mm: cma: indefinitely retry allocations in cma

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Date:   Fri, 11 Sep 2020 20:24:38 +0000
From:   Chris Goldsworthy <cgoldswo@...eaurora.org>
To:     linux-kernel@...r.kernel.org
Cc:     Chris Goldsworthy <cgoldswo@...eaurora.org>,
        Vinayak Menon <vinmenon@...eaurora.org>
Subject: [PATCH v2] mm: cma: indefinitely retry allocations in cma_alloc

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.

One example of where _refcount can be greater than _mapcount for a page we
would not expect to be pinned 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. If the process doing copy_one_pte() is context switched out after
incrementing _refcount but before incrementing _mapcount, then the page
will be temporarily pinned.

So, inside of cma_alloc(), instead of giving up when alloc_contig_range()
returns -EBUSY after having scanned a whole CMA-region bitmap, perform
retries indefinitely, with sleeps, to give the system an opportunity to
unpin any pinned pages.

Signed-off-by: Chris Goldsworthy <cgoldswo@...eaurora.org>
Co-developed-by: Vinayak Menon <vinmenon@...eaurora.org>
Signed-off-by: Vinayak Menon <vinmenon@...eaurora.org>
---
 mm/cma.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/mm/cma.c b/mm/cma.c
index 7f415d7..90bb505 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -32,6 +32,7 @@
 #include <linux/highmem.h>
 #include <linux/io.h>
 #include <linux/kmemleak.h>
+#include <linux/delay.h>
 #include <trace/events/cma.h>

 #include "cma.h"
@@ -442,8 +443,28 @@ 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) {
+				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 100ms and retry the allocation.
+				 */
+				start = 0;
+				ret = -ENOMEM;
+				msleep(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);
 		/*
-- 
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