linux-kernel - [PATCH] percpu: clean up all mappings when pcpu_map

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Message-ID: <20240311194346.2291333-1-yosryahmed@google.com>
Date: Mon, 11 Mar 2024 19:43:46 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>, 
	Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Yosry Ahmed <yosryahmed@...gle.com>
Subject: [PATCH] percpu: clean up all mappings when pcpu_map_pages() fails

In pcpu_map_pages(), if __pcpu_map_pages() fails on a CPU, we call
__pcpu_unmap_pages() to clean up mappings on all CPUs where mappings
were created, but not on the CPU where __pcpu_map_pages() fails.

__pcpu_map_pages() and __pcpu_unmap_pages() are wrappers around
vmap_pages_range_noflush() and vunmap_range_noflush(). All other callers
of vmap_pages_range_noflush() call vunmap_range_noflush() when mapping
fails, except pcpu_map_pages(). The reason could be that partial
mappings may be left behind from a failed mapping attempt.

Call __pcpu_unmap_pages() for the failed CPU as well in
pcpu_map_pages().

This was found by code inspection, no failures or bugs were observed.

Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
---

Perhaps the reason __pcpu_unmap_pages() is not currently being called
for the failed CPU is that the size and alignment requirements make sure
we never leave any partial mappings behind? I have no idea. Nonetheless,
I think we want this change as that could be fragile, and is
inconsistent with other callers.

---
 mm/percpu-vm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
index 2054c9213c433..cd69caf6aa8d8 100644
--- a/mm/percpu-vm.c
+++ b/mm/percpu-vm.c
@@ -231,10 +231,10 @@ static int pcpu_map_pages(struct pcpu_chunk *chunk,
 	return 0;
 err:
 	for_each_possible_cpu(tcpu) {
-		if (tcpu == cpu)
-			break;
 		__pcpu_unmap_pages(pcpu_chunk_addr(chunk, tcpu, page_start),
 				   page_end - page_start);
+		if (tcpu == cpu)
+			break;
 	}
 	pcpu_post_unmap_tlb_flush(chunk, page_start, page_end);
 	return err;
-- 
2.44.0.278.ge034bb2e1d-goog