[PATCH] dma-buf: heaps: Clear CMA pages with clear_pages()

[PATCH] dma-buf: heaps: Clear CMA pages with clear_pages()

[Linus Walleij]

As of commit 62a9f5a85b98
"mm: introduce clear_pages() and clear_user_pages()" we can
clear a range of pages with a potentially assembly-optimized
call.

Instead of using a memset, use this helper to clear the whole
range of pages from the CMA allocation.

Signed-off-by: Linus Walleij <linusw@kernel.org>
---
 drivers/dma-buf/heaps/cma_heap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/cma_heap.c
index bd3370b9a3f6..f0bacf25ed9d 100644
--- a/drivers/dma-buf/heaps/cma_heap.c
+++ b/drivers/dma-buf/heaps/cma_heap.c
@@ -343,7 +343,7 @@ static struct dma_buf *cma_heap_allocate(struct dma_heap *heap,
 			nr_clear_pages--;
 		}
 	} else {
-		memset(page_address(cma_pages), 0, size);
+		clear_pages(page_address(cma_pages), pagecount);
 	}
 
 	buffer->pages = kmalloc_objs(*buffer->pages, pagecount);

---
base-commit: 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f
change-id: 20260303-cma-heap-clear-pages-540f3ac9f734

Best regards,
-- 
Linus Walleij <linusw@kernel.org>

Claude review: dma-buf: heaps: Clear CMA pages with clear_pages()

[Claude Code Review Bot]

Patch Review

**Correctness of the API change**: The conversion from `memset(..., 0, size)` to `clear_pages(addr, pagecount)` is semantically correct — both zero out the same memory range. The `page_address()` return value is suitable for `clear_pages()`.

**Preemption latency concern**: The `clear_pages()` documentation in `include/linux/mm.h:4410-4417` explicitly states:

> clear_pages() does not (and on architectures where it reduces to a few long-running instructions, might not be able to) call cond_resched() to check if rescheduling is required.
> ...the caller is expected to limit @npages to no more than PROCESS_PAGES_NON_PREEMPT_BATCH.

Here, `pagecount` is derived directly from the user-supplied `len` parameter at line 303-304:

```c
size_t size = PAGE_ALIGN(len);
pgoff_t pagecount = size >> PAGE_SHIFT;
```

CMA allocations can be quite large (tens or hundreds of MB). On architectures that define `clear_pages`, `PROCESS_PAGES_NON_PREEMPT_BATCH` is `SZ_32M >> PAGE_SHIFT` (8192 pages). If a user requests, say, a 256MB buffer, `pagecount` would be 65536, far exceeding the recommended batch limit.

The original `memset()` had the same theoretical issue (no `cond_resched()`), so this isn't a regression in that sense. But adopting `clear_pages()` while ignoring its documented calling convention is worth noting. Consider batching the clear in a loop with `cond_resched()` checks, similar to how the HighMem path iterates page-by-page with a `fatal_signal_pending()` check. For example:

```c
unsigned long remaining = pagecount;
void *addr = page_address(cma_pages);
while (remaining > 0) {
    unsigned int batch = min_t(unsigned long, remaining,
                               PROCESS_PAGES_NON_PREEMPT_BATCH);
    clear_pages(addr, batch);
    addr += batch * PAGE_SIZE;
    remaining -= batch;
    cond_resched();
    if (fatal_signal_pending(current))
        goto free_cma;
}
```

This would also bring parity with the HighMem path's `fatal_signal_pending()` check, which the non-HighMem path currently lacks (and has always lacked, even before this patch).

**Missing include**: Verify that `cma_heap.c` pulls in the `clear_pages()` declaration. It's defined in `<linux/mm.h>`, which is likely already included transitively, but it's worth confirming.

**Overall**: The patch is a reasonable micro-optimization. The main suggestion is to respect the `PROCESS_PAGES_NON_PREEMPT_BATCH` limit documented in the `clear_pages()` API, and optionally add the `fatal_signal_pending()` check that the HighMem path already has.

---
Generated by Claude Code Patch Reviewer

Claude review: dma-buf: heaps: Clear CMA pages with clear_pages()

[Claude Code Review Bot]

Overall Series Review

Subject: dma-buf: heaps: Clear CMA pages with clear_pages()
Author: Linus Walleij <linusw@kernel.org>
Patches: 1
Reviewed: 2026-03-05T14:00:37.741553

---

This is a single-patch series that replaces `memset(page_address(cma_pages), 0, size)` with `clear_pages(page_address(cma_pages), pagecount)` in the CMA heap allocator's non-highmem path. The change leverages the newer `clear_pages()` API (introduced by commit 62a9f5a85b98) which can use architecture-optimized page-clearing instructions.

The conversion is straightforward and functionally correct. However, there is one concern regarding preemption latency that should be addressed.

---
Generated by Claude Code Patch Reviewer