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

[Claude Code Review Bot <claude-review@example.com>]

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