[PATCH 3/9] sparc32: do not flush a NULL address in flush_dcache_folio

[Magnus Lindholm <linmag7@gmail.com>]

sparc_flush_folio_to_ram() is sparc32's flush_dcache_folio() and takes the
address to flush from folio_address(). For a highmem folio that is NULL,
so it flushes virtual address 0, and 0 + PAGE_SIZE and so on for a multi-
page folio.

On HyperSPARC, hypersparc_flush_page_to_ram() probes with
ASI_M_FLUSH_PROBE, finds nothing mapped at 0 and returns, so nothing is
flushed at all.

Returning early would be wrong: flush_dcache_folio() is also the barrier
taken before reading a folio userspace may have dirtied, as in
filemap_read() when the folio is writably mapped. Fall back to the whole-
cache operation, which reaches user aliases at any colour and on SMP is
cross-called to every cpu.

Do that only when the folio is mapped by userspace; otherwise the only
other mapping is the kmap_local slot, which arch_kmap_local_pre_unmap()
writes back on teardown. Without that test block_write_end() makes every
buffered write discard 512 KB of VAC, which cost 106 s against 56 s on a
64 MB dd.

With the mapping live and a reader on another cpu, 150 of 150 reads
returned stale data before and 0 of 150 after, every first mismatch on a
32-byte boundary, the VAC line size.

The separate HyperSPARC mapped-folio fix completes the lowmem and PKMAP
colour handling. The highmem fallback can be expensive for GUP: each
mapped folio may require whole-cache maintenance on all CPUs.

Fixes: 665f64029454 ("sparc32: implement the new page table range API")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/sparc/mm/init_32.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/sparc/mm/init_32.c b/arch/sparc/mm/init_32.c
index 7b53ee9f061f..9cf0065fc2d4 100644
--- a/arch/sparc/mm/init_32.c
+++ b/arch/sparc/mm/init_32.c
@@ -131,6 +131,16 @@ void sparc_flush_folio_to_ram(struct folio *folio)
 	unsigned long vaddr = (unsigned long)folio_address(folio);
 	unsigned int i, nr = folio_nr_pages(folio);
 
+	/*
+	 * No kernel address to aim at. Only a user alias can be incoherent
+	 * here; the kmap the kernel used is flushed when it is unmapped.
+	 */
+	if (!vaddr) {
+		if (folio_mapped(folio))
+			flush_cache_all();
+		return;
+	}
+
 	for (i = 0; i < nr; i++)
 		__flush_page_to_ram(vaddr + i * PAGE_SIZE);
 }
-- 
2.43.0