firmware_loader: Replace kmap() with kmap_local_page()

The use of kmap() is being deprecated in favor of kmap_local_page().

Two main problems with kmap(): (1) It comes with an overhead as mapping
space is restricted and protected by a global lock for synchronization and
(2) kmap() also requires global TLB invalidation when the kmap’s pool
wraps and it might block when the mapping space is fully utilized until a
slot becomes available.

kmap_local_page() is preferred over kmap() and kmap_atomic(). Where it
cannot mechanically replace the latters, code refactor should be considered
(special care must be taken if kernel virtual addresses are aliases in
different contexts).

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).

Call kmap_local_page() in firmware_loader wherever kmap() is currently
used. In firmware_rw() use the helpers copy_{from,to}_page() instead of
open coding the local mappings + memcpy().

Successfully tested with "firmware" selftests on a QEMU/KVM 32-bits VM
with 4GB RAM, booting a kernel with HIGHMEM64GB enabled.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20220714235030.12732-1-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: xp4ns3

drivers/base/firmware_loader/main.c

@@ -435,11 +435,11 @@ static int fw_decompress_xz_pages(struct device *dev, struct fw_priv *fw_priv,
 
 		/* decompress onto the new allocated page */
 		page = fw_priv->pages[fw_priv->nr_pages - 1];
-		xz_buf.out = kmap(page);
+		xz_buf.out = kmap_local_page(page);
 		xz_buf.out_pos = 0;
 		xz_buf.out_size = PAGE_SIZE;
 		xz_ret = xz_dec_run(xz_dec, &xz_buf);
-		kunmap(page);
+		kunmap_local(xz_buf.out);
 		fw_priv->size += xz_buf.out_pos;
 		/* partial decompression means either end or error */
 		if (xz_buf.out_pos != PAGE_SIZE)

drivers/base/firmware_loader/sysfs.c

@@ -242,19 +242,17 @@ static void firmware_rw(struct fw_priv *fw_priv, char *buffer,
 			loff_t offset, size_t count, bool read)
 {
 	while (count) {
-		void *page_data;
 		int page_nr = offset >> PAGE_SHIFT;
 		int page_ofs = offset & (PAGE_SIZE - 1);
 		int page_cnt = min_t(size_t, PAGE_SIZE - page_ofs, count);
 
-		page_data = kmap(fw_priv->pages[page_nr]);
-
 		if (read)
-			memcpy(buffer, page_data + page_ofs, page_cnt);
+			memcpy_from_page(buffer, fw_priv->pages[page_nr],
+					 page_ofs, page_cnt);
 		else
-			memcpy(page_data + page_ofs, buffer, page_cnt);
+			memcpy_to_page(fw_priv->pages[page_nr], page_ofs,
+				       buffer, page_cnt);
 
-		kunmap(fw_priv->pages[page_nr]);
 		buffer += page_cnt;
 		offset += page_cnt;
 		count -= page_cnt;