On Thu, 19 Jul 2007 13:28:56 +0400 Evgeniy Polyakov [off-list ref] wrote:

Hi.

On Thu, Jul 19, 2007 at 01:33:04AM -0700, Andrew Morton (akpm@linux-foundation.org) wrote:

quoted

I don't think the 2-year-old Vaio has offload engine support ;) Dan, this:

+		if (flags & ASYNC_TX_KMAP_DST)
+			dest_buf = kmap_atomic(dest, KM_USER0) + dest_offset;
+		else
+			dest_buf = page_address(dest) + dest_offset;
+
+		if (flags & ASYNC_TX_KMAP_SRC)
+			src_buf = kmap_atomic(src, KM_USER0) + src_offset;
+		else
+			src_buf = page_address(src) + src_offset;
+
+		memcpy(dest_buf, src_buf, len);
+
+		if (flags & ASYNC_TX_KMAP_DST)
+			kunmap_atomic(dest_buf, KM_USER0);
+
+		if (flags & ASYNC_TX_KMAP_SRC)
+			kunmap_atomic(src_buf, KM_USER0);
+

is very wrong if both ASYNC_TX_KMAP_DST and ASYNC_TX_KMAP_SRC can ever be
set.  We'll end up using the same kmap slot for both src add dest and we
get either corrupted data or a BUG.

So far it can not since the only user is raid code, which only allows to
perform either reading from bio or writing into one, which requires only
one mapping.

hm, so we got lucky?

Btw, shouldn't it always be kmap_atomic() even if flag is not set.
That pages are usual one returned by alloc_page().

The code would work OK if the kmap_atomic()s were unconditional, but it
would be a bit more expensive if the page is in highmem and we don't
actually intend to access it with the CPU.

kmap_atomic() against a non-highmem page is basically free: just an
additional test_bit().