On Thu, Jul 19, 2007 at 02:38:31AM -0700, Andrew Morton (akpm@linux-foundation.org) wrote:

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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?

I would say it was intentionally, current code can perform only one
operation in a time. Of course changing KM_USER from 0 to 1 in second
kmap_atomic will not force oceans to run out of coasts.

Kind of:

diff --git a/crypto/async_tx/async_memcpy.c b/crypto/async_tx/async_memcpy.c
index a973f4e..a48c7f3 100644
--- a/crypto/async_tx/async_memcpy.c
+++ b/crypto/async_tx/async_memcpy.c

@@ -94,7 +94,7 @@ async_memcpy(struct page *dest, struct page *src, unsigned int dest_offset,
                        dest_buf = page_address(dest) + dest_offset;

                if (flags & ASYNC_TX_KMAP_SRC)
-                       src_buf = kmap_atomic(src, KM_USER0) + src_offset;
+                       src_buf = kmap_atomic(src, KM_USER1) + src_offset;
                else
                        src_buf = page_address(src) + src_offset;

@@ -104,7 +104,7 @@ async_memcpy(struct page *dest, struct page *src, unsigned int dest_offset,
                        kunmap_atomic(dest_buf, KM_USER0);

                if (flags & ASYNC_TX_KMAP_SRC)
-                       kunmap_atomic(src_buf, KM_USER0);
+                       kunmap_atomic(src_buf, KM_USER1);

                async_tx_sync_epilog(flags, depend_tx, cb_fn, cb_param);
        }

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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().