David Miller wrote:	[Sun Mar 22 2015, 01:36:03PM EDT]

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 21 Mar 2015 11:49:12 -0700

quoted

Davem? I don't read sparc assembly, so I'm *really* not going to try
to verify that (a) all the memcpy implementations always copy
low-to-high and (b) that I even read the address comparisons in
memmove.S right.

All of the sparc memcpy implementations copy from low to high.
I'll eat my hat if they don't. :-)

The guard tests at the beginning of memmove() are saying:

	if (dst <= src)
		memcpy(...);
	if (src + len <= dst)
		memcpy(...);

And then the reverse copy loop (and we do have to copy in reverse for
correctness) is basically:

	src = (src + len - 1);
	dst = (dst + len - 1);

1:	tmp = *(u8 *)src;
	len -= 1;
	src -= 1;
	*(u8 *)dst = tmp;
	dst -= 1;
	if (len != 0)
		goto 1b;

And then we return the original 'dst' pointer.

So at first glance it looks at least correct.

memmove() is a good idea to look into though, as SLAB and SLUB are the
only really heavy users of it, and they do so with overlapping
contents.

And they end up using that byte-at-a-time code, since SLAB and SLUB
do mmemove() calls of the form:

	memmove(X + N, X, LEN);

In which case neither of the memcpy() guard tests will pass.

Maybe there is some subtle bug in there I just don't see right now.

My original pursuit of this issue focused on transfers to and from the shared
array. Basically substituting memcpy-s with a primitive unsigned long memory
mover. This might have been incorrect.

There were substantial doubts because of large modifications to 2.6.39 too.
Unstabile hardware cause(d|s) issue too.

Eliminating the shared array functions correctly. Though this removal changes
performance and timing dramatically.

This afternoon I included modification of two memmove-s and no issue thus far.
The issue APPEARS to come from memmove-s within cache_flusharray() and/or
drain_array(). Now we are covering moves within an array_cache.

The above was done on 2.6.39.

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