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SubjectRe: 4.0.0-rc4: panic in free_block
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
>
> > 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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