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    SubjectRe: Oops with DCACHE_WORD_ACCESS and ocfs2, autofs4
    On Thu, May 3, 2012 at 9:15 AM, Linus Torvalds
    <torvalds@linux-foundation.org> wrote:
    >
    > So I guess I need to do the exception handling that I was hoping I
    > wouldn't have to. Give me a jiffy.

    Ok, that took longer than a jiffy, the asm was just nasty to get right
    with all the proper suffixes for 32-bit vs 64-bit, and the fact that
    gas apparently really needs %cl for the shift count, and doesn't like
    %rcx. Silly assembler.

    Also, the asm would have been much simpler if I didn't care so much
    about the regular fast-path. I wanted the fast-path for the asm to be
    a single load, with no downside, and everything fixed up in the
    exception case.

    And it's close. It's a single load, and the only downside is that
    register '%rcx' is marked as used, because *if* the exception happens,
    we want to use %rcx do the alignment fixup.

    Peter, in particular, can you double (and triple-) check my asm, to
    see if I missed anything? It does that "lea" of the address into %rcx
    twice, because that way we don't need any other register temporaries.

    On 32-bit, this results in:

    - fast-path single-instruction unaligned load (with gcc free to pick
    registers and addressing modes):

    movl (%edi,%edx),%eax

    - with the exception fixup code becoming:

    leal (%edi,%edx),%ecx
    andl $-4,%ecx
    movl (%ecx),%eax
    leal (%edi,%edx),%ecx
    andl $3,%ecx
    shll $3,%ecx
    shll %cl,%eax
    shrl %cl,%eax
    jmp 2b

    which looks ok. I don't worry about the efficiency of the fixup code,
    because if that code is ever entered we will have taken a page fault
    etc, so the only thing to worry about is that the fixup doesn't
    need/fix any unnecessary extra registers so that the fast-path case
    doesn't get less flexible.

    Does anybody see anything wrong with this?

    Anyway, with this, I guess we could enable word-at-a-time even with
    CONFIG_DEBUG_PAGEALLOC on x86, and that might even be a good idea for
    coverage.

    Jana - does the attached patch work for you?

    Linus
    [unhandled content-type:application/octet-stream]
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