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    SubjectRe: kerneloops.org: 2.6.26-rc possible regression in ext3


    On Wed, 18 Jun 2008, Linus Torvalds wrote:
    >
    > and that "lea" is doing an address computation of "eax+2*ebx-2". Which
    > does *not* look like an address to a 32-bit entity, but to a 16-bit one.
    > Yeah, it's not conclusive, but it is suggestive.

    I'm wrong, that's just "eax+ebx-2". The *2 was just a brainfart on my
    part.

    But I think I have pinpointed where it comes from: it's the

    struct dx_map_entry *map;

    which is a structure like this:

    struct dx_map_entry
    {
    u32 hash;
    u16 offs;
    u16 size;
    };

    and it does look like it's the

    if (size + map[i].size/2 > blocksize/2)

    calculation, where "i" counts backwards from "count-1" to 0.

    In particular, the code

    27: 8d 4c 18 fe lea 0xfffffffe(%eax,%ebx,1),%ecx
    2b:* 8b 19 mov (%ecx),%ebx <-- trapping instruction
    2d: 83 e9 08 sub $0x8,%ecx
    30: 89 d8 mov %ebx,%eax
    32: 66 d1 e8 shr %ax
    38: 8d 04 02 lea (%edx,%eax,1),%eax

    seems to be that "size + map[i].size/2" calculation, but I have a hard
    time trying to line it up with wat _my_ compiler gives me. But the nearest
    match I have is:

    movw 6(%ecx), %bx # <variable>.size, D.21305
    subl $8, %ecx #, ivtmp.921
    movl -104(%ebp), %edx # blocksize, tmp179
    movl %ebx, %eax # D.21305, tmp176
    shrw %ax # tmp176
    movzwl %ax, %eax # tmp176, tmp177
    leal (%esi,%eax), %eax #, tmp178

    which seems to be largely the same thing (except I have a "movw" to load
    the size, and %ecx is offset by one 'map' entry - so the offset is 6 (in
    the memop) instead of that "-2" (from the lea).

    I think I'll give up, but that's the closest match I can find. No
    guarantees, but it seems to support the notion of "wrong 32-bit load where
    it should have used a 16-bit one".

    Linus


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