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    SubjectRe: Make sure we populate the initroot filesystem late enough
    Date
    On Feb 27, 2007, at 2:24 AM, David Woodhouse wrote:
    > On Sun, 2007-02-25 at 20:13 -0800, Linus Torvalds wrote:
    >> On Sun, 25 Feb 2007, David Woodhouse wrote:
    >>>> Can you try adding something like
    >>>>
    >>>> memset(start, 0xf0, end - start);
    >>>
    >>> Yeah, I did that before giving up on it for the day and going in
    >>> search
    >>> of dinner. It changes the failure mode to a BUG() in
    >>> cache_free_debugcheck(), at line 2876 of mm/slab.c
    >>
    >> Ok, that's just strange.
    >
    > In this case I hadn't left the 'return' in free_initrd_mem(). I was
    > poisoning the pages and then returning them to the pool as usual.
    >
    > If I poison the pages and _don't_ return them to the pool, it boots
    > fine. PageReserved is set on every page in the initrd region; total
    > page_count() is equal to the number of pages (which doesn't
    > _necessarily_ mean that page_count() for every page is equal to 1 but
    > it's a strong hint that that's the case).
    >
    > Looking in /dev/mem after it boots, I see that my poison is still
    > present throughout the whole region.
    >
    >> One obvious thing to do would be to remove all the "__initdata"
    >> entries in
    >> mm/slab.c..
    >
    > This is biting us long before we call free_initmem().
    >
    >> But I'd also like to see the full backtrace for the BUG_ON(),
    >> in case that gives any clues at all.
    >
    > I'll see if I can find a camera.
    >
    >>> It smells like the pages weren't actually reserved in the first place
    >>> and we were blithely allocating them. The only problem with that
    >>> theory
    >>> is that the initrd doesn't seem to be getting corrupted -- and if we
    >>> were handing out its pages like that then surely _something_ would
    >>> have
    >>> scribbled on it before we tried to read it.
    >>
    >> Yeah, I don't think it's necessarily initrd itself, I'd be more
    >> inclined
    >> to think that the reason you see this change with the initrd
    >> unpacking is
    >> simply that it does a lot of allocations for the initrd files, so I
    >> think
    >> it is only indirectly involved - just because it ends up being a slab
    >> user.
    >
    > Whatever happens, initrd as a 'slab user' is fine. The crashes happen
    > _later_, when someone else is using the memory which used to belong to
    > the initrd. In that 'BUG at slab.c:2876' I mentioned above, r3 was
    > within the initrd region. As I said, I'll try to find a camera.


    Just a thought,

    Any chance you are using one of the unusal code paths, like the
    bootloader
    moving the initrd or using a kernel-crash region?


    milton

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