lkml.org 
[lkml]   [2008]   [Dec]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: early fixmap causes kmap breakage
    On Mon, Dec 29, 2008 at 10:22:12PM -0800, Eric W. Biederman wrote:
    > Nick Piggin <npiggin@suse.de> writes:
    >
    > > On Mon, Dec 29, 2008 at 03:17:31PM -0800, Andrew Morton wrote:
    > >> On Thu, 18 Dec 2008 22:15:43 +0100
    > >> Nick Piggin <npiggin@suse.de> wrote:
    > >>
    > >> > Hi,
    > >> >
    > >> > I've debugged a problem where i386+pae systems with more than a few CPUs
    > >> > blow up at boot in the kmap_atomic code.
    > >>
    > >> ping?
    > >
    > > No further progress here, I'm waiting on input for how to fix this
    > > "nicely". Meantime, clearing the early fixmap pte I guess works, but
    > > you lose a page... is it possible to put it into .initdata or is
    > > there some issue with that? (I guess on a PAE kernel, 4K isn't a
    > > big deal).
    > >
    > >
    > >> > The problem is that the kmap_atomic pte pages all need to be contiguous
    > >> > memory because the pte is calculated via the first kmap pte page + an
    > >> > offset (so as not to have to walk the page tables every time).
    > >> >
    > >> > The fixmap setup code crudely allocates contiguous pte pages, which is fine,
    > >> > but if it finds an already populated pmd entry, then it will not switch it
    > >> > to a new, contiguous pte page. So the early fixmap introduces a discontig
    > >> > page table right in the middle of the kmap atomic fixmaps.
    >
    > Where is this?

    Where is what?


    > >> > Commenting out the eaarly fixmap setup in head_32.S gets everything working
    > >> > properly. What would be the best way to fix this? Could we put the early
    > >> > fixmap page table in initdata, and then have the fixmap setup proper first
    > >> > clear its corresponding pmd entry?
    >
    > Why would we want or need to?

    So the fixmap allocator allocates contiguous pagetables. Wasn't my
    description clear enough?


    > >> How come users/testers aren't reporting this?
    > >
    > > Because apparently nobody tests 32-bit PAE systems with more than a couple
    > > of CPUs anymore. This bug comes from HW vendor doing testing of SLES11.
    >
    > Hmm.
    >
    > I have taken a quick skim and I am not seeming the part of the code you are
    > talking about. Is the problem code in mainline?
    >
    > I'm guessing it has something to do with reserve_top_address() being called
    > with a bad value in the normal case. But I don't see it being called
    > at all in the normal case.

    Yes I think so. one_page_table_init will not allocate a new pte page if
    one already exists, so it will not be contiguous with previous/subsequent
    pages with the early fixmap pte there. page_table_range_init explains why
    that is a problem (this code seems terribly fragile and prone to breakage
    but I don't want to expend theeffort on it -- still, I have a patch to
    panic the kernel if it breaks its guarantee of contiguous page tables,
    which makes breakage easily visible).




    \
     
     \ /
      Last update: 2008-12-30 07:37    [W:0.029 / U:0.604 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site