Ingo Molnar <[email protected]> wrote:
> FYI, i have forward ported your MAX_ARG_PAGES limit removal patch to
> 2.6.20-rc2 and have included it in the -rt kernel. It's working great -
> i can now finally do a "ls -t patches/*.patch" in my patch repository -
> something i havent been able to do for years ;-)
I get a NULL kernel pointer reference with this patch with FRV in MMU mode.
It's whilst the kernel is attempting to execute init:
(gdb) bt
#0 0xc0063d54 in do_anonymous_page (mm=0xc05c1a04, vma=0xc05c884c, address=3221225457, page_table=0xdc00bffc, pmd=0xc05c6f00, write_access=0) at pgtable.h:525
#1 0xc006468c in __handle_mm_fault (mm=0xbffffff1, vma=0xc05c884c, address=3691036668, write_access=1) at memory.c:2404
#2 0xc0061c18 in get_user_pages (tsk=0xc034eba0, mm=0xc05c1a04, start=3221225457, len=1, write=1, force=0, pages=0xc0353b80, vmas=0x0) at memory.c:1064
#3 0xc0084c7c in copy_strings (argc=0, argv=0xc0388c3c, bprm=0xc0388b88) at exec.c:326
#4 0xc0084dac in copy_strings_kernel (argc=1, argv=0xc05c884c, bprm=0xbffffff1) at exec.c:365
#5 0xc0085f5c in do_execve (filename=0xc0387000 "/sbin/init", argv=0xc026e8c8, envp=0xc026e950, regs=0xc0353bf8) at exec.c:1158
#6 0xc000f55c in sys_execve (name=0x1 <Address 0x1 out of bounds>, argv=0xc026e8c8, envp=0xc026e950) at process.c:263
#7 0xc000e1e0 in __syscall_call () at arch/frv/kernel/entry.S:878
#8 0xc000e1e0 in __syscall_call () at arch/frv/kernel/entry.S:878
The problem appears to be that current->mm is NULL. It's a bit difficult to
say for sure because gdb or gcc determined that the error is in pgtable.h, but
doesn't seem to have determined which one.
However, looking at the code, I think it must be this:
static int do_anonymous_page(struct mm_struct *mm, ...)
{
...
update_mmu_cache(vma, address, entry);
...
}
But that uses current->mm, which is NULL, via this function:
[include/asm-frv/pgtable.h]
/* * preload information about a newly instantiated PTE into the SCR0/SCR1 PGE
cache */
static inline void update_mmu_cache(struct vm_area_struct *vma,
unsigned long address, pte_t pte)
{
unsigned long ampr;
pgd_t *pge = pgd_offset(current->mm, address);
pud_t *pue = pud_offset(pge, address);
pmd_t *pme = pmd_offset(pue, address);
ampr = pme->ste[0] & 0xffffff00;
ampr |= xAMPRx_L | xAMPRx_SS_16Kb | xAMPRx_S | xAMPRx_C | xAMPRx_V;
asm volatile("movgs %0,scr0\n"
"movgs %0,scr1\n"
"movgs %1,dampr4\n"
"movgs %1,dampr5\n"
:
: "r"(address), "r"(ampr)
);
}
This really oughtn't to be called in this situation, I suspect. I could just
skip the operation if current->mm is NULL, but I'm not sure that that's the
right thing to do. Maybe I should be using current->active_mm instead.
Everything works fine without the patch.
David
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