Re: [BUG] kernel BUG at arch/i386/mm/highmem.c:15! on 2.6.23-rc8/rc9

[Hugh Dickins]

On Thu, 4 Oct 2007, gurudas pai wrote:
> Nick Piggin wrote:
> > > > While running Oracle database test on x86/6GB RAM machine panics with
> > > > following messages.
> > >
> > > Hmm, seems like something in sys_remap_file_pages might have broken.
> > > It's a bit hard to work out from the backtrace, though.
> > >
> > > Is it possible you can strace to find the arguments for the
> > > remap_file_pages that goes wrong?
> > 
> > Ahh, I think it's just underflowing the preempt count somewhere, which
> > is leading highmem.c:15 to just *think* it is in an interrupt.
> > 
> > But you aren't running a preemptible kernel, which makes it unusual...
> > it would have to be coming from interrupt code (or just random corruption).
> > Still, preempt debugging should catch those cases as well.
> > 
> > So, can you disregard my last message, and instead compile a kernel
> > with CONFIG_PREEMPT and CONFIG_DEBUG_PREEMPT, and see what
> > messages come up?
> With CONFIG_PREEMPT and CONFIG_DEBUG_PREEMPT set I got following messages on
> rc9.
> 
> BUG: using smp_processor_id() in preemptible [00000001] code: oracle/3631
> caller is kunmap_atomic+0xb/0x82
>  [<c04ec241>] debug_smp_processor_id+0xa1/0xb4
>  [<c0420dd0>] kunmap_atomic+0xb/0x82
>  [<c045fae3>] __do_fault+0x55/0x35b
>  [<c04623e8>] handle_mm_fault+0x4d0/0x909
>  [<c0460624>] follow_page+0x1d9/0x228
>  [<c0462a71>] get_user_pages+0x250/0x332
>  [<c0462bce>] make_pages_present+0x7b/0x90
>  [<c045f06a>] sys_remap_file_pages+0x2de/0x330
>  [<c0404f0e>] syscall_call+0x7/0xb
>  [<c0620000>] ioctl_standard_call+0x209/0x2ce

Very helpful, thanks.  Guru, please try the appended patch, I think
you'll find it fixes it for you (it did for me, once I'd puzzled out
why I was failing to reproduce the problem - tests on ext3 don't work).
Thank you so much for reporting this just in time!


[PATCH] fix sys_remap_file_pages BUG at highmem.c:15!

Gurudas Pai reports kernel BUG at arch/i386/mm/highmem.c:15! below
sys_remap_file_pages, while running Oracle database test on x86 in 6GB RAM:
kunmap thinks we're in_interrupt because the preempt count has wrapped.

That's because __do_fault expected to unmap page_table, but one of its two
callers do_nonlinear_fault already unmapped it: let do_linear_fault unmap
it first too, and then there's no need to pass the page_table arg down.

Why have we been so slow to notice this?  Probably through forgetting
that the mapping_cap_account_dirty test means that sys_remap_file_pages
nowadays only goes the full nonlinear vma route on a few memory-backed
filesystems like ramfs, tmpfs and hugetlbfs.

Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>

--- 2.6.23-rc9/mm/memory.c	2007-07-26 19:49:58.000000000 +0100
+++ linux/mm/memory.c	2007-10-04 15:42:20.000000000 +0100
@@ -2307,13 +2307,14 @@ oom:
  * do not need to flush old virtual caches or the TLB.
  *
  * We enter with non-exclusive mmap_sem (to exclude vma changes,
- * but allow concurrent faults), and pte mapped but not yet locked.
+ * but allow concurrent faults), and pte neither mapped nor locked.
  * We return with mmap_sem still held, but pte unmapped and unlocked.
  */
 static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
-		unsigned long address, pte_t *page_table, pmd_t *pmd,
+		unsigned long address, pmd_t *pmd,
 		pgoff_t pgoff, unsigned int flags, pte_t orig_pte)
 {
+	pte_t *page_table;
 	spinlock_t *ptl;
 	struct page *page;
 	pte_t entry;
@@ -2327,7 +2328,6 @@ static int __do_fault(struct mm_struct *
 	vmf.flags = flags;
 	vmf.page = NULL;
 
-	pte_unmap(page_table);
 	BUG_ON(vma->vm_flags & VM_PFNMAP);
 
 	if (likely(vma->vm_ops->fault)) {
@@ -2468,8 +2468,8 @@ static int do_linear_fault(struct mm_str
 			- vma->vm_start) >> PAGE_CACHE_SHIFT) + vma->vm_pgoff;
 	unsigned int flags = (write_access ? FAULT_FLAG_WRITE : 0);
 
-	return __do_fault(mm, vma, address, page_table, pmd, pgoff,
-							flags, orig_pte);
+	pte_unmap(page_table);
+	return __do_fault(mm, vma, address, pmd, pgoff, flags, orig_pte);
 }
 
 
@@ -2552,9 +2552,7 @@ static int do_nonlinear_fault(struct mm_
 	}
 
 	pgoff = pte_to_pgoff(orig_pte);
-
-	return __do_fault(mm, vma, address, page_table, pmd, pgoff,
-							flags, orig_pte);
+	return __do_fault(mm, vma, address, pmd, pgoff, flags, orig_pte);
 }
 
 /*
-
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