Re: [bug] SLOB, tipc_init(), WARNING: at arch/x86/mm/highmem_32.c:52 kmap_atomic_prot()

[Ingo Molnar <mingo@elte.hu>]

* Matt Mackall <mpm@selenic.com> wrote:

> On Thu, Nov 29, 2007 at 03:39:20PM +0100, Ingo Molnar wrote:
> > 
> > * Ingo Molnar <mingo@elte.hu> wrote:
> > 
> > > i'm getting this on 32-bit (with the kmap-atomic debugging patch 
> > > applied):
> > > 
> > >  ---------------->
> > >  Calling initcall 0x78b67c00: tipc_init+0x0/0xc0()
> > >  TIPC: Activated (version 1.6.2 compiled Nov 29 2007 15:04:36)
> > >  WARNING: at arch/x86/mm/highmem_32.c:52 kmap_atomic_prot()
> > >  Pid: 1, comm: swapper Not tainted 2.6.24-rc3-cfs-v24 #45
> > >   [<78107272>] show_trace_log_lvl+0x12/0x40
> > >   [<781072ad>] show_trace+0xd/0x20
> > >   [<781086f8>] dump_stack+0x58/0x60
> > >   [<7811541f>] kmap_atomic_prot+0x1bf/0x240
> > >   [<781154ae>] kmap_atomic+0xe/0x20
> > >   [<78157be5>] get_page_from_freelist+0x225/0x420
> > >   [<78157e4d>] __alloc_pages+0x6d/0x3a0
> > >   [<78169a7b>] slob_new_page+0x1b/0x60
> > >   [<78169be4>] slob_alloc+0x124/0x1e0
> > >   [<78169e0f>] __kmalloc_node+0x6f/0xa0
> > >   [<7884f7c2>] reg_init+0x42/0x80
> > >   [<7884f80a>] tipc_reg_start+0xa/0x40
> > >   [<7883e6c6>] tipc_core_start+0x66/0xc0
> > >   [<78b67c81>] tipc_init+0x81/0xc0
> > 
> > this is due to the kzalloc() here:
> > 
> > 0x7884f1d0 is in reg_init (net/tipc/user_reg.c:88).
> > 83              spin_lock_bh(&reg_lock);
> > 84              if (!users) {
> > 85                      users = kzalloc(USER_LIST_SIZE, GFP_ATOMIC);
> > 86                      if (users) {
> > 87                              for (i = 1; i <= MAX_USERID; i++) {
> > 88                                      users[i].next = i - 1;
> > 
> > which does a:
> > 
> > 120     static inline void clear_highpage(struct page *page)
> > 121     {
> > 122             void *kaddr = kmap_atomic(page, KM_USER0);
> > 123             clear_page(kaddr);
> > 124             kunmap_atomic(kaddr, KM_USER0);
> > 125     }
> > 
> > but ... why does the debug code think it's in softirq context?
> > 
> > plus, and this is a slob question i guess, how come we drop into 
> > clear_highpage() for a kzalloc()??
> 
> Good question. Looks like kzalloc switched from doing a memset to
> passing a GFP_ZERO flag down to kmalloc. Slob didn't get completely
> updated to reflect this, so it blindly propagates the flag onto
> __alloc_pages and does a harmless double-clear.
> 
> Someone should remind us what the point of moving the kzalloc memset
> down into the allocators was. We now have all three allocators doing:
> 
>         if (unlikely((flags & __GFP_ZERO) && ptr))
>                 memset(ptr, 0, obj_size(cachep));
> 
> and needing to mask flags before passing them to page allocators,
> which hardly seems better than kzalloc unconditionally doing the
> memset. Wouldn't it be better/faster/smaller to make kzalloc a
> non-inline?
> 
> Slob also has a nice second path for large kmallocs where it just 
> calls the page allocator directly which also needs this treatment. 
> Which does the right thing with non-highmem systems, but can hit this 
> bug otherwise.
> 
> Below is a totally untested patch. Alternately, we could simply tweak 
> clear_highpage to remove the limitation, but that would leave slob 
> doing an extra clear.

ok, this fixes the debug warning.

	Ingo
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