From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Tue, 07 Aug 2007 15:06:27 +0200
From: Jan Kiszka <jan.kiszka@domain.hid>
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Subject: [Xenomai-core] [COW-BUG] __alloc_pages called from atomic context
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To: Philippe Gerum <rpm@xenomai.org>, Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: adeos-main <adeos-main@gna.org>, xenomai-core <xenomai@xenomai.org>

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Hi all,

we are getting a lot of

BUG: sleeping function called from invalid context at mm/page_alloc.c:122=
5
in_atomic():1, irqs_disabled():0
 [<c010305d>] show_trace_log_lvl+0x1a/0x2f
 [<c0103156>] show_trace+0x12/0x14
 [<c0103915>] dump_stack+0x16/0x18
 [<c010c4ab>] __might_sleep+0xcd/0xd3
 [<c0149488>] __alloc_pages+0x32/0x281
 [<c014fdd2>] copy_page_range+0x221/0x41e
 [<c010ec18>] copy_process+0x9e1/0xfe2
 [<c010f415>] do_fork+0x99/0x176
 [<c0100e75>] sys_clone+0x33/0x39
 [<c0102aaf>] syscall_call+0x7/0xb
 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

here due to a Xenomai program issuing system() calls.

After once again dissecting the "nice" mm code (sigh...), the reason
turned out to be plain simple:

copy_pte_range(...);
  spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
  copy_one_pte(...);
    if (is_cow_mapping(vm_flags))
      alloc_page_vma(GFP_HIGHUSER, ...);
        __alloc_pages(...)
	  might_sleep_if(gfp_mask & __GFP_WAIT);

And this is true due to #define GFP_HIGHUSER (__GFP_WAIT | ...

So the bad news is that the COW code in likely all i-pipe versions is
broken. But the good new is that this might be easily fixable by
providing the right gfp_mask. GFP_ATOMIC?

Jan


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