Gilles Chanteperdrix wrote:
> On 8/7/07, Jan Kiszka <[EMAIL PROTECTED]> wrote:
>> Gilles Chanteperdrix wrote:
>>> Jan Kiszka wrote:
>>>> Hi all,
>>>> we are getting a lot of
>>>> BUG: sleeping function called from invalid context at mm/page_alloc.c:1225
>>>> 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
>>>>  =======================
>>>> 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?
>>> It does not look like a good solution, you are going to empty the
>>> GFP_ATOMIC pools. The proper solution would rather be to look at the
>>> real mm code (I mean not the one I wrote) and see how they cope with
>>> this issue.
>> Mmpf. What are the chances for a quick fix within the next days? We have
>> to consider alternatives right now here because the whole system is
>> meant for production purpose next week (C-ELROB '07).
>> OK, I'm already finding myself inside the code :-/. What about this
>> approach: We try to alloc with GFP_ATOMIC. Once this fails, we break
>> out, drop all locks (just like it happens in case of need_resched()),
>> try to fill up the pool, and restart then. What would reliably make
>> Linux refill its atomic pool?
>> Alternative approach: preallocate the required pages before entering the
>> loop in copy_pte_range. But that may require more code changes.
> I would say the real fix is to drop momentarily the spinlock(s?) for 
> allocating.

Are you sure it's safe to drop locks in the (logical) middle of
copy_one_pte()? I can't tell yet from the few glances I took. It's just
my feeling that says "no" so far.


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