[Barrelfish-users] A Weird Bug about Page Fault

[Simon Peter]

Hi Jinghao,

It seems this is SCC specific. I just ran your test-case on QEMU on both 
x86-64 and -32 platforms and it seems to work just fine (i.e. I get the 
"all good" output).

Simon

On 12/03/2012 12:47 AM, Shi Jinghao wrote:
> Hi Andrew,
>
> Thanks for your reply. The two different exceptions you mentioned is
> insightful I tried your suggestion. But that does not help. The NaN
> errors still occur. I also tried to put extra dummy float point
> operations in page fault handler. And that does not help, either.
>
> Thanks,
> Jinghao
>
> On Sun, Dec 2, 2012 at 2:06 AM, Andrew Baumann
> <Andrew.Baumann at microsoft.com <mailto:Andrew.Baumann at microsoft.com>> wrote:
>
>     Hi Jinghao,____
>
>     __ __
>
>     I notice that the first time you use floating point in this program
>     is when writing to the array. There should be two different
>     exceptions raised and handled here: one for the page fault, and one
>     for the first use of the floating point hardware (which we lazily
>     context-switch). My guess is that the page-fault path, which is not
>     heavily exercised, does not interact well with the floating point
>     save/restore code.____
>
>     __ __
>
>     If you initialise the floating point hardware by doing some other
>     floating point operations (or writing to a statically allocated
>     variable) beforehand, does the problem go away?____
>
>     __ __
>
>     Andrew____
>
>     __ __
>
>     *From:* Shi Jinghao [mailto:jhshi at cs.hku.hk <mailto:jhshi at cs.hku.hk>]
>     *Sent:* Saturday, 1 December 2012 02:20
>     *To:* barrelfish-users at lists.inf.ethz.ch
>     <mailto:barrelfish-users at lists.inf.ethz.ch>
>     *Subject:* [Barrelfish-users] A Weird Bug about Page Fault____
>
>     __ __
>
>     Hi,____
>
>     __ __
>
>     I've been developing a memory management library on Barrelfish
>     (SCC). Recently I bumped into a very weird bug about page fault. I
>     attached a minimal case (pgfault_test.tgz) that can reproduce this
>     bug.____
>
>     __ __
>
>     The work flow of the test case is as simple as following:____
>
>     __ __
>
>     1) Allocate an array of doubles as read-only, using frame_alloc and
>     vspace_map_one_frame_attr (or pmap->f.map, this doesn't matter)____
>
>     __ __
>
>     2) Initiate the array, this will generate page fault____
>
>     __ __
>
>     3) In page fault handler, remap the faulted page as read-write,
>     using pmap->f.modify_flags____
>
>     __ __
>
>     The weird thing is: the first touch of this array will not result in
>     a proper value, but just NaN!____
>
>     __ __
>
>     I've conducted several runs and found the following:____
>
>     __ __
>
>     1) This bug will occur when the array type is double or float.
>     Everything is fine if it's a integer array.____
>
>     __ __
>
>     2) Only the item that caused the page fault will end in a NaN value,
>     others items are just fine. And this applies when the faulted be
>     anywhere within that page, not just the page start.____
>
>     __ __
>
>     3) If you assign each array value with a constant value (say 1.0),
>     or a int/double variable, then all items will end up with a right
>     value. It seems only when we assign a[i] with i (or any expression
>     contains i) will produce this bug.____
>
>     __ __
>
>     I tested the attached code in release2012-05-25 (the revision I work
>     on) and the latest revision (release2012-10-03).____
>
>     __ __
>
>     I've also composed a minimal test case in sccLinux (write_fault.c).
>     It turns out that everything is all good. No annoying NaN values.____
>
>     __ __
>
>     This bug has bothered me for quite a few days. Really appreciate if
>     someone can give a hint on this.____
>
>     __ __
>
>     Thanks,____
>
>     Jinghao____
>
>
>
>
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