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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Hi Jinghao,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">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?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Andrew<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif""> Shi Jinghao [mailto:jhshi@cs.hku.hk]
<br>
<b>Sent:</b> Saturday, 1 December 2012 02:20<br>
<b>To:</b> barrelfish-users@lists.inf.ethz.ch<br>
<b>Subject:</b> [Barrelfish-users] A Weird Bug about Page Fault<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hi,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The work flow of the test case is as simple as following:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">2) Initiate the array, this will generate page fault<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">3) In page fault handler, remap the faulted page as read-write, using pmap->f.modify_flags<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The weird thing is: the first touch of this array will not result in a proper value, but just NaN!<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I've conducted several runs and found the following:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">1) This bug will occur when the array type is double or float. Everything is fine if it's a integer array.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I tested the attached code in release2012-05-25 (the revision I work on) and the latest revision (release2012-10-03).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">This bug has bothered me for quite a few days. Really appreciate if someone can give a hint on this.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Jinghao<o:p></o:p></p>
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