Re: nvdimm,pmem: makedumpfile: __vtop4_x86_64: Can't get a valid pte.

[Eliot Moss <moss@roc.cs.umass.edu>]

Glad you found it. Any thoughts/reactions?  EM

Sent from my iPhone

> On Nov 29, 2022, at 12:17 AM, lizhijian@fujitsu.com wrote:
> 
> 
> 
>> On 28/11/2022 23:03, Eliot Moss wrote:
>>> On 11/28/2022 9:46 AM, lizhijian@fujitsu.com wrote:
>>> 
>>> 
>>> On 28/11/2022 20:53, Eliot Moss wrote:
>>>> On 11/28/2022 7:04 AM, lizhijian@fujitsu.com wrote:
>>>>> Hi folks,
>>>>> 
>>>>> I'm going to make crash coredump support pmem region. So
>>>>> I have modified kexec-tools to add pmem region to PT_LOAD of vmcore.
>>>>> 
>>>>> But it failed at makedumpfile, log are as following:
>>>>> 
>>>>> In my environment, i found the last 512 pages in pmem region will cause the error.
>>>> 
>>>> I wonder if an issue I reported is related: when set up to map
>>>> 2Mb (huge) pages, the last 2Mb of a large region got mapped as
>>>> 4Kb pages, and then later, half of a large region was treated
>>>> that way.
>>>> 
>>> Could you share the url/link ? I'd like to take a look
>> 
>> It was in a previous email to the nvdimm list.  the title was:
>> 
>> "Possible PMD (huge pages) bug in fs dax"
>> 
>> And here is the body.  I just sent directly to the list so there
>> is no URL (if I should be engaging in a different way, please let me know):
> 
> I found it :) at
> https://www.mail-archive.com/nvdimm@lists.linux.dev/msg02743.html
> 
> 
>> ================================================================================
>> Folks - I posted already on nvdimm, but perhaps the topic did not quite grab
>> anyone's attention.  I had had some trouble figuring all the details to get
>> dax mapping of files from an xfs file system with underlying Optane DC memory
>> going, but now have that working reliably.  But there is an odd behavior:
>> 
>> When first mapping a file, I request mapping a 32 Gb range, aligned on a 1 Gb
>> (and thus clearly on a 2 Mb) boundary.
>> 
>> For each group of 8 Gb, the first 4095 entries map with a 2 Mb huge (PMD)
>> page.  The 4096th one does FALLBACK.  I suspect some problem in
>> dax.c:grab_mapping_entry or its callees, but am not personally well enough
>> versed in either the dax code or the xarray implementation to dig further.
>> 
>> 
>> If you'd like a second puzzle 😄 ... after completing this mapping, another
>> thread accesses the whole range sequentially.  This results in NOPAGE fault
>> handling for the first 4095+4095 2M regions that previously resulted in
>> NOPAGE -- so far so good.  But it gives FALLBACK for the upper 16 Gb (except
>> the two PMD regions it alrady gave FALLBACK for).
>> 
>> 
>> I can provide trace output from a run if you'd like and all the ndctl, gdisk
>> -l, fdisk -l, and xfs_info details if you like.
>> 
>> 
>> In my application, it would be nice if dax.c could deliver 1 Gb PUD size
>> mappings as well, though it would appear that that would require more surgery
>> on dax.c.  It would be somewhat analogous to what's already there, of course,
>> but I don't mean to minimize the possible trickiness of it.  I realize I
>> should submit that request as a separate thread 😄 which I intend to do
>> later.
>> ================================================================================
>> 
>> Regards - Eliot Moss