Re: [PATCH 1/1] mm:hugetlbfs: Fix hwpoison reserve accounting

[Mike Kravetz <mike.kravetz@oracle.com>]

On 10/19/2017 07:30 PM, Naoya Horiguchi wrote:
> On Thu, Oct 19, 2017 at 04:00:07PM -0700, Mike Kravetz wrote:
> 
> Thank you for addressing this. The patch itself looks good to me, but
> the reported issue (negative reserve count) doesn't reproduce in my trial
> with v4.14-rc5, so could you share the exact procedure for this issue?

Sure, but first one question on your test scenario below.

> 
> When error handler runs over a huge page, the reserve count is incremented
> so I'm not sure why the reserve count goes negative.

I'm not sure I follow.  What specific code is incrementing the reserve
count?  

>                                                      My operation is like below:
> 
>   $ sysctl vm.nr_hugepages=10
>   $ grep HugePages_ /proc/meminfo
>   HugePages_Total:      10
>   HugePages_Free:       10
>   HugePages_Rsvd:        0
>   HugePages_Surp:        0
>   $ ./test_alloc_generic -B hugetlb_file -N1 -L "mmap access memory_error_injection:error_type=madv_hard"  // allocate a 2MB file on hugetlbfs, then madvise(MADV_HWPOISON) on it.
>   $ grep HugePages_ /proc/meminfo
>   HugePages_Total:      10
>   HugePages_Free:        9
>   HugePages_Rsvd:        1  // reserve count is incremented
>   HugePages_Surp:        0

This is confusing to me.  I can not create a test where there is a reserve
count after poisoning page.

I tried to recreate your test.  Running unmodified 4.14.0-rc5.

Before test
-----------
HugePages_Total:       1
HugePages_Free:        1
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB

After open(creat) and mmap of 2MB hugetlbfs file
------------------------------------------------
HugePages_Total:       1
HugePages_Free:        1
HugePages_Rsvd:        1
HugePages_Surp:        0
Hugepagesize:       2048 kB

Reserve count is 1 as expected/normal

After madvise(MADV_HWPOISON) of the single huge page in mapping/file
--------------------------------------------------------------------
HugePages_Total:       1
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB

In this case, the reserve (and free) count were decremented.  Note that
before the poison operation the page was not associated with the mapping/
file.  I did not look closely at the code, but assume the madvise may
cause the page to be 'faulted in'.

The counts remain the same when the program exits
-------------------------------------------------
HugePages_Total:       1
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB

Remove the file (rm /var/opt/oracle/hugepool/foo)
-------------------------------------------------
HugePages_Total:       1
HugePages_Free:        0
HugePages_Rsvd:    18446744073709551615
HugePages_Surp:        0
Hugepagesize:       2048 kB

I am still confused about how your test maintains a reserve count after
poisoning.  It may be a good idea for you to test my patch with your
test scenario as I can not recreate here.

-- 
Mike Kravetz

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