Re: Is MADV_HWPOISON supposed to work only on faulted-in pages?

On 26 Feb 2017, at 19:20, Naoya Horiguchi wrote:

On Sat, Feb 25, 2017 at 10:28:15AM +0800, Yisheng Xie wrote:

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hi Naoya,

On 2017/2/23 11:23, Naoya Horiguchi wrote:

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On Mon, Feb 20, 2017 at 05:00:17AM +0000, Horiguchi Naoya(堀口 直也) wrote:

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On Tue, Feb 14, 2017 at 04:41:29PM +0100, Jan Stancek wrote:

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Hi,

code below (and LTP madvise07 [1]) doesn't produce SIGBUS,
unless I touch/prefault page before call to madvise().

Is this expected behavior?

Thank you for reporting.

madvise(MADV_HWPOISON) triggers page fault when called on the address
over which no page is faulted-in, so I think that SIGBUS should be
called in such case.

But it seems that memory error handler considers such a page as "reserved
kernel page" and recovery action fails (see below.)

  [  383.371372] Injecting memory failure for page 0x1f10 at 0x7efcdc569000
  [  383.375678] Memory failure: 0x1f10: reserved kernel page still referenced by 1 users
  [  383.377570] Memory failure: 0x1f10: recovery action for reserved kernel page: Failed

I'm not sure how/when this behavior was introduced, so I try to understand.

I found that this is a zero page, which is not recoverable for memory
error now.

quoted

IMO, the test code below looks valid to me, so no need to change.

I think that what the testcase effectively does is to test whether memory
handling on zero pages works or not.
And the testcase's failure seems acceptable, because it's simply not-implemented yet.
Maybe recovering from error on zero page is possible (because there's no data
loss for memory error,) but I'm not sure that code might be simple enough and/or
it's worth doing ...

I question about it,  if a memory error happened on zero page, it will
cause all of data read from zero page is error, I mean no-zero, right?

Hi Yisheng,

Yes, the impact is serious (could affect many processes,) but it's possibility
is very low because there's only one page in a system that is used for zero page.
There are many other pages which are not recoverable for memory error like
slab pages, so I'm not sure how I prioritize it (maybe it's not a
top-priority thing, nor low-hanging fruit.)

quoted

And can we just use re-initial it with zero data maybe by memset ?

Maybe it's not enoguh. Under a real hwpoison, we should isolate the error
page to prevent the access on the broken data.
But zero page is statically defined as an array of global variable, so
it's not trival to replace it with a new zero page at runtime.

Anyway, it's in my todo list, so hopefully revisited in the future.

Hi Naoya,

The test case tries to HWPOISON a range of virtual addresses that do not
map to any physical pages.

I expected either madvise should fail because HWPOISON does not work on
non-existing physical pages or madvise_hwpoison() should populate
some physical pages for that virtual address range and poison them.

As I tested it on kernel v4.10, the test application exited at
madvise, because madvise returns -1 and error message is
"Device or resource busy". I think this is a proper behavior.

There might be some confusion in madvise's man page on MADV_HWPOISON.
If you add some text saying madvise fails if any page is not mapped in
the given address range, that can eliminate the confusion.


--
Best Regards
Yan Zi