On 27 Feb 2017, at 0:33, Naoya Horiguchi wrote: > On Sun, Feb 26, 2017 at 10:27:02PM -0600, Zi Yan wrote: >> On 26 Feb 2017, at 19:20, Naoya Horiguchi wrote: >> >>> On Sat, Feb 25, 2017 at 10:28:15AM +0800, Yisheng Xie wrote: >>>> hi Naoya, >>>> >>>> On 2017/2/23 11:23, Naoya Horiguchi wrote: >>>>> On Mon, Feb 20, 2017 at 05:00:17AM +0000, Horiguchi Naoya(堀口 直也) wrote: >>>>>> On Tue, Feb 14, 2017 at 04:41:29PM +0100, Jan Stancek wrote: >>>>>>> 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. >>>>> >>>>>> 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.) >>> >>>> 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. >> > > Hi Yan, > >> 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. > > The latter is the current behavior. It just comes from get_user_pages_fast() > which not only finds the page and takes refcount, but also touch the page. > > madvise(MADV_HWPOISON) is a test feature, and calling it for address backed > by no page doesn't simulate anything real. IOW, the behavior is undefined. > So I don't have a strong opinion about how it should behave. > >> >> 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. > > yes, maybe we see the same thing, you can see in dmesg "recovery action > for reserved kernel page: Failed" message. > >> >> 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* > > Writing it down to man page makes readers think this behavior is a part of > specification, that might not be good now because the failure in error > handling of zero page is not the eventually fixed behavior. > I mean that if zero page handles hwpoison properly in the future, madvise > will succeed without any confusion. > So I feel that we don't have to update man page for this issue. You are right, I missed the part that get_user_pages_fast() will actually fault in the madvised pages with zero_page. Thanks for clarifying this. -- Best Regards Yan Zi
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