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. Thanks, Naoya Horiguchi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href
