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. I still think that this is a worth of documenting in the manual page, since the call failed silently before 4.10 right? I guess that we may as well add a BUGS section and document at least that. -- Cyril Hrubis chrubis@xxxxxxx -- 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
