On Tue, Aug 27, 2013 at 09:17:29AM +0800, Wanpeng Li wrote: > On Mon, Aug 26, 2013 at 08:46:54PM -0400, Naoya Horiguchi wrote: > >On Tue, Aug 27, 2013 at 08:21:05AM +0800, Wanpeng Li wrote: > >> Hi Naoya, > >> On Mon, Aug 26, 2013 at 08:12:29PM -0400, Naoya Horiguchi wrote: > >> >Hi Wanpeng, > >> > > >> >On Tue, Aug 27, 2013 at 07:26:04AM +0800, Wanpeng Li wrote: > >> >> Hi Naoya, > >> >> On Mon, Aug 26, 2013 at 11:45:37AM -0400, Naoya Horiguchi wrote: > >> >> >On Mon, Aug 26, 2013 at 04:46:12PM +0800, Wanpeng Li wrote: > >> >> >> madvise hwpoison inject will poison the read-only empty zero page if there is > >> >> >> no write access before poison. Empty zero page reference count will be increased > >> >> >> for hwpoison, subsequent poison zero page will return directly since page has > >> >> >> already been set PG_hwpoison, however, page reference count is still increased > >> >> >> by get_user_pages_fast. The unpoison process will unpoison the empty zero page > >> >> >> and decrease the reference count successfully for the fist time, however, > >> >> >> subsequent unpoison empty zero page will return directly since page has already > >> >> >> been unpoisoned and without decrease the page reference count of empty zero page. > >> >> >> This patch fix it by decrease page reference count for empty zero page which has > >> >> >> already been unpoisoned and page count > 1. > >> >> > > >> >> >I guess that fixing on the madvise side looks reasonable to me, because this > >> >> >refcount mismatch happens only when we poison with madvise(). The root cause > >> >> >is that we can get refcount multiple times on a page, even if memory_failure() > >> >> >or soft_offline_page() can do its work only once. > >> >> > > >> >> > >> >> I think this just happen in read-only before poison case against empty > >> >> zero page. > >> > > >> >OK. I agree. > >> > > >> >> Hi Andrew, > >> >> > >> >> I see you have already merged the patch, which method you prefer? > >> >> > >> >> >How about making madvise_hwpoison() put a page and return immediately > >> >> >(without calling memory_failure() or soft_offline_page()) when the page > >> >> >is already hwpoisoned? > >> >> >I hope it also helps us avoid meaningless printk flood. > >> >> > > >> >> > >> >> Btw, Naoya, how about patch 10/10, any input are welcome! ;-) > >> > > >> >No objection if you (and Andrew) decide to go with current approach. > >> > >> Andrew prefer your method, I will resend the patch w/ your suggested-by. ;-) > > > >Thanks you :) > > > >> >But I think that if we shift to fix this problem in madvise(), > >> >we don't need 10/10 any more. So it looks simpler to me. > >> > >> I don't think it's same issue. There is just one page in my test case. > >> #define PAGES_TO_TEST 1 > >> If I miss something? > > > >Ah, OK. > > I complete do it in madvise codes, however, the bug mentioned in patch > 10/10 is still there. ;-) > > > > >BTW, in my understanding, zero pages are not exist physically (I mean that > >no real page is allocated to store 4096 bytes of 0.) So there can't happen > >any real MCE SRAO on zero page. So one possible solution might be that we > >completely ignore all of madvise(MADV_HWPOISON) over zero pages. > > What's the userland visible difference against mmap w/o write access before poison > you expect? In this case the userland is a test program like mce-test, so my expectation is that the test program shouldn't detect false test failures when it accidentally calls madvise(MADV_HWPOISON) on zero pages, because there's no real test target associated with such testcases. So I think just returning with success return code without doing anything looks good. 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>