On Tue, Dec 04, 2018 at 09:48:26AM +0100, Michal Hocko wrote: > On Tue 04-12-18 07:21:16, Naoya Horiguchi wrote: > > On Mon, Dec 03, 2018 at 11:03:09AM +0100, Michal Hocko wrote: > > > From: Michal Hocko <mhocko@xxxxxxxx> > > > > > > We have received a bug report that an injected MCE about faulty memory > > > prevents memory offline to succeed. The underlying reason is that the > > > HWPoison page has an elevated reference count and the migration keeps > > > failing. There are two problems with that. First of all it is dubious > > > to migrate the poisoned page because we know that accessing that memory > > > is possible to fail. Secondly it doesn't make any sense to migrate a > > > potentially broken content and preserve the memory corruption over to a > > > new location. > > > > > > Oscar has found out that it is the elevated reference count from > > > memory_failure that is confusing the offlining path. HWPoisoned pages > > > are isolated from the LRU list but __offline_pages might still try to > > > migrate them if there is any preceding migrateable pages in the pfn > > > range. Such a migration would fail due to the reference count but > > > the migration code would put it back on the LRU list. This is quite > > > wrong in itself but it would also make scan_movable_pages stumble over > > > it again without any way out. > > > > > > This means that the hotremove with hwpoisoned pages has never really > > > worked (without a luck). HWPoisoning really needs a larger surgery > > > but an immediate and backportable fix is to skip over these pages during > > > offlining. Even if they are still mapped for some reason then > > > try_to_unmap should turn those mappings into hwpoison ptes and cause > > > SIGBUS on access. Nobody should be really touching the content of the > > > page so it should be safe to ignore them even when there is a pending > > > reference count. > > > > > > Debugged-by: Oscar Salvador <osalvador@xxxxxxxx> > > > Cc: stable > > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > > > --- > > > Hi, > > > I am sending this as an RFC now because I am not fully sure I see all > > > the consequences myself yet. This has passed a testing by Oscar but I > > > would highly appreciate a review from Naoya about my assumptions about > > > hwpoisoning. E.g. it is not entirely clear to me whether there is a > > > potential case where the page might be still mapped. > > > > One potential case is ksm page, for which we give up unmapping and leave > > it unmapped. Rather than that I don't have any idea, but any new type of > > page would be potentially categorized to this class. > > Could you be more specific why hwpoison code gives up on ksm pages while > we can safely unmap here? Actually no big reason. Ksm pages never dominate memory, so we simply didn't have strong motivation to save the pages. > [...] > > > > I think this looks OK (no better idea.) > > > > Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> > > Thanks! > > > I wondered why I didn't find this for long, and found that my testing only > > covered the case where PageHWPoison is the first page of memory block. > > scan_movable_pages() considers PageHWPoison as non-movable, so do_migrate_range() > > started with pfn after the PageHWPoison and never tried to migrate it > > (so effectively ignored every PageHWPoison as the above code does.) > > Yeah, it seems that the hotremove worked only by chance in presence of > hwpoison pages so far. The specific usecase which triggered this patch > is a heavily memory utilized system with in memory database IIRC. So it > is quite likely that hwpoison pages are punched to otherwise used > memory. > > Thanks for the review Naoya! Your welcome, and thank you for reporting/fixing the issue. - Naoya
