On 29.07.20 19:31, Mike Kravetz wrote: > On 6/30/20 7:26 AM, David Hildenbrand wrote: >> Right now, if we have two isolations racing, we might trigger the >> WARN_ON_ONCE() and to dump_page(NULL), dereferencing NULL. Let's just >> return directly. > > Just curious, what call path has the WARN_ON_ONCE()/dump_page(NULL)? See below, two set_migratetype_isolate() caller racing. > >> >> In the future, we might want to report -EAGAIN to the caller instead, as >> this could indicate a temporary isolation failure only. >> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> Cc: Michal Hocko <mhocko@xxxxxxxx> >> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> > > Hi David, > > That 'return -EAGAIN' was added as a sort of synchronization mechanism. > See commit message for 2c7452a075d4d. Before adding the 'return -EAGAIN', > I could create races which would abandon isolated pageblocks. Repeating > those races over and over would result in a good chunk of system memory > being isolated and unusable. It's actually -EBUSY, it should maybe later be changed to -EAGAIN (see comment), so caller can decide to retry immediately. Other discussion. > > Admittedly, these races are rare and I had to work really hard to produce > them. I'll try to find my testing mechanism. My concern is reintroducing > this abandoning of pageblocks. I have not looked further in your series > to see if this potentially addressed later. If not, then we should not > remove the return code. > Memory offlining could race with alloc_contig_range(), e.g., called when allocating gigantic pages, or when virtio-mem tries to unplug memory. The latter two could also race. We are getting more alloc_contig_range() users, which is why these races will become more relevant. I have no clue what you mean with "reintroducing this abandoning of pageblocks". All this patch is changing is not doing the dump_page() - or am I missing something important? -- Thanks, David / dhildenb
