> On Jan 14, 2020, at 6:53 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > >> On Jan 14, 2020, at 4:02 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: >>> >>> Yeah, that was a long discussion with a lot of lockdep false positives. >>> I believe I have made it clear that the console code shouldn't depend on >>> memory allocation because that is just too fragile. If that is not >>> possible for some reason then it has to be mentioned in the changelog. >>> I really do not want us to add kludges to the MM code just because of >>> printk deficiencies unless that is absolutely inevitable. >> >> I don't know how to convince you, but both random number generator and >> printk() maintainers agreed to get ride of printk() with zone->lock >> held as you can see in the approved commit mentioned in this patch >> description because it is a whac-a-mole to fix other places. In other >> word, the patch alone fixes quite a few false positives and potential >> real deadlocks. Maybe Andrew please has a look at this directly? >> > > Well, a few things. > > The changelog is quite poor. It doesn't describe the problem (console > drivers allocating memory) not does it describe the solution > (deferring the dump_page() until after release of zone->lock). > > So I changed it to this: > > : Some console drivers can perform memory allocation at inappropriate times, > : which can result in lockdep warnings (and presumably deadlocks) if printk > : is called with zone->lock held. > : > : By far the best fix is to reeducate those console drivers to not perform > : these allocations, but this is proving difficult. … but this is proving difficult because even if we fixed that directly, lockdep Is still able to find an indirect dependency chain, for example [1] CPU1: console_owner —> port_lock_key CPU2: port_lock_key —> (&port->lock)->rlock CPU3: (&port->lock)->rlock —> zone->lock which will trigger a splat with zone->lock —> console_owner [1] https://lore.kernel.org/linux-mm/1570460350.5576.290.camel@xxxxxx/ > : > : Another but poorer approach is to call printk_deferred() when holding > : zone->lock, but memory offline will call dump_page() which needs to defer > : after the lock. > : > : So change has_unmovable_pages() so that it no longer calls dump_page() > : itself - instead it passes the page's descripton (as a string) back to the > : caller so that in the case of a has_unmovable_pages() failure, the caller > : can call dump_page() after releasing zone->lock. > : > : While at it, remove a similar but unnecessary debug printk() as well. > > But I see a couple of other issues. > >> @@ -8290,8 +8290,10 @@ bool has_unmovable_pages(struct zone *zo >> return false; >> unmovable: >> WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE); >> - if (flags & REPORT_FAILURE) >> - dump_page(pfn_to_page(pfn + iter), reason); >> + if (flags & REPORT_FAILURE) { >> + page = pfn_to_page(pfn + iter); > > This statement appears to be unnecessary. dump_page() in set_migratetype_isolate() needs that “page”. > >> + strscpy(dump, reason, 64); >> + } > > > Also, that whole `reason' thing in has_unmovable_pages() is just there > to tell us whether it was an "unmovable page" or a "CMA page". This > doesn't seem terribly useful to me. Also, I expect that the > dump_page() output will permit the user to determine that it was a CMA > page anyway. If not, we can change dump_page() to add that info. > > So how about we remove that whole `reason' thing and possibly enhance > dump_page()? The patch then becomes much simpler. Sounds like a good idea. I’ll send a v2.
