On 10/22/19 12:57 PM, Michal Hocko wrote: > [Cc Mel] > > On Tue 22-10-19 12:21:56, Waiman Long wrote: >> The pagetypeinfo_showfree_print() function prints out the number of >> free blocks for each of the page orders and migrate types. The current >> code just iterates the each of the free lists to get counts. There are >> bug reports about hard lockup panics when reading the /proc/pagetyeinfo >> file just because it look too long to iterate all the free lists within >> a zone while holing the zone lock with irq disabled. >> >> Given the fact that /proc/pagetypeinfo is readable by all, the possiblity >> of crashing a system by the simple act of reading /proc/pagetypeinfo >> by any user is a security problem that needs to be addressed. > Should we make the file 0400? It is a useful thing when debugging but > not something regular users would really need for life. > I am not against doing that, but it may break existing applications that somehow need to read pagetypeinfo. That is why I didn't try to advocate about changing protection. >> There is a free_area structure associated with each page order. There >> is also a nr_free count within the free_area for all the different >> migration types combined. Tracking the number of free list entries >> for each migration type will probably add some overhead to the fast >> paths like moving pages from one migration type to another which may >> not be desirable. > Have you tried to measure that overhead? I haven't tried to measure the performance impact yet. I did thought about tracking nr_free for each of the migration types within a free_area. That will require auditing the code to make sure that all the intra-free_area migrations are properly accounted for. I can work on it if people prefer this alternative. > >> we can actually skip iterating the list of one of the migration types >> and used nr_free to compute the missing count. Since MIGRATE_MOVABLE >> is usually the largest one on large memory systems, this is the one >> to be skipped. Since the printing order is migration-type => order, we >> will have to store the counts in an internal 2D array before printing >> them out. >> >> Even by skipping the MIGRATE_MOVABLE pages, we may still be holding the >> zone lock for too long blocking out other zone lock waiters from being >> run. This can be problematic for systems with large amount of memory. >> So a check is added to temporarily release the lock and reschedule if >> more than 64k of list entries have been iterated for each order. With >> a MAX_ORDER of 11, the worst case will be iterating about 700k of list >> entries before releasing the lock. > But you are still iterating through the whole free_list at once so if it > gets really large then this is still possible. I think it would be > preferable to use per migratetype nr_free if it doesn't cause any > regressions. > Yes, it is still theoretically possible. I will take a further look at having per-migrate type nr_free. BTW, there is one more place where the free lists are being iterated with zone lock held - mark_free_pages(). Cheers, Longman
