Re: [PATCH] mm/vmstat: Reduce zone lock hold time when reading /proc/pagetypeinfo

[Qian Cai <cai@lca.pw>]

On Wed, 2019-10-23 at 10:30 -0400, Waiman Long wrote:
> On 10/22/19 5:59 PM, Andrew Morton wrote:
> > On Tue, 22 Oct 2019 12:21:56 -0400 Waiman Long <longman@redhat.com> 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.
> > 
> > Yes.
> > 
> > > 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.
> > > 
> > > 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.
> > > 
> > > ...
> > > 
> > > --- a/mm/vmstat.c
> > > +++ b/mm/vmstat.c
> > > @@ -1373,23 +1373,54 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
> > >  					pg_data_t *pgdat, struct zone *zone)
> > >  {
> > >  	int order, mtype;
> > > +	unsigned long nfree[MAX_ORDER][MIGRATE_TYPES];
> > 
> > 600+ bytes is a bit much.  I guess it's OK in this situation.
> > 
> 
> This function is called by reading /proc/pagetypeinfo. The call stack is
> rather shallow:
> 
> PID: 58188  TASK: ffff938a4d4f1fa0  CPU: 2   COMMAND: "sosreport"
>  #0 [ffff9483bf488e48] crash_nmi_callback at ffffffffb8c551d7
>  #1 [ffff9483bf488e58] nmi_handle at ffffffffb931d8cc
>  #2 [ffff9483bf488eb0] do_nmi at ffffffffb931dba8
>  #3 [ffff9483bf488ef0] end_repeat_nmi at ffffffffb931cd69
>     [exception RIP: pagetypeinfo_showfree_print+0x73]
>     RIP: ffffffffb8db7173  RSP: ffff938b9fcbfda0  RFLAGS: 00000006
>     RAX: fffff0c9946d7020  RBX: ffff96073ffd5528  RCX: 0000000000000000
>     RDX: 00000000001c7764  RSI: ffffffffb9676ab1  RDI: 0000000000000000
>     RBP: ffff938b9fcbfdd0   R8: 000000000000000a   R9: 00000000fffffffe
>     R10: 0000000000000000  R11: ffff938b9fcbfc36  R12: ffff942b97758240
>     R13: ffffffffb942f730  R14: ffff96073ffd5000  R15: ffff96073ffd5180
>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
> --- <NMI exception stack> ---
>  #4 [ffff938b9fcbfda0] pagetypeinfo_showfree_print at ffffffffb8db7173
>  #5 [ffff938b9fcbfdd8] walk_zones_in_node at ffffffffb8db74df
>  #6 [ffff938b9fcbfe20] pagetypeinfo_show at ffffffffb8db7a29
>  #7 [ffff938b9fcbfe48] seq_read at ffffffffb8e45c3c
>  #8 [ffff938b9fcbfeb8] proc_reg_read at ffffffffb8e95070
>  #9 [ffff938b9fcbfed8] vfs_read at ffffffffb8e1f2af
> #10 [ffff938b9fcbff08] sys_read at ffffffffb8e2017f
> #11 [ffff938b9fcbff50] system_call_fastpath at ffffffffb932579b
> 
> So we should not be in any risk of overflowing the stack.
> 
> > > -	for (mtype = 0; mtype < MIGRATE_TYPES; mtype++) {
> > > -		seq_printf(m, "Node %4d, zone %8s, type %12s ",
> > > -					pgdat->node_id,
> > > -					zone->name,
> > > -					migratetype_names[mtype]);
> > > -		for (order = 0; order < MAX_ORDER; ++order) {
> > > +	lockdep_assert_held(&zone->lock);
> > > +	lockdep_assert_irqs_disabled();
> > > +
> > > +	/*
> > > +	 * MIGRATE_MOVABLE is usually the largest one in large memory
> > > +	 * systems. We skip iterating that list. Instead, we compute it by
> > > +	 * subtracting the total of the rests from free_area->nr_free.
> > > +	 */
> > > +	for (order = 0; order < MAX_ORDER; ++order) {
> > > +		unsigned long nr_total = 0;
> > > +		struct free_area *area = &(zone->free_area[order]);
> > > +
> > > +		for (mtype = 0; mtype < MIGRATE_TYPES; mtype++) {
> > >  			unsigned long freecount = 0;
> > > -			struct free_area *area;
> > >  			struct list_head *curr;
> > >  
> > > -			area = &(zone->free_area[order]);
> > > -
> > > +			if (mtype == MIGRATE_MOVABLE)
> > > +				continue;
> > >  			list_for_each(curr, &area->free_list[mtype])
> > >  				freecount++;
> > > -			seq_printf(m, "%6lu ", freecount);
> > > +			nfree[order][mtype] = freecount;
> > > +			nr_total += freecount;
> > >  		}
> > > +		nfree[order][MIGRATE_MOVABLE] = area->nr_free - nr_total;
> > > +
> > > +		/*
> > > +		 * If we have already iterated more than 64k of list
> > > +		 * entries, we might have hold the zone lock for too long.
> > > +		 * Temporarily release the lock and reschedule before
> > > +		 * continuing so that other lock waiters have a chance
> > > +		 * to run.
> > > +		 */
> > > +		if (nr_total > (1 << 16)) {
> > > +			spin_unlock_irq(&zone->lock);
> > > +			cond_resched();
> > > +			spin_lock_irq(&zone->lock);
> > > +		}
> > > +	}
> > > +
> > > +	for (mtype = 0; mtype < MIGRATE_TYPES; mtype++) {
> > > +		seq_printf(m, "Node %4d, zone %8s, type %12s ",
> > > +					pgdat->node_id,
> > > +					zone->name,
> > > +					migratetype_names[mtype]);
> > > +		for (order = 0; order < MAX_ORDER; ++order)
> > > +			seq_printf(m, "%6lu ", nfree[order][mtype]);
> > >  		seq_putc(m, '\n');
> > 
> > This is not exactly a thing of beauty :( Presumably there might still
> > be situations where the irq-off times remain excessive.
> 
> Yes, that is still possible.
> > 
> > Why are we actually holding zone->lock so much?  Can we get away with
> > holding it across the list_for_each() loop and nothing else?  If so,
> 
> We can certainly do that with the risk that the counts will be less
> reliable for a given order. I can send a v2 patch if you think this is
> safer.
> 
> 
> > this still isn't a bulletproof fix.  Maybe just terminate the list
> > walk if freecount reaches 1024.  Would anyone really care?
> > 
> > Sigh.  I wonder if anyone really uses this thing for anything
> > important.  Can we just remove it all?
> > 
> 
> Removing it will be a breakage of kernel API.

Who cares about breaking this part of the API that essentially nobody will use
this file?

> 
> Another alternative is to mark the migration type in the page structure
> so that we can do per-migration type nr_free tracking. That will be a
> major change to the mm code.
> 
> I consider this patch lesser of the two evils. 
> 
> Cheers,
> Longman
> 
>