On Wed, Oct 23, 2019 at 10:48:13AM -0400, Qian Cai wrote:
> 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@xxxxxxxxxx> 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?
>
Seems that _some_ are using it, aren't they? Otherwise we wouldn't be
seeing complaints. Moving it out of /proc to /sys/kernel/debug looks
like a reasonable compromise with those that care about the interface.
