Martin J. Bligh wrote:
But temp_priority should be set to 0 at that point.
It that were true, it'd be great. But how?
This is everything that touches it:
0 mmzone.h <global> 208 int temp_priority;
1 page_alloc.c free_area_init_core 2019 zone->temp_priority =
zone->prev_priority = DEF_PRIORITY;
2 vmscan.c shrink_zones 937 zone->temp_priority = priority;
3 vmscan.c try_to_free_pages 987 zone->temp_priority = DEF_PRIORITY;
4 vmscan.c try_to_free_pages 1031 zone->prev_priority =
zone->temp_priority;
5 vmscan.c balance_pgdat 1081 zone->temp_priority = DEF_PRIORITY;
6 vmscan.c balance_pgdat 1143 zone->temp_priority = priority;
7 vmscan.c balance_pgdat 1189 zone->prev_priority =
zone->temp_priority;
8 vmstat.c zoneinfo_show 593 zone->temp_priority,
Only thing that looks interesting here is shrink_zones.
For try_to_free_pages, shrink_zones will continue to be called until
priority reaches 0. So temp_priority and prev_priority are now 0. When
it breaks out of the loop, prev_priority gets assigned temp_priority.
Both of which are zero *unless you've hit the temp_priority race*. As
I said, getting rid of temp_priority and somehow tracking it locally
will close this race. I agree this race is a bug and would be happy to
see it fixed. This might be what your patch inadvertently fixes.
But your loops are not exactly per reclaimer either. Granted there
is a large race window in the current code, but this patch isn't the
way to fix that particular problem.
Why not? Perhaps it's not a panacea, but it's a definite improvement.
OK it is an improvement for the cases when we hit priority = 0. It would
be nice to fix the race for medium priorities as well though. Hmm, OK,
if we can't do that easily then I would be OK with this approach for the
time being.
Please don't duplicate that whole loop again in try_to_free_pages, though.
Moreover, whilst try_to_free_pages calls shrink_zones, balance_pgdat
does not. Nothing else I can see sets temp_priority.
balance_pgdat.
That's only called from kswapd. If we're in balance_pgdat, we ARE
kswapd. We can't fix ourself. So effectively we're doing:
while (priority--) {
if (we reclaimed OK)
goto out;
}
out:
prev_priority = DEF_PRIORITY;
We've just walked the whole bloody list with priority set to 0.
We failed to reclaim a few pages.
We know the world is in deep pain.
Why the hell would we elevate prev_priority?
No. If we've walked the whole bloody list and failed to reclaim any
pages, we do not set prev_priority to DEF_PRIORITY. Read the code, it
does the same thing with the priorities as shrink_zones.
Unnecesary and indicates something else is broken if you are seeing
problems here.
You think we should set prev_priority up, when we've just walked the
whole list at prio 0 and can't reclaim anything? Unless so, I fail
to see how the patch is unnecessary.
And yes, I'm sure other things are broken, but again, this fixes a
clear bug.
AFAIKS there is no bug that have identified here or in your changelog.
There is a race, there are many of tolerable races in reclaim. I can
accept this races is intolerable for you, so I am OK with fixing it.
> So do you still see the problem on upstream kernel
without your patches applied?
I can't slap an upstream bleeding edge kernel across a few thousand
production machines, and wait to see if the world blows up, sorry.
If I can make a reproduce test case, I'll send it out, but thus far
we've been unsuccessful.
No problem, I didn't ask you to do that. But if you want this patch
in the upstream kerenl, then I will keep asking whether it fixes a
problem in the upstream kernel.
But I can see it happening in earlier versions, and I can read the
code in 2.6.18, and see obvious bugs.
I can't see any besides the temp_priority race.
--
SUSE Labs, Novell Inc.
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