Re: [PATCH 2/2] sysvshm: SHM_LOCK use lru_add_drain_all_async()

[Hugh Dickins]

On Tue, 3 Jan 2012, KOSAKI Motohiro wrote:
> (1/3/12 8:51 PM), Hugh Dickins wrote:
> > 
> > In testing my fix for that, I find that there has been no attempt to
> > keep the Unevictable count accurate on SysVShm: SHM_LOCK pages get
> > marked unevictable lazily later as memory pressure discovers them -
> > which perhaps mirrors the way in which SHM_LOCK makes no attempt to
> > instantiate pages, unlike mlock.
> 
> Ugh, you are right. I'm recovering my remember gradually. Lee implemented
> immediate lru off logic at first and I killed it
> to close a race. I completely forgot. So, yes, now SHM_LOCK has no attempt to
> instantiate pages. I'm ashamed.

Why ashamed?  The shmctl man-page documents "The caller must fault in any
pages that are required to be present after locking is enabled."  That's
just how it behaves.

> > (But in writing this, realize I still don't quite understand why
> > the Unevictable count takes a second or two to get back to 0 after
> > SHM_UNLOCK: perhaps I've more to discover.)
> 
> Interesting. I'm looking at this too.

In case you got distracted before you found it, mm/vmstat.c's

static DEFINE_PER_CPU(struct delayed_work, vmstat_work);
int sysctl_stat_interval __read_mostly = HZ;

static void vmstat_update(struct work_struct *w)
{
	refresh_cpu_vm_stats(smp_processor_id());
	schedule_delayed_work(&__get_cpu_var(vmstat_work),
		round_jiffies_relative(sysctl_stat_interval));
}

would be why, I think.  And that implies to me that your
lru_add_drain_all_async() is not necessary, you'd get just as good
an effect, more cheaply, by doing a local lru_add_drain() before the
refresh in vmstat_update().

But it would still require your changes to ____pagevec_lru_add_fn(),
if those turn out to help more than they hurt.

Hugh
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