Re: [patch]vmscan: protect exectuable page from inactive list scan

[Wu Fengguang <fengguang.wu@intel.com>]

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On Thu, Sep 30, 2010 at 10:27:04AM +0800, KOSAKI Motohiro wrote:
> > On Wed, 2010-09-29 at 18:17 +0800, Johannes Weiner wrote:
> > > On Wed, Sep 29, 2010 at 10:57:40AM +0800, Shaohua Li wrote:
> > > > With commit 645747462435, pte referenced file page isn't activated in inactive
> > > > list scan. For VM_EXEC page, if it can't get a chance to active list, the
> > > > executable page protect loses its effect. We protect such page in inactive scan
> > > > here, now such page will be guaranteed cached in a full scan of active and
> > > > inactive list, which restores previous behavior.
> > > 
> > > This change was in the back of my head since the used-once detection
> > > was merged but there were never any regressions reported that would
> > > indicate a requirement for it.
> > The executable page protect is to improve responsibility. I would expect
> > it's hard for user to report such regression. 
> 
> Seems strange. 8cab4754d24a0f was introduced for fixing real world problem.
> So, I wonder why current people can't feel the same lag if it is.
> 
> 
> > > Does this patch fix a problem you observed?
> > No, I haven't done test where Fengguang does in commit 8cab4754d24a0f.
> 
> But, I am usually not against a number. If you will finished to test them I'm happy :)

Yeah, it needs good numbers for adding such special case code.
I attached the scripts used for 8cab4754d24a0f, hope this helps.

Note that the test-mmap-exec-prot.sh used /proc/sys/fs/suid_dumpable
as an indicator whether the extra logic is enabled. This is a convenient
trick I sometimes play with new code:

+                       extern int suid_dumpable;
+                       if (suid_dumpable)
                        if ((vm_flags & VM_EXEC) && !PageAnon(page)) {
                                list_add(&page->lru, &l_active);
                                continue;

> > 
> > > > --- a/mm/vmscan.c
> > > > +++ b/mm/vmscan.c
> > > > @@ -608,8 +608,15 @@ static enum page_references page_check_references(struct page *page,
> > > >  		 * quickly recovered.
> > > >  		 */
> > > >  		SetPageReferenced(page);
> > > > -
> > > > -		if (referenced_page)
> > > > +		/*
> > > > +		 * Identify pte referenced and file-backed pages and give them
> > > > +		 * one trip around the active list. So that executable code get
> > > > +		 * better chances to stay in memory under moderate memory
> > > > +		 * pressure. JVM can create lots of anon VM_EXEC pages, so we
> > > > +		 * ignore them here.
> > > > +               if (referenced_page || ((vm_flags & VM_EXEC) &&
> > > > +                   page_is_file_cache(page)))
> > > >                         return PAGEREF_ACTIVATE;

> > > 
> > > PTE-referenced PageAnon() pages are activated unconditionally a few
> > > lines further up, so the page_is_file_cache() check filters only shmem
> > > pages.  I doubt this was your intention...?
> > This is intented. the executable page protect is just to protect
> > executable file pages. please see 8cab4754d24a0f.
> 
> 8cab4754d24a0f was using !PageAnon() but your one are using page_is_file_cache.
> 8cab4754d24a0f doesn't tell us the reason of the change, no?

What if the executable file happen to be on tmpfs?  The !PageAnon()
test also covers that case. The page_is_file_cache() test here seems
unnecessary. And it looks better to move the VM_EXEC test above the
SetPageReferenced() line to avoid possible side effects.

Thanks,
Fengguang

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