Re: [PATCH 2/6] mm: mlocking in try_to_unmap_one

[KOSAKI Motohiro]

> On Wed, 11 Nov 2009, KOSAKI Motohiro wrote:
> 
> Though it doesn't quite answer your question,
> I'll just reinsert the last paragraph of my description here...
> 
> > > try_to_unmap_file()'s TTU_MUNLOCK nonlinear handling was particularly
> > > amusing: once unravelled, it turns out to have been choosing between
> > > two different ways of doing the same nothing.  Ah, no, one way was
> > > actually returning SWAP_FAIL when it meant to return SWAP_SUCCESS.
> 
> ... 
> > > @@ -1081,45 +1053,23 @@ static int try_to_unmap_file(struct page
> ...
> > >
> > > -	if (list_empty(&mapping->i_mmap_nonlinear))
> > > +	/* We don't bother to try to find the munlocked page in nonlinears */
> > > +	if (MLOCK_PAGES && TTU_ACTION(flags) == TTU_MUNLOCK)
> > >  		goto out;
> > 
> > I have dumb question.
> > Does this shortcut exiting code makes any behavior change?
> 
> Not dumb.  My intention was to make no behaviour change with any of
> this patch; but in checking back before completing the description,
> I suddenly realized that that shortcut intentionally avoids the
> 
> 	if (max_nl_size == 0) {	/* all nonlinears locked or reserved ? */
> 		ret = SWAP_FAIL;
> 		goto out;
> 	}
> 
> (which doesn't show up in the patch: you'll have to look at rmap.c),
> which used to have the effect of try_to_munlock() returning SWAP_FAIL
> in the case when there were one or more VM_NONLINEAR vmas of the file,
> but none of them (and none of the covering linear vmas) VM_LOCKED.
> 
> That should have been a SWAP_SUCCESS case, or with my changes
> another SWAP_AGAIN, either of which would make munlock_vma_page()
> 				count_vm_event(UNEVICTABLE_PGMUNLOCKED);
> which would be correct; but the SWAP_FAIL meant that count was not
> incremented in this case.

Ah, correct.
Then, we lost the capability unevictability of non linear mapping pages, right.
if so, following additional patch makes more consistent?


> 
> Actually, I've double-fixed that, because I also changed
> munlock_vma_page() to increment the count whenever ret != SWAP_MLOCK;
> which seemed more appropriate, but would have been a no-op if
> try_to_munlock() only returned SWAP_SUCCESS or SWAP_AGAIN or SWAP_MLOCK
> as it claimed.
> 
> But I wasn't very inclined to boast of fixing that bug, since my testing
> didn't give confidence that those /proc/vmstat unevictable_pgs_*lock*
> counts are being properly maintained anyway - when I locked the same
> pages in two vmas then unlocked them in both, I ended up with mlocked
> bigger than munlocked (with or without my 2/6 patch); which I suspect
> is wrong, but rather off my present course towards KSM swapping...

Ah, vmstat inconsistent is weird. I'll try to debug it later.
Thanks this notice.


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