Re: [PATCH v2] mm, hugetlb: set PageLRU for in-use/active hugepages

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Tue, Feb 17, 2015 at 03:57:44PM -0800, Andrew Morton wrote:
> On Tue, 17 Feb 2015 09:32:08 +0000 Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> wrote:
> 
> > Currently we are not safe from concurrent calls of isolate_huge_page(),
> > which can make the victim hugepage in invalid state and results in BUG_ON().
> > 
> > The root problem of this is that we don't have any information on struct page
> > (so easily accessible) about the hugepage's activeness. Note that hugepages'
> > activeness means just being linked to hstate->hugepage_activelist, which is
> > not the same as normal pages' activeness represented by PageActive flag.
> > 
> > Normal pages are isolated by isolate_lru_page() which prechecks PageLRU before
> > isolation, so let's do similarly for hugetlb. PageLRU is unused on hugetlb now,
> > so the change is mostly just inserting Set/ClearPageLRU (no conflict with
> > current usage.) And the other changes are justified like below:
> > - __put_compound_page() calls __page_cache_release() to do some LRU works,
> >   but this is obviously for thps and assumes that hugetlb has always !PageLRU.
> >   This assumption is not true any more, so this patch simply adds if (!PageHuge)
> >   to avoid calling __page_cache_release() for hugetlb.
> > - soft_offline_huge_page() now just calls list_move(), but generally callers
> >   of page migration should use the common routine in isolation, so let's
> >   replace the list_move() with isolate_huge_page() rather than inserting
> >   ClearPageLRU.
> > 
> > Set/ClearPageLRU should be called within hugetlb_lock, but hugetlb_cow() and
> > hugetlb_no_page() don't do this. This is justified because in these function
> > SetPageLRU is called right after the hugepage is allocated and no other thread
> > tries to isolate it.
> 
> Whoa.
> 
> So if I'm understanding this correctly, hugepages never have PG_lru set
> and so you are overloading that bit on hugepages to indicate that the
> page is present on hstate->hugepage_activelist?

Right, that's my intention.

> This is somewhat of a big deal and the patch doesn't make it very clear
> at all.  We should
> 
> - document PG_lru, for both of its identities

OK, I'll do this.

> - consider adding a new PG_hugepage_active(?) flag which has the same
>   value as PG_lru (see how PG_savepinned was done).

I thought of this at first, but didn't do just to avoid complexity for
the first patch. I know this is necessary finally, so I'll do this next.

Maybe I'll name it as PG_hugetlb_active, because just stating "hugepage"
might cause some confusion between hugetlb and thp in the future.

> - create suitable helper functions for the new PG_lru meaning. 
>   Simply calling PageLRU/SetPageLRU for pages which *aren't on the LRU*
>   is lazy and misleading.  Create a name for the new concept
>   (hugepage_active?) and document it and use it consistently.

OK.

> 
> > @@ -75,7 +76,8 @@ static void __put_compound_page(struct page *page)
> >  {
> >  	compound_page_dtor *dtor;
> >  
> > -	__page_cache_release(page);
> > +	if (!PageHuge(page))
> > +		__page_cache_release(page);
> >  	dtor = get_compound_page_dtor(page);
> >  	(*dtor)(page);
> 
> And this needs a good comment - there's no way that a reader can work
> out why this code is here unless he goes dumpster diving in the git
> history.

OK.

Thanks,
Naoya Horiguchi
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