On Mon, Jun 06, 2011 at 11:32:16AM +0100, Mel Gorman wrote:
> On Fri, Jun 03, 2011 at 08:07:30PM +0200, Andrea Arcangeli wrote:
> > On Fri, Jun 03, 2011 at 07:37:07PM +0200, Andrea Arcangeli wrote:
> > > On Fri, Jun 03, 2011 at 08:01:44AM +0900, Minchan Kim wrote:
> > > > Do you want this? (it's almost pseudo-code)
> > >
> > > Yes that's good idea so we at least take into account if we isolated
> > > something big, and it's pointless to insist wasting CPU on the tail
> > > pages and even trace a fail because of tail pages after it.
> > >
> > > I introduced a __page_count to increase readability. It's still
> > > hackish to work on subpages in vmscan.c but at least I added a comment
> > > and until we serialize destroy_compound_page vs compound_head, I guess
> > > there's no better way. I didn't attempt to add out of order
> > > serialization similar to what exists for split_huge_page vs
> > > compound_trans_head yet, as the page can be allocated or go away from
> > > under us, in split_huge_page vs compound_trans_head it's simpler
> > > because both callers are required to hold a pin on the page so the
> > > page can't go be reallocated and destroyed under it.
> >
> > Sent too fast... had to shuffle a few things around... trying again.
> >
> > ===
> > Subject: mm: no page_count without a page pin
> >
> > From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> >
> > It's unsafe to run page_count during the physical pfn scan because
> > compound_head could trip on a dangling pointer when reading page->first_page if
> > the compound page is being freed by another CPU. Also properly take into
> > account if we isolated a compound page during the scan and break the loop if
> > we've isolated enoguh. Introduce __page_count to cleanup some atomic_read from
> > &page->_count in common code to cleanup.
> >
> > Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>
> This patch is pulling in stuff from Minchan. Minimally his patch should
> be kept separate to preserve history or his Signed-off should be
> included on this patch.
>
> > ---
> > arch/powerpc/mm/gup.c | 2 -
> > arch/powerpc/platforms/512x/mpc512x_shared.c | 2 -
> > arch/x86/mm/gup.c | 2 -
> > fs/nilfs2/page.c | 2 -
> > include/linux/mm.h | 13 ++++++----
> > mm/huge_memory.c | 4 +--
> > mm/internal.h | 2 -
> > mm/page_alloc.c | 6 ++--
> > mm/swap.c | 4 +--
> > mm/vmscan.c | 35 ++++++++++++++++++++-------
> > 10 files changed, 47 insertions(+), 25 deletions(-)
> >
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1047,7 +1047,7 @@ static unsigned long isolate_lru_pages(u
> > for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
> > struct page *page;
> > unsigned long pfn;
> > - unsigned long end_pfn;
> > + unsigned long start_pfn, end_pfn;
> > unsigned long page_pfn;
> > int zone_id;
> >
> > @@ -1087,9 +1087,9 @@ static unsigned long isolate_lru_pages(u
> > */
> > zone_id = page_zone_id(page);
> > page_pfn = page_to_pfn(page);
> > - pfn = page_pfn & ~((1 << order) - 1);
> > - end_pfn = pfn + (1 << order);
> > - for (; pfn < end_pfn; pfn++) {
> > + start_pfn = page_pfn & ~((1 << order) - 1);
> > + end_pfn = start_pfn + (1 << order);
> > + for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> > struct page *cursor_page;
> >
> > /* The target page is in the block, ignore it. */
> > @@ -1116,16 +1116,33 @@ static unsigned long isolate_lru_pages(u
> > break;
> >
> > if (__isolate_lru_page(cursor_page, mode, file) == 0) {
> > + unsigned int isolated_pages;
> > list_move(&cursor_page->lru, dst);
> > mem_cgroup_del_lru(cursor_page);
> > - nr_taken += hpage_nr_pages(page);
> > - nr_lumpy_taken++;
> > + isolated_pages = hpage_nr_pages(page);
> > + nr_taken += isolated_pages;
> > + nr_lumpy_taken += isolated_pages;
> > if (PageDirty(cursor_page))
> > - nr_lumpy_dirty++;
> > + nr_lumpy_dirty += isolated_pages;
> > scan++;
> > + pfn += isolated_pages-1;
>
> Ah, here is the isolated_pages - 1 which is necessary. Should have read
> the whole thread before responding to anything :).
>
> I still think this optimisation is rare and only applies if we are
> encountering huge pages during the linear scan. How often are we doing
> that really?
>
> > + VM_BUG_ON(!isolated_pages);
>
> This BUG_ON is overkill. hpage_nr_pages would have to return 0.
>
> > + VM_BUG_ON(isolated_pages > MAX_ORDER_NR_PAGES);
>
> This would require order > MAX_ORDER_NR_PAGES to be passed into
> isolate_lru_pages or for a huge page to be unaligned to a power of
> two. The former is very unlikely and the latter is not supported by
> any CPU.
>
> > } else {
> > - /* the page is freed already. */
> > - if (!page_count(cursor_page))
> > + /*
> > + * Check if the page is freed already.
> > + *
> > + * We can't use page_count() as that
> > + * requires compound_head and we don't
> > + * have a pin on the page here. If a
> > + * page is tail, we may or may not
> > + * have isolated the head, so assume
> > + * it's not free, it'd be tricky to
> > + * track the head status without a
> > + * page pin.
> > + */
> > + if (!PageTail(cursor_page) &&
> > + !__page_count(cursor_page))
> > continue;
> > break;
>
> Ack to this part.
>
> I'm not keen on __page_count() as __ normally means the "unlocked"
> version of a function although I realise that rule isn't universal
Yes. It's not univeral.
I have thought about it as it's just private function or utility function
(ie, not-exportable).
So I don't mind the name.
> either. I can't think of a better name though.
Me, too.
--
Kind regards
Minchan Kim
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