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SubjectRe: [patch 1/3] vmscan: factor out page reference checks
Hello Minchan,

On Tue, Feb 23, 2010 at 10:38:23PM +0900, Minchan Kim wrote:
> Hi, Hannes.
>
> On Mon, 2010-02-22 at 20:49 +0100, Johannes Weiner wrote:
> > Moving the big conditional into its own predicate function makes the
> > code a bit easier to read and allows for better commenting on the
> > checks one-by-one.
> >
> > This is just cleaning up, no semantics should have been changed.
> >
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > ---
> > mm/vmscan.c | 53 ++++++++++++++++++++++++++++++++++++++++-------------
> > 1 files changed, 40 insertions(+), 13 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index c26986c..c2db55b 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -579,6 +579,37 @@ redo:
> > put_page(page); /* drop ref from isolate */
> > }
> >
> > +enum page_references {
> > + PAGEREF_RECLAIM,
> > + PAGEREF_RECLAIM_CLEAN,
> > + PAGEREF_ACTIVATE,
> > +};
> > +
> > +static enum page_references page_check_references(struct page *page,
> > + struct scan_control *sc)
> > +{
> > + unsigned long vm_flags;
> > + int referenced;
> > +
> > + referenced = page_referenced(page, 1, sc->mem_cgroup, &vm_flags);
> > + if (!referenced)
> > + return PAGEREF_RECLAIM;
> > +
> > + /* Lumpy reclaim - ignore references */
> > + if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> > + return PAGEREF_RECLAIM;
> > +
> > + /* Mlock lost isolation race - let try_to_unmap() handle it */
>
> How doest try_to_unamp handle it?
>
> /* Page which PG_mlocked lost isolation race - let try_to_unmap() move
> the page to unevitable list */
>
> The point is to move the page into unevictable list in case of race.
> Let's write down comment more clearly.
> As it was, it was clear, I think. :)

Okay, I do not feel strongly about it. I just figured it would be enough
at this point to say 'page is special, pass it on to try_to_unmap(), it
knows how to handle it. We do not care.'. But maybe you are right.

I attached an incremental patch below.

> > + if (vm_flags & VM_LOCKED)
> > + return PAGEREF_RECLAIM;
> > +
> > + if (page_mapping_inuse(page))
> > + return PAGEREF_ACTIVATE;
> > +
> > + /* Reclaim if clean, defer dirty pages to writeback */
> > + return PAGEREF_RECLAIM_CLEAN;
> > +}
> > +
> > /*
> > * shrink_page_list() returns the number of reclaimed pages
> > */
> > @@ -590,16 +621,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > struct pagevec freed_pvec;
> > int pgactivate = 0;
> > unsigned long nr_reclaimed = 0;
> > - unsigned long vm_flags;
> >
> > cond_resched();
> >
> > pagevec_init(&freed_pvec, 1);
> > while (!list_empty(page_list)) {
> > + enum page_references references;
> > struct address_space *mapping;
> > struct page *page;
> > int may_enter_fs;
> > - int referenced;
> >
> > cond_resched();
> >
> > @@ -641,17 +671,14 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > goto keep_locked;
> > }
> >
> > - referenced = page_referenced(page, 1,
> > - sc->mem_cgroup, &vm_flags);
> > - /*
> > - * In active use or really unfreeable? Activate it.
> > - * If page which have PG_mlocked lost isoltation race,
> > - * try_to_unmap moves it to unevictable list
> > - */
> > - if (sc->order <= PAGE_ALLOC_COSTLY_ORDER &&
> > - referenced && page_mapping_inuse(page)
> > - && !(vm_flags & VM_LOCKED))
> > + references = page_check_references(page, sc);
> > + switch (references) {
> > + case PAGEREF_ACTIVATE:
> > goto activate_locked;
> > + case PAGEREF_RECLAIM:
> > + case PAGEREF_RECLAIM_CLEAN:
> > + ; /* try to reclaim the page below */
> > + }
> >
> > /*
> > * Anonymous process memory has backing store?
> > @@ -685,7 +712,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > }
> >
> > if (PageDirty(page)) {
> > - if (sc->order <= PAGE_ALLOC_COSTLY_ORDER && referenced)
> > + if (references == PAGEREF_RECLAIM_CLEAN)
>
> How equal PAGEREF_RECLAIM_CLEAN and sc->order <= PAGE_ALLOC_COSTLY_ORDER
> && referenced by semantic?

It is encoded in page_check_references(). When
sc->order <= PAGE_ALLOC_COSTLY_ORDER && referenced
it returns PAGEREF_RECLAIM_CLEAN.

So

- PageDirty() && order < COSTLY && referenced
+ PageDirty() && references == PAGEREF_RECLAIM_CLEAN

is an equivalent transformation. Does this answer your question?

Hannes

---
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: vmscan: improve comment on mlocked page in reclaim
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 674a78b..819fff7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -578,7 +578,10 @@ static enum page_references page_check_references(struct page *page,
if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
return PAGEREF_RECLAIM;

- /* Mlock lost isolation race - let try_to_unmap() handle it */
+ /*
+ * Mlock lost the isolation race with us. Let try_to_unmap()
+ * move the page to the unevictable list.
+ */
if (vm_flags & VM_LOCKED)
return PAGEREF_RECLAIM;



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