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SubjectRe: [PATCH 11/11] mm: Isolate pages for immediate reclaim on their own LRU
On Tue, Dec 20, 2011 at 04:10:26PM +0900, Minchan Kim wrote:
> > > > * Writeback is about to end against a page which has been marked for immediate
> > > > * reclaim. If it still appears to be reclaimable, move it to the tail of the
> > > > * inactive list.
> > > > */
> > > > void rotate_reclaimable_page(struct page *page)
> > > > {
> > > > + struct zone *zone = page_zone(page);
> > > > + struct list_head *page_list;
> > > > + struct pagevec *pvec;
> > > > + unsigned long flags;
> > > > +
> > > > + page_cache_get(page);
> > > > + local_irq_save(flags);
> > > > + __mod_zone_page_state(zone, NR_IMMEDIATE, -1);
> > > > +
> > >
> > > I am not sure underflow never happen.
> > > We do SetPageReclaim at several places but dont' increase NR_IMMEDIATE.
> > >
> >
> > In those cases, we do not move the page to the immedate list either.
>
> That's my concern.
> We didn't move the page to immediate list but set SetPageReclaim. It means
> we don't increate NR_IMMEDIATE.
> If end_page_writeback have called that page, rotate_reclimable_page would be called.
> Eventually, __mod_zone_page_state(zone, NR_IMMEDIATE, -1) is called.
> But I didn't look into the code yet for confirming it's possbile or not.
>

Ah, now I see your concern. The key is that they get moved to the
immediate LRU later although it is not obvious. This should be double
checked but when I was implementing this, I looked at the different
places that called SetPageReclaim.

mm/swap.c:lru_deactivate_fn() calls SetPageReclaim but also moves the
page to the immediate LRU list so no problem with accounting
there.

mm/vmscan.c:pageout() calls SetPageReclaim but does not move the page
explicitly as such. Instead, it gets picked up by
putback_lru_pages() later which checks for inactive LRU pages
that are marked PageReclaim and selects the immediate LRU in
this case. The counter gets incremented for the appropriate
LRU list by __add_page_to_lru_list(). Even if we do have
an active page with PageReclaim set, it should not cause an
accounting difficulty

mm/vmscan.c:shrink_page_list() calls SetPageReclaim but like pageout(),
it gets picked up by putback_lru_pages() later

Did I miss anything?

> > During one test I was recording /proc/vmstat every 10 seconds and never
> > saw an underflow.
>
> If it's very rare, it would be very hard to see it.
>

But once it happened, I would not expect it to recover. The nr_immediate
value usually reads as 0.

> > > > <SNIP>
> > > > static void update_page_reclaim_stat(struct zone *zone, struct page *page,
> > > > @@ -475,6 +532,13 @@ static void lru_deactivate_fn(struct page *page, void *arg)
> > > > * is _really_ small and it's non-critical problem.
> > > > */
> > > > SetPageReclaim(page);
> > > > +
> > > > + /*
> > > > + * Move to the LRU_IMMEDIATE list to avoid being scanned
> > > > + * by page reclaim uselessly.
> > > > + */
> > > > + list_move_tail(&page->lru, &zone->lru[LRU_IMMEDIATE].list);
> > > > + __mod_zone_page_state(zone, NR_IMMEDIATE, 1);
> > >
> > > It mekes below count of PGDEACTIVATE wrong in lru_deactivate_fn.
> > > Before this patch, all is from active to inacive so it was right.
> > > But with this patch, it can be from acdtive to immediate.
> > >
> >
> > I do not quite understand. PGDEACTIVATE is incremented if the page was
> > active and this is checked before the move to the immediate LRU. Whether
> > it moves to the immediate LRU or the end of the inactive list, it is
> > still a deactivation. What's wrong with incrementing the count if it
>
> Hmm, I have thought deactivation is only from active to deactive.

This is a matter of definition really. The page is going from active
to inactive. The immediate list is similar to the inactive list in
this case, at least from a deactivation point of view.

> I might be wrong but if we perhaps move page from active to unevictable list,
> is it deactivation, too?

I would consider it a deactivate if PageActive got cleared. Here we are
talking about the lru_deactivate_fn function. Whether it moves to the
immediate list or the end of the inactive list, the page is being
deactivated.

> Maybe we need consistent count.
>

In this case, I think we are being consistent. The page is deactivated,
we increase the PFDEACTIVATE counter.

Thanks very much for reviewing this closely, I appreciate it.

--
Mel Gorman
SUSE Labs


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