Messages in this thread | | | Date | Fri, 6 Jun 2008 18:04:30 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH -mm 04/25] free swap space on swap-in/activation |
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On Fri, 06 Jun 2008 16:28:42 -0400 Rik van Riel <riel@redhat.com> wrote:
> From: Rik van Riel <riel@redhat.com> > > Free swap cache entries when swapping in pages if vm_swap_full() > [swap space > 1/2 used]. Uses new pagevec to reduce pressure > on locks. > > Signed-off-by: Rik van Riel <riel@redhat.com> > Signed-off-by: Lee Schermerhorn <Lee.Schermerhorn@hp.com> > Signed-off-by: MinChan Kim <minchan.kim@gmail.com> > > --- > include/linux/pagevec.h | 1 + > include/linux/swap.h | 6 ++++++ > mm/swap.c | 18 ++++++++++++++++++ > mm/swapfile.c | 25 ++++++++++++++++++++++--- > mm/vmscan.c | 7 +++++++ > 5 files changed, 54 insertions(+), 3 deletions(-) > > Index: linux-2.6.26-rc2-mm1/mm/vmscan.c > =================================================================== > --- linux-2.6.26-rc2-mm1.orig/mm/vmscan.c 2008-05-23 14:21:33.000000000 -0400 > +++ linux-2.6.26-rc2-mm1/mm/vmscan.c 2008-05-23 14:21:33.000000000 -0400 > @@ -619,6 +619,9 @@ free_it: > continue; > > activate_locked: > + /* Not a candidate for swapping, so reclaim swap space. */ > + if (PageSwapCache(page) && vm_swap_full())
The patch puts rather a lot of pressure onto vm_swap_full(). We might want to look into optimising that.
- Is the 50% thing optimum? Could go higher and perhaps should be based on amount-of-memory.
- Can precalculate the fraction rather than doing it inline all the time.
- Can make total_swap_pages __read_mostly and have a think about nr_swap_pages too.
- Can completely optimise the thing away if !CONFIG_SWAP.
Has all this code been tested with CONFIG_SWAP=n?
> Index: linux-2.6.26-rc2-mm1/mm/swap.c > =================================================================== > --- linux-2.6.26-rc2-mm1.orig/mm/swap.c 2008-05-23 14:21:33.000000000 -0400 > +++ linux-2.6.26-rc2-mm1/mm/swap.c 2008-05-23 14:21:33.000000000 -0400 > @@ -443,6 +443,24 @@ void pagevec_strip(struct pagevec *pvec) > } > } > > +/* > + * Try to free swap space from the pages in a pagevec > + */ > +void pagevec_swap_free(struct pagevec *pvec) > +{ > + int i; > + > + for (i = 0; i < pagevec_count(pvec); i++) { > + struct page *page = pvec->pages[i]; > + > + if (PageSwapCache(page) && !TestSetPageLocked(page)) { > + if (PageSwapCache(page)) > + remove_exclusive_swap_page_ref(page); > + unlock_page(page); > + } > + } > +}
What's going on here.
Normally we'll bump a page's refcount to account for its presence in a pagevec. This code doesn't do that.
Is it safe? If so, how come?
Suitable code comments should be added which explain this unusual and dangerous optimisation. Or fix the bug :)
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