Re: [PATCH 1/2] mm: store zero pages to be swapped out in a bitmap

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, May 30, 2024 at 5:27 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, May 30, 2024 at 11:19:07AM +0100, Usama Arif wrote:
> > Approximately 10-20% of pages to be swapped out are zero pages [1].
> > Rather than reading/writing these pages to flash resulting
> > in increased I/O and flash wear, a bitmap can be used to mark these
> > pages as zero at write time, and the pages can be filled at
> > read time if the bit corresponding to the page is set.
> > With this patch, NVMe writes in Meta server fleet decreased
> > by almost 10% with conventional swap setup (zswap disabled).
> >
> > [1]https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/
> >
> > Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>
> This is awesome.
>
> > ---
> >  include/linux/swap.h |  1 +
> >  mm/page_io.c         | 86 ++++++++++++++++++++++++++++++++++++++++++--
> >  mm/swapfile.c        | 10 ++++++
> >  3 files changed, 95 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index a11c75e897ec..e88563978441 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -299,6 +299,7 @@ struct swap_info_struct {
> >       signed char     type;           /* strange name for an index */
> >       unsigned int    max;            /* extent of the swap_map */
> >       unsigned char *swap_map;        /* vmalloc'ed array of usage counts */
> > +     unsigned long *zeromap;         /* vmalloc'ed bitmap to track zero pages */
>
> One bit per swap slot, so 1 / (4096 * 8) = 0.003% static memory
> overhead for configured swap space. That seems reasonable for what
> appears to be a fairly universal 10% reduction in swap IO.
>
> An alternative implementation would be to reserve a bit in
> swap_map. This would be no overhead at idle, but would force
> continuation counts earlier on heavily shared page tables, and AFAICS
> would get complicated in terms of locking, whereas this one is pretty
> simple (atomic ops protect the map, swapcache lock protects the bit).
>
> So I prefer this version. But a few comments below:

I am wondering if it's even possible to take this one step further and
avoid reclaiming zero-filled pages in the first place. Can we just
unmap them and let the first read fault allocate a zero'd page like
uninitialized memory, or point them at the zero page and make them
read-only, or something? Then we could free them directly without
going into the swap code to begin with.

That's how I thought about it initially when I attempted to support
only zero-filled pages in zswap. It could be a more complex
implementation though.

[..]
> > +
> > +static void swap_zeromap_folio_set(struct folio *folio)
> > +{
> > +     struct swap_info_struct *sis = swp_swap_info(folio->swap);
> > +     swp_entry_t entry;
> > +     unsigned int i;
> > +
> > +     for (i = 0; i < folio_nr_pages(folio); i++) {
> > +             entry = page_swap_entry(folio_page(folio, i));
> > +             bitmap_set(sis->zeromap, swp_offset(entry), 1);
>
> This should be set_bit(). bitmap_set() isn't atomic, so it would
> corrupt the map on concurrent swapping of other zero pages. And you
> don't need a range op here anyway.

It's a shame there is no range version of set_bit(). I suspect we can
save a few atomic operations on large folios if we write them in
chunks rather than one by one.