Date:   Thu, 2 Mar 2023 12:25:31 -0500
From:   Brian Geffon <bgeffon@...gle.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PM: hibernate: don't store zero pages in the image file.

On Thu, Feb 9, 2023 at 2:44 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Sat, Jan 14, 2023 at 1:15 AM Brian Geffon <bgeffon@...gle.com> wrote:
> >
> > On ChromeOS we've observed a considerable number of in-use pages filled
> > with zeros. Today with hibernate it's entirely possible that saveable
> > pages are just zero filled. Since we're already copying pages
> > word-by-word in do_copy_page it becomes almost free to determine if a
> > page was completely filled with zeros.
> >
> > This change introduces a new bitmap which will track these zero pages.
> > If a page is zero it will not be included in the saved image, instead to
> > track these zero pages in the image file we will introduce a new flag
> > which we will set on the packed PFN list. When reading back in the image
> > file we will detect these zero page PFNs and rebuild the zero page bitmap.
> >
> > When the image is being loaded through calls to snapshot_write_next if we
> > encounter a zero page we will silently memset it to 0 and then continue on
> > to the next page. Given the implementation in snapshot_read_next and
> > snapshot_write_next this change  will be transparent to non-compressed,
> > compressed, and swsusp modes of operation.
> >
> > To provide some concrete numbers from simple ad-hoc testing, on a device
> > which was lightly in use we saw that:
> >
> >  PM: hibernation: Image created (964408 pages copied, 548304 zero pages)
> >
> > Of the approximately 6.2GB of saveable pages 2.2GB (36%) were just zero
> > filled and could be tracked entirely within the packed PFN list. The
> > savings would obviously be much lower for lzo compressed images, but even
> > in the case of compression not copying pages across to the compression
> > threads will still speed things up. It's also possible that we would see
> > better overall compression ratios as larger regions of "real data" would
> > improve the compressibility.
> >
> > Finally, such an approach could dramatically improve swsusp performance
> > as each one of those zero pages requires a write syscall to reload, by
> > handling it as part of the packed PFN list we're able to fully avoid
> > that.
> >
> > patch v2:
> >         - correct a minor issue when rebasing.
>
> I need some more time to go through this in more detail, so it is
> likely to miss 6.3.  Sorry about that.

Hi Rafael,
I mailed a v3 patch which fixed an issue where the zero bm wasn't
being allocated in safe pages
on resume. This patch now appears to be pretty stable, I haven't seen
any issues so far. If you would
be so kind,  I'd appreciate any feedback you have.

Thanks in advance!

>
> > Signed-off-by: Brian Geffon <bgeffon@...gle.com>
> > ---
> >  kernel/power/snapshot.c | 129 ++++++++++++++++++++++++++++++----------
> >  1 file changed, 99 insertions(+), 30 deletions(-)
> >
> > diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> > index cd8b7b35f1e8..8d0ba36b0218 100644
> > --- a/kernel/power/snapshot.c
> > +++ b/kernel/power/snapshot.c
> > @@ -404,6 +404,7 @@ struct bm_position {
> >         struct mem_zone_bm_rtree *zone;
> >         struct rtree_node *node;
> >         unsigned long node_pfn;
> > +       unsigned long cur_pfn;
> >         int node_bit;
> >  };
> >
> > @@ -589,6 +590,7 @@ static void memory_bm_position_reset(struct memory_bitmap *bm)
> >         bm->cur.node = list_entry(bm->cur.zone->leaves.next,
> >                                   struct rtree_node, list);
> >         bm->cur.node_pfn = 0;
> > +       bm->cur.cur_pfn = BM_END_OF_MAP;
> >         bm->cur.node_bit = 0;
> >  }
> >
> > @@ -850,6 +852,11 @@ static void memory_bm_clear_current(struct memory_bitmap *bm)
> >         clear_bit(bit, bm->cur.node->data);
> >  }
> >
> > +static unsigned long memory_bm_get_current(struct memory_bitmap *bm)
> > +{
> > +       return bm->cur.cur_pfn;
> > +}
> > +
> >  static int memory_bm_test_bit(struct memory_bitmap *bm, unsigned long pfn)
> >  {
> >         void *addr;
> > @@ -929,10 +936,12 @@ static unsigned long memory_bm_next_pfn(struct memory_bitmap *bm)
> >                 if (bit < bits) {
> >                         pfn = bm->cur.zone->start_pfn + bm->cur.node_pfn + bit;
> >                         bm->cur.node_bit = bit + 1;
> > +                       bm->cur.cur_pfn = pfn;
> >                         return pfn;
> >                 }
> >         } while (rtree_next_node(bm));
> >
> > +       bm->cur.cur_pfn = BM_END_OF_MAP;
> >         return BM_END_OF_MAP;
> >  }
> >
> > @@ -1371,14 +1380,18 @@ static unsigned int count_data_pages(void)
> >
> >  /*
> >   * This is needed, because copy_page and memcpy are not usable for copying
> > - * task structs.
> > + * task structs. Returns 1 if a page was filled with only zeros, otherwise 0.
> >   */
> > -static inline void do_copy_page(long *dst, long *src)
> > +static inline int do_copy_page(long *dst, long *src)
> >  {
> >         int n;
> > +       long z = 0;
> >
> > -       for (n = PAGE_SIZE / sizeof(long); n; n--)
> > +       for (n = PAGE_SIZE / sizeof(long); n; n--) {
> > +               z |= *src;
> >                 *dst++ = *src++;
> > +       }
> > +       return !z;
> >  }
> >
> >  /**
> > @@ -1389,15 +1402,17 @@ static inline void do_copy_page(long *dst, long *src)
> >   * CONFIG_ARCH_HAS_SET_DIRECT_MAP is not set. In that case kernel_page_present()
> >   * always returns 'true'.
> >   */
> > -static void safe_copy_page(void *dst, struct page *s_page)
> > +static int safe_copy_page(void *dst, struct page *s_page)
> >  {
> > +       int ret;
> >         if (kernel_page_present(s_page)) {
> > -               do_copy_page(dst, page_address(s_page));
> > +               ret = do_copy_page(dst, page_address(s_page));
> >         } else {
> >                 hibernate_map_page(s_page);
> > -               do_copy_page(dst, page_address(s_page));
> > +               ret = do_copy_page(dst, page_address(s_page));
> >                 hibernate_unmap_page(s_page);
> >         }
> > +       return ret;
> >  }
> >
> >  #ifdef CONFIG_HIGHMEM
> > @@ -1407,17 +1422,18 @@ static inline struct page *page_is_saveable(struct zone *zone, unsigned long pfn
> >                 saveable_highmem_page(zone, pfn) : saveable_page(zone, pfn);
> >  }
> >
> > -static void copy_data_page(unsigned long dst_pfn, unsigned long src_pfn)
> > +static int copy_data_page(unsigned long dst_pfn, unsigned long src_pfn)
> >  {
> >         struct page *s_page, *d_page;
> >         void *src, *dst;
> > +       int ret;
> >
> >         s_page = pfn_to_page(src_pfn);
> >         d_page = pfn_to_page(dst_pfn);
> >         if (PageHighMem(s_page)) {
> >                 src = kmap_atomic(s_page);
> >                 dst = kmap_atomic(d_page);
> > -               do_copy_page(dst, src);
> > +               ret = do_copy_page(dst, src);
> >                 kunmap_atomic(dst);
> >                 kunmap_atomic(src);
> >         } else {
> > @@ -1426,30 +1442,32 @@ static void copy_data_page(unsigned long dst_pfn, unsigned long src_pfn)
> >                          * The page pointed to by src may contain some kernel
> >                          * data modified by kmap_atomic()
> >                          */
> > -                       safe_copy_page(buffer, s_page);
> > +                       ret = safe_copy_page(buffer, s_page);
> >                         dst = kmap_atomic(d_page);
> >                         copy_page(dst, buffer);
> >                         kunmap_atomic(dst);
> >                 } else {
> > -                       safe_copy_page(page_address(d_page), s_page);
> > +                       ret = safe_copy_page(page_address(d_page), s_page);
> >                 }
> >         }
> > +       return ret;
> >  }
> >  #else
> >  #define page_is_saveable(zone, pfn)    saveable_page(zone, pfn)
> >
> > -static inline void copy_data_page(unsigned long dst_pfn, unsigned long src_pfn)
> > +static inline int copy_data_page(unsigned long dst_pfn, unsigned long src_pfn)
> >  {
> > -       safe_copy_page(page_address(pfn_to_page(dst_pfn)),
> > +       return safe_copy_page(page_address(pfn_to_page(dst_pfn)),
> >                                 pfn_to_page(src_pfn));
> >  }
> >  #endif /* CONFIG_HIGHMEM */
> >
> >  static void copy_data_pages(struct memory_bitmap *copy_bm,
> > -                           struct memory_bitmap *orig_bm)
> > +                           struct memory_bitmap *orig_bm,
> > +                           struct memory_bitmap *zero_bm)
> >  {
> >         struct zone *zone;
> > -       unsigned long pfn;
> > +       unsigned long pfn, copy_pfn;
> >
> >         for_each_populated_zone(zone) {
> >                 unsigned long max_zone_pfn;
> > @@ -1462,11 +1480,18 @@ static void copy_data_pages(struct memory_bitmap *copy_bm,
> >         }
> >         memory_bm_position_reset(orig_bm);
> >         memory_bm_position_reset(copy_bm);
> > +       copy_pfn = memory_bm_next_pfn(copy_bm);
> >         for(;;) {
> >                 pfn = memory_bm_next_pfn(orig_bm);
> >                 if (unlikely(pfn == BM_END_OF_MAP))
> >                         break;
> > -               copy_data_page(memory_bm_next_pfn(copy_bm), pfn);
> > +               if (copy_data_page(copy_pfn, pfn)) {
> > +                       memory_bm_set_bit(zero_bm, pfn);
> > +
> > +                       /* We will reuse this copy_pfn for a real 'nonzero' page. */
> > +                       continue;
> > +               }
> > +               copy_pfn = memory_bm_next_pfn(copy_bm);
> >         }
> >  }
> >
> > @@ -1494,6 +1519,9 @@ static struct memory_bitmap orig_bm;
> >   */
> >  static struct memory_bitmap copy_bm;
> >
> > +/* Memory bitmap which tracks which saveable pages were zero filled. */
> > +static struct memory_bitmap zero_bm;
> > +
> >  /**
> >   * swsusp_free - Free pages allocated for hibernation image.
> >   *
> > @@ -1756,6 +1784,12 @@ int hibernate_preallocate_memory(void)
> >                 goto err_out;
> >         }
> >
> > +       error = memory_bm_create(&zero_bm, GFP_IMAGE, PG_ANY);
> > +       if (error) {
> > +               pr_err("Cannot allocate zero bitmap\n");
> > +               goto err_out;
> > +       }
> > +
> >         alloc_normal = 0;
> >         alloc_highmem = 0;
> >
> > @@ -2013,11 +2047,12 @@ static int swsusp_alloc(struct memory_bitmap *copy_bm,
> >
> >  asmlinkage __visible int swsusp_save(void)
> >  {
> > -       unsigned int nr_pages, nr_highmem;
> > +       unsigned int nr_pages, nr_highmem, nr_zero_pages;
> >
> >         pr_info("Creating image:\n");
> >
> >         drain_local_pages(NULL);
> > +       nr_zero_pages = 0;
> >         nr_pages = count_data_pages();
> >         nr_highmem = count_highmem_pages();
> >         pr_info("Need to copy %u pages\n", nr_pages + nr_highmem);
> > @@ -2037,19 +2072,23 @@ asmlinkage __visible int swsusp_save(void)
> >          * Kill them.
> >          */
> >         drain_local_pages(NULL);
> > -       copy_data_pages(&copy_bm, &orig_bm);
> > +       copy_data_pages(&copy_bm, &orig_bm, &zero_bm);
> >
> >         /*
> >          * End of critical section. From now on, we can write to memory,
> >          * but we should not touch disk. This specially means we must _not_
> >          * touch swap space! Except we must write out our image of course.
> >          */
> > +       memory_bm_position_reset(&zero_bm);
> > +       while (memory_bm_next_pfn(&zero_bm) != BM_END_OF_MAP)
> > +               nr_zero_pages++;
> >
> >         nr_pages += nr_highmem;
> > -       nr_copy_pages = nr_pages;
> > +       /* We don't actually copy the zero pages */
> > +       nr_copy_pages = nr_pages - nr_zero_pages;
> >         nr_meta_pages = DIV_ROUND_UP(nr_pages * sizeof(long), PAGE_SIZE);
> >
> > -       pr_info("Image created (%d pages copied)\n", nr_pages);
> > +       pr_info("Image created (%d pages copied, %d zero pages)\n", nr_copy_pages, nr_zero_pages);
> >
> >         return 0;
> >  }
> > @@ -2094,15 +2133,22 @@ static int init_header(struct swsusp_info *info)
> >         return init_header_complete(info);
> >  }
> >
> > +#define ENCODED_PFN_ZERO_FLAG (1UL << (BITS_PER_LONG - 1))
> > +#define ENCODED_PFN_MASK (~ENCODED_PFN_ZERO_FLAG)
> > +
> >  /**
> >   * pack_pfns - Prepare PFNs for saving.
> >   * @bm: Memory bitmap.
> >   * @buf: Memory buffer to store the PFNs in.
> > + * @zero_bm: Memory bitmap containing PFNs of zero pages.
> >   *
> >   * PFNs corresponding to set bits in @bm are stored in the area of memory
> > - * pointed to by @buf (1 page at a time).
> > + * pointed to by @buf (1 page at a time). Pages which were filled with only
> > + * zeros will have the highest bit set in the packed format to distinguish
> > + * them from PFNs which will be contained in the image file.
> >   */
> > -static inline void pack_pfns(unsigned long *buf, struct memory_bitmap *bm)
> > +static inline void pack_pfns(unsigned long *buf, struct memory_bitmap *bm,
> > +               struct memory_bitmap *zero_bm)
> >  {
> >         int j;
> >
> > @@ -2110,6 +2156,8 @@ static inline void pack_pfns(unsigned long *buf, struct memory_bitmap *bm)
> >                 buf[j] = memory_bm_next_pfn(bm);
> >                 if (unlikely(buf[j] == BM_END_OF_MAP))
> >                         break;
> > +               if (memory_bm_test_bit(zero_bm, buf[j]))
> > +                       buf[j] |= ENCODED_PFN_ZERO_FLAG;
> >         }
> >  }
> >
> > @@ -2151,7 +2199,7 @@ int snapshot_read_next(struct snapshot_handle *handle)
> >                 memory_bm_position_reset(&copy_bm);
> >         } else if (handle->cur <= nr_meta_pages) {
> >                 clear_page(buffer);
> > -               pack_pfns(buffer, &orig_bm);
> > +               pack_pfns(buffer, &orig_bm, &zero_bm);
> >         } else {
> >                 struct page *page;
> >
> > @@ -2247,24 +2295,32 @@ static int load_header(struct swsusp_info *info)
> >   * unpack_orig_pfns - Set bits corresponding to given PFNs in a memory bitmap.
> >   * @bm: Memory bitmap.
> >   * @buf: Area of memory containing the PFNs.
> > + * @zero_bm: Memory bitmap which will be populated with the PFNs of zero pages.
> >   *
> >   * For each element of the array pointed to by @buf (1 page at a time), set the
> > - * corresponding bit in @bm.
> > + * corresponding bit in @bm. If the page was originally populated with only
> > + * zeros then a corresponding bit will also be set in @zero_bm.
> >   */
> > -static int unpack_orig_pfns(unsigned long *buf, struct memory_bitmap *bm)
> > +static int unpack_orig_pfns(unsigned long *buf, struct memory_bitmap *bm,
> > +               struct memory_bitmap *zero_bm)
> >  {
> > -       int j;
> > +       int j, zero;
> > +       unsigned long decoded_pfn;
> >
> >         for (j = 0; j < PAGE_SIZE / sizeof(long); j++) {
> >                 if (unlikely(buf[j] == BM_END_OF_MAP))
> >                         break;
> >
> > -               if (pfn_valid(buf[j]) && memory_bm_pfn_present(bm, buf[j])) {
> > -                       memory_bm_set_bit(bm, buf[j]);
> > +               zero = !!(buf[j] & ENCODED_PFN_ZERO_FLAG);
> > +               decoded_pfn = buf[j] & ENCODED_PFN_MASK;
> > +               if (pfn_valid(decoded_pfn) && memory_bm_pfn_present(bm, decoded_pfn)) {
> > +                       memory_bm_set_bit(bm, decoded_pfn);
> > +                       if (zero)
> > +                               memory_bm_set_bit(zero_bm, decoded_pfn);
> >                 } else {
> > -                       if (!pfn_valid(buf[j]))
> > +                       if (!pfn_valid(decoded_pfn))
> >                                 pr_err(FW_BUG "Memory map mismatch at 0x%llx after hibernation\n",
> > -                                      (unsigned long long)PFN_PHYS(buf[j]));
> > +                                      (unsigned long long)PFN_PHYS(decoded_pfn));
> >                         return -EFAULT;
> >                 }
> >         }
> > @@ -2631,6 +2687,7 @@ int snapshot_write_next(struct snapshot_handle *handle)
> >         static struct chain_allocator ca;
> >         int error = 0;
> >
> > +next:
> >         /* Check if we have already loaded the entire image */
> >         if (handle->cur > 1 && handle->cur > nr_meta_pages + nr_copy_pages)
> >                 return 0;
> > @@ -2657,9 +2714,13 @@ int snapshot_write_next(struct snapshot_handle *handle)
> >                 if (error)
> >                         return error;
> >
> > +               error = memory_bm_create(&zero_bm, GFP_ATOMIC, PG_ANY);
> > +               if (error)
> > +                       return error;
> > +
> >                 hibernate_restore_protection_begin();
> >         } else if (handle->cur <= nr_meta_pages + 1) {
> > -               error = unpack_orig_pfns(buffer, &copy_bm);
> > +               error = unpack_orig_pfns(buffer, &copy_bm, &zero_bm);
> >                 if (error)
> >                         return error;
> >
> > @@ -2686,6 +2747,14 @@ int snapshot_write_next(struct snapshot_handle *handle)
> >                         handle->sync_read = 0;
> >         }
> >         handle->cur++;
> > +
> > +       /* Zero pages were not included in the image, memset it and move on. */
> > +       if ((handle->cur > nr_meta_pages + 1) &&
> > +                       memory_bm_test_bit(&zero_bm, memory_bm_get_current(&orig_bm))) {
> > +               memset(handle->buffer, 0, PAGE_SIZE);
> > +               goto next;
> > +       }
> > +
> >         return PAGE_SIZE;
> >  }
> >
> > --
> > 2.39.0.314.g84b9a713c41-goog
> >

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