Messages in this thread | | | From | Brian Geffon <> | Date | Mon, 27 Mar 2023 10:20:14 -0400 | Subject | Re: [PATCH v3] PM: hibernate: don't store zero pages in the image file. |
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On Thu, Mar 23, 2023 at 12:19 PM Matthias Kaehlcke <mka@chromium.org> wrote: >
Hi Matthias, Thanks for taking a look.
> On Thu, Mar 02, 2023 at 12:13:48PM -0500, Brian Geffon 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 write_next_page 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/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 -> v3: > > - Use nr_zero_pages rather than walking each pfn to count. > > - Make sure zero_bm is allocated in safe pages on resume. > > When reading in the pfn list and building the zero page bm > > we don't know which pages are unsafe yet so we will need to > > copy this bm to safe pages after the metadata has been read. > > > > Patch v1 -> v2: > > - minor code mistake from rebasing corrected. > > > > Signed-off-by: Brian Geffon <bgeffon@google.com> > > --- > > kernel/power/snapshot.c | 169 +++++++++++++++++++++++++++++++--------- > > 1 file changed, 133 insertions(+), 36 deletions(-) > > > > diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c > > index cd8b7b35f1e8b..a2c4fe17f9067 100644 > > --- a/kernel/power/snapshot.c > > +++ b/kernel/power/snapshot.c > > ... > > > @@ -1371,14 +1381,18 @@ 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. > > nit: s/a page/the page/
Ack. will fix in follow up version.
> > > */ > > -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; > > } > > ... > > > -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, > > + unsigned int *zero_count) > > { > > struct zone *zone; > > - unsigned long pfn; > > + unsigned long pfn, copy_pfn; > > > > for_each_populated_zone(zone) { > > unsigned long max_zone_pfn; > > @@ -1462,11 +1482,20 @@ 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); > > + if (zero_count) > > This check is not needed. The function is called only once, with a pointer. The kernel > trusts itself if the pointer is supposed to be always != NULL. > > Or better: use a local counter and have copy_data_pages() return the number of pages > that were actually copied, which is what the caller is interested in.
That makes a lot of sense, I'll switch to this approach in next iteration.
> > > + (*zero_count)++; > > + > > + /* We will reuse this copy_pfn for a real 'nonzero' page. */ > > + continue; > > + } > > + copy_pfn = memory_bm_next_pfn(copy_bm); > > } > > } > > ... > > > @@ -2247,24 +2299,34 @@ 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 the page was originally populated with only > > + * zeros then a corresponding bit will also be set in @zero_bm. > > s/the the/the/
Ack.
> > ... > > > @@ -2486,6 +2548,7 @@ static inline void free_highmem_data(void) {} > > * prepare_image - Make room for loading hibernation image. > > * @new_bm: Uninitialized memory bitmap structure. > > * @bm: Memory bitmap with unsafe pages marked. > > + * @zero_bm: Memory bitmap containing the zero pages. > > That sounds as if the memory bitmap actually contained zero pages. I suggest to > change it to something like the comment for 'bm' above, i.e. "... with zero > pages marked"
Sure that makes sense, will reword it.
> > > * > > * Use @bm to mark the pages that will be overwritten in the process of > > * restoring the system memory state from the suspend image ("unsafe" pages) > > @@ -2496,8 +2559,12 @@ static inline void free_highmem_data(void) {} > > * pages will be used for just yet. Instead, we mark them all as allocated and > > * create a lists of "safe" pages to be used later. On systems with high > > * memory a list of "safe" highmem pages is created too. > > + * > > + * Because we didn't know which pages were unsafe when we created the zero bm we > > + * will make a copy of it and recreate it within safe pages. > > nit: s/we will make/we make/
Ack.
> > > */ > > -static int prepare_image(struct memory_bitmap *new_bm, struct memory_bitmap *bm) > > +static int prepare_image(struct memory_bitmap *new_bm, struct memory_bitmap *bm, > > + struct memory_bitmap *zero_bm) > > { > > unsigned int nr_pages, nr_highmem; > > struct linked_page *lp; > > @@ -2516,6 +2583,20 @@ static int prepare_image(struct memory_bitmap *new_bm, struct memory_bitmap *bm) > > > > duplicate_memory_bitmap(new_bm, bm); > > memory_bm_free(bm, PG_UNSAFE_KEEP); > > + error = memory_bm_create(bm, GFP_ATOMIC, PG_ANY); > > + if (error) > > + goto Free; > > + > > + /* use bm as storage while we rebuild zero_bm using safe pages */ > > Re-using the 'bm' parameter is confusing, it should be avoided IMO unless there > is a real benefit. struct memory_bitmap isn't that big, why not create a local > variable 'zero_mb_tmp' or similar as a temporary store for the zero page bitmap?
Sure, I'll send an updated patch which does that.
> > > + duplicate_memory_bitmap(bm, zero_bm); > > + memory_bm_free(zero_bm, PG_UNSAFE_KEEP); > > + error = memory_bm_create(zero_bm, GFP_ATOMIC, PG_SAFE); > > + if (error) > > + goto Free; > > + duplicate_memory_bitmap(zero_bm, bm); > > + memory_bm_free(bm, PG_UNSAFE_KEEP); > > + /* at this point zero_bm is in safe pages and we can use it while restoring */ > > + > > if (nr_highmem > 0) { > > error = prepare_highmem_image(bm, &nr_highmem); > > if (error)
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