On 04/09/2024 06:55, Barry Song wrote: > On Thu, Jun 13, 2024 at 12:48 AM Usama Arif <usamaarif642@xxxxxxxxx> 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@xxxxxxxxx> >> --- >> include/linux/swap.h | 1 + >> mm/page_io.c | 114 ++++++++++++++++++++++++++++++++++++++++++- >> mm/swapfile.c | 24 ++++++++- >> 3 files changed, 136 insertions(+), 3 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 */ >> struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */ >> struct swap_cluster_list free_clusters; /* free clusters list */ >> unsigned int lowest_bit; /* index of first free in swap_map */ >> diff --git a/mm/page_io.c b/mm/page_io.c >> index a360857cf75d..39fc3919ce15 100644 >> --- a/mm/page_io.c >> +++ b/mm/page_io.c >> @@ -172,6 +172,88 @@ int generic_swapfile_activate(struct swap_info_struct *sis, >> goto out; >> } >> >> +static bool is_folio_page_zero_filled(struct folio *folio, int i) >> +{ >> + unsigned long *data; >> + unsigned int pos, last_pos = PAGE_SIZE / sizeof(*data) - 1; >> + bool ret = false; >> + >> + data = kmap_local_folio(folio, i * PAGE_SIZE); >> + if (data[last_pos]) >> + goto out; >> + for (pos = 0; pos < PAGE_SIZE / sizeof(*data); pos++) { >> + if (data[pos]) >> + goto out; >> + } >> + ret = true; >> +out: >> + kunmap_local(data); >> + return ret; >> +} >> + >> +static bool is_folio_zero_filled(struct folio *folio) >> +{ >> + unsigned int i; >> + >> + for (i = 0; i < folio_nr_pages(folio); i++) { >> + if (!is_folio_page_zero_filled(folio, i)) >> + return false; >> + } >> + return true; >> +} >> + >> +static void folio_zero_fill(struct folio *folio) >> +{ >> + unsigned int i; >> + >> + for (i = 0; i < folio_nr_pages(folio); i++) >> + clear_highpage(folio_page(folio, i)); >> +} >> + >> +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)); >> + set_bit(swp_offset(entry), sis->zeromap); >> + } >> +} >> + >> +static void swap_zeromap_folio_clear(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)); >> + clear_bit(swp_offset(entry), sis->zeromap); >> + } >> +} >> + >> +/* >> + * Return the index of the first subpage which is not zero-filled >> + * according to swap_info_struct->zeromap. >> + * If all pages are zero-filled according to zeromap, it will return >> + * folio_nr_pages(folio). >> + */ >> +static unsigned int swap_zeromap_folio_test(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)); >> + if (!test_bit(swp_offset(entry), sis->zeromap)) >> + return i; >> + } >> + return i; >> +} >> + >> /* >> * We may have stale swap cache pages in memory: notice >> * them here and get rid of the unnecessary final write. >> @@ -195,6 +277,13 @@ int swap_writepage(struct page *page, struct writeback_control *wbc) >> folio_unlock(folio); >> return ret; >> } >> + >> + if (is_folio_zero_filled(folio)) { >> + swap_zeromap_folio_set(folio); >> + folio_unlock(folio); >> + return 0; >> + } >> + swap_zeromap_folio_clear(folio); >> if (zswap_store(folio)) { >> folio_start_writeback(folio); >> folio_unlock(folio); >> @@ -426,6 +515,26 @@ static void sio_read_complete(struct kiocb *iocb, long ret) >> mempool_free(sio, sio_pool); >> } >> >> +static bool swap_read_folio_zeromap(struct folio *folio) >> +{ >> + unsigned int idx = swap_zeromap_folio_test(folio); >> + >> + if (idx == 0) >> + return false; >> + >> + /* >> + * Swapping in a large folio that is partially in the zeromap is not >> + * currently handled. Return true without marking the folio uptodate so >> + * that an IO error is emitted (e.g. do_swap_page() will sigbus). >> + */ >> + if (WARN_ON_ONCE(idx < folio_nr_pages(folio))) >> + return true; > > Hi Usama, Yosry, > > I feel the warning is wrong as we could have the case where idx==0 > is not zeromap but idx=1 is zeromap. idx == 0 doesn't necessarily > mean we should return false. > > What about the below change which both fixes the warning and unblocks > large folios swap-in? > Hi Barry, I remembered when resending the zeromap series about the comment Yosry had made earlier, but checked that the mTHP swap-in was not in mm-unstable. I should have checked the mailing list and commented! I have not tested the below diff yet (will do in a few hours). But there might be a small issue with it. Have commented inline. > diff --git a/mm/page_io.c b/mm/page_io.c > index 4bc77d1c6bfa..7d7ff7064e2b 100644 > --- a/mm/page_io.c > +++ b/mm/page_io.c > @@ -226,26 +226,6 @@ static void swap_zeromap_folio_clear(struct folio *folio) > } > } > > -/* > - * Return the index of the first subpage which is not zero-filled > - * according to swap_info_struct->zeromap. > - * If all pages are zero-filled according to zeromap, it will return > - * folio_nr_pages(folio). > - */ > -static unsigned int swap_zeromap_folio_test(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)); > - if (!test_bit(swp_offset(entry), sis->zeromap)) > - return i; > - } > - return i; > -} > - > /* > * We may have stale swap cache pages in memory: notice > * them here and get rid of the unnecessary final write. > @@ -524,9 +504,10 @@ static void sio_read_complete(struct kiocb *iocb, long ret) > > static bool swap_read_folio_zeromap(struct folio *folio) > { > - unsigned int idx = swap_zeromap_folio_test(folio); > + unsigned int nr_pages = folio_nr_pages(folio); > + unsigned int nr = swap_zeromap_entries_count(folio->swap, nr_pages); > > - if (idx == 0) > + if (nr == 0) > return false; > > /* > @@ -534,7 +515,7 @@ static bool swap_read_folio_zeromap(struct folio *folio) > * currently handled. Return true without marking the folio uptodate so > * that an IO error is emitted (e.g. do_swap_page() will sigbus). > */ > - if (WARN_ON_ONCE(idx < folio_nr_pages(folio))) > + if (WARN_ON_ONCE(nr < nr_pages)) > return true; > > folio_zero_range(folio, 0, folio_size(folio)); > diff --git a/mm/swap.h b/mm/swap.h > index f8711ff82f84..2d59e9d89e95 100644 > --- a/mm/swap.h > +++ b/mm/swap.h > @@ -80,6 +80,32 @@ static inline unsigned int folio_swap_flags(struct folio *folio) > { > return swp_swap_info(folio->swap)->flags; > } > + > +/* > + * Return the number of entries which are zero-filled according to > + * swap_info_struct->zeromap. It isn't precise if the return value > + * is larger than 0 and smaller than nr to avoid extra iterations, > + * In this case, it means entries haven't consistent zeromap. > + */ > +static inline unsigned int swap_zeromap_entries_count(swp_entry_t entry, int nr) > +{ > + struct swap_info_struct *sis = swp_swap_info(entry); > + unsigned long offset = swp_offset(entry); > + unsigned int type = swp_type(entry); > + unsigned int n = 0; > + > + for (int i = 0; i < nr; i++) { > + entry = swp_entry(type, offset + i); > + if (test_bit(offset + i, sis->zeromap)) { Should this be if (test_bit(swp_offset(entry), sis->zeromap)) Also, are you going to use this in alloc_swap_folio? You mentioned above that this unblocks large folios swap-in, but I don't see it in the diff here. I am guessing there is some change in alloc_swap_info that uses swap_zeromap_entries_count? Thanks Usama > + if (i != n) > + return i; > + n++; > + } > + } > + > + return n; > +} > + > #else /* CONFIG_SWAP */ > struct swap_iocb; > static inline void swap_read_folio(struct folio *folio, struct swap_iocb **plug) > @@ -171,6 +197,11 @@ static inline unsigned int folio_swap_flags(struct folio *folio) > { > return 0; > } > + > +static inline unsigned int swap_zeromap_entries_count(swp_entry_t entry, int nr) > +{ > + return 0; > +} > #endif /* CONFIG_SWAP */ > > #endif /* _MM_SWAP_H */ >
