As shown in the patchseries that introduced the zswap same-filled optimization [1], 10-20% of the pages stored in zswap are same-filled. This is also observed across Meta's server fleet. By using VM counters in swap_writepage (not included in this patchseries) it was found that less than 1% of the same-filled pages to be swapped out are non-zero pages. For conventional swap setup (without zswap), rather than reading/writing these pages to flash resulting in increased I/O and flash wear, the pte can be cleared for those addresses at unmap time while shrinking folio list. When this causes a page fault, do_pte_missing will take care of this page. When using zswap, this also means that a zswap_entry does not need to be allocated for zero filled pages resulting in memory savings. A similar attempt was made earlier in [2] where zswap would only track zero-filled pages instead of same-filled. This patchseries adds zero-filled pages optimization by default (hence it can be used even if zswap is disabled) and removes the same-filled code from zswap (as only 1% of the same-filled pages are non-zero), simplifying code. This patchseries is based on mm-unstable. [1] https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/ [2] https://lore.kernel.org/lkml/20240325235018.2028408-1-yosryahmed@xxxxxxxxxx/ --- v1 -> v2: - instead of using a bitmap in swap, clear pte for zero pages and let do_pte_missing handle this page at page fault. (Yosry and Matthew) - Check end of page first when checking if folio is zero filled as it could lead to better performance. (Yosry) Usama Arif (2): mm: clear pte for folios that are zero filled mm: remove code to handle same filled pages include/linux/rmap.h | 1 + mm/rmap.c | 163 ++++++++++++++++++++++--------------------- mm/vmscan.c | 89 ++++++++++++++++------- mm/zswap.c | 86 +++-------------------- 4 files changed, 158 insertions(+), 181 deletions(-) -- 2.43.0
