Our type system does not currently distinguish between tail pages and head or single pages. This is a problem because we call compound_head() multiple times (and the compiler cannot optimise it out), bloating the kernel. It also makes programming hard as it is often unclear whether a function operates on an individual page, or an entire compound page. This patch series introduces the struct folio, which is a type that represents an entire compound page. This initial set reduces the kernel size by approximately 6kB, although its real purpose is adding infrastructure to enable further use of the folio. The big correctness proof that exists in this patch series is that we never lock or wait for writeback on a tail page. This is important as we would miss wakeups due to being on the wrong page waitqueue if we ever did. I analysed the text size reduction using a config based on Oracle UEK with all modules changed to built-in. That's obviously not a kernel which makes sense to run, but it serves to compare the effects on (many common) filesystems & drivers, not just the core. add/remove: 33510/33499 grow/shrink: 1831/1898 up/down: 888762/-894719 (-5957) For a Debian config, just comparing vmlinux.o gives a reduction of 3828 bytes of text and 72 bytes of data: text data bss dec hex filename 16125879 4421122 1846344 22393345 155b201 linus/vmlinux.o 16122051 4421050 1846344 22389445 155a2c5 folio/vmlinux.o For nfs (a module I happened to notice was particularly affected), it's a reduction of 588 bytes of text and 16 bytes of data: 257142 59228 408 316778 4d56a linus/fs/nfs/nfs.o 256554 59212 408 316174 4d30e folio/fs/nfs/nfs.o Current tree at: https://git.infradead.org/users/willy/pagecache.git/shortlog/refs/heads/folio (contains another ~70 patches on top of this batch) v4: - Rebase on current Linus tree (including swap fix) - Analyse each patch in terms of its effects on kernel text size. A few were modified to improve their effect. In particular, where pushing calls to page_folio() into the callers resulted in unacceptable size increases, the wrapper was placed in mm/folio-compat.c. This lets us see all the places which are good targets for conversion to folios. - Some of the patches were reordered, split or merged in order to make more logical sense. - Use nth_page() for folio_next() if we're using SPARSEMEM and not VMEMMAP (Zi Yan) - Increment and decrement page stats in units of pages instead of units of folios (Zi Yan) v3: - Rebase on next-20210127. Two major sources of conflict, the generic_file_buffered_read refactoring (in akpm tree) and the fscache work (in dhowells tree). v2: - Pare patch series back to just infrastructure and the page waiting parts. Matthew Wilcox (Oracle) (25): mm: Introduce struct folio mm: Add folio_pgdat and folio_zone mm/vmstat: Add functions to account folio statistics mm/debug: Add VM_BUG_ON_FOLIO and VM_WARN_ON_ONCE_FOLIO mm: Add put_folio mm: Add get_folio mm: Create FolioFlags mm: Handle per-folio private data mm: Add folio_index, folio_page and folio_contains mm/util: Add folio_mapping and folio_file_mapping mm/memcg: Add folio wrappers for various memcontrol functions mm/filemap: Add unlock_folio mm/filemap: Add lock_folio mm/filemap: Add lock_folio_killable mm/filemap: Convert lock_page_async to lock_folio_async mm/filemap: Convert end_page_writeback to end_folio_writeback mm/filemap: Add wait_on_folio_locked & wait_on_folio_locked_killable mm/page-writeback: Add wait_on_folio_writeback mm/page-writeback: Add wait_for_stable_folio mm/filemap: Convert wait_on_page_bit to wait_on_folio_bit mm/filemap: Add __lock_folio_or_retry mm/filemap: Convert wake_up_page_bit to wake_up_folio_bit mm/page-writeback: Convert test_clear_page_writeback to take a folio mm/filemap: Convert page wait queues to be folios cachefiles: Switch to wait_page_key fs/afs/write.c | 21 ++-- fs/cachefiles/rdwr.c | 13 +-- fs/io_uring.c | 2 +- include/linux/fscache.h | 5 + include/linux/memcontrol.h | 30 +++++ include/linux/mm.h | 88 ++++++++++----- include/linux/mm_types.h | 33 ++++++ include/linux/mmdebug.h | 20 ++++ include/linux/page-flags.h | 106 ++++++++++++++---- include/linux/pagemap.h | 197 +++++++++++++++++++++++---------- include/linux/vmstat.h | 83 ++++++++++++++ mm/Makefile | 2 +- mm/filemap.c | 218 ++++++++++++++++++------------------- mm/folio-compat.c | 37 +++++++ mm/memory.c | 8 +- mm/page-writeback.c | 58 +++++----- mm/swapfile.c | 6 +- mm/util.c | 20 ++-- 18 files changed, 666 insertions(+), 281 deletions(-) create mode 100644 mm/folio-compat.c -- 2.30.0
