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Subject[PATCH v4 00/25] Page folios
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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

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