[RFC PATCH 0/5] mm: sub-folio dirty tracking for PTE-mapped mmap writes

[Kiryl Shutsemau <kirill@shutemov.name>]

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

A store through a shared file mapping dirties the whole folio. With large
page cache folios that turns a 4K store into 2M of writeback: one dirty
bit per folio, and writeback has no way to know which part changed.

XFS already knows better. iomap tracks dirty state per block and
iomap_writeback_folio() submits only the dirty ranges, and the buffered
write path sets just the range it copied. Only the mmap path throws that
away, because iomap_dirty_folio() covers the whole folio.

Narrowing the dirtying at page_mkwrite() time does not work on its own:
set_pte_range() batch-maps a whole folio writable on the first shared
write fault, so the stores that follow never fault and never reach the
filesystem.

So harvest the hardware instead. folio_clear_dirty_for_io() already calls
folio_mkclean(), whose rmap walk reads pte_dirty() for every entry of the
folio and throws it away. Those bits are the only record of which parts
of a large folio were written through a mapping. Collect them there and
hand the filesystem the runs that were dirty, through a new
a_ops->dirty_folio_range().

All of this is about PTE-mapped folios. A PMD-mapped folio has a single
dirty bit for the 2M it maps, so there is nothing finer to harvest, and
it keeps writing back whole. Keeping shared write faults off PMDs is a
separate patch and not part of this posting.

On a 512M file in 2M folios on XFS, storing one byte per folio and
calling msync() wrote 512M before and writes 1M after, with identical
minor fault counts.

Not addressed here:

 - Dirty accounting stays folio-granular. A 4K store still counts as 2M
   against dirty_ratio and balance_dirty_pages().
 - iomap_page_mkwrite() still allocates blocks for the whole folio.
 - Filesystems without per-block dirty state see no change.

Kiryl Shutsemau (Meta) (5):
  mm: let folio_mkclean() report which pages had dirty PTEs
  mm: add a_ops->dirty_folio_range() and use the mkclean dirty harvest
  mm: keep the mmap dirty range down to the faulting page
  iomap: narrow page_mkwrite() dirtying to the faulting page
  xfs: track mmap dirty state per block

 fs/iomap/buffered-io.c | 37 +++++++++++++-----
 fs/xfs/xfs_aops.c      |  2 +-
 include/linux/fs.h     |  3 ++
 include/linux/iomap.h  |  2 +
 include/linux/mm.h     |  1 +
 include/linux/rmap.h   |  7 ++++
 mm/memory.c            | 58 ++++++++++++++++++++++++++--
 mm/page-writeback.c    | 87 +++++++++++++++++++++++++++++++++++++++---
 mm/rmap.c              | 56 +++++++++++++++++++++------
 9 files changed, 222 insertions(+), 31 deletions(-)


base-commit: 0a0d1d55dad570724bf8c7ea83409639cfb4be9b
-- 
2.54.0