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

[Usama Arif <usama.arif@linux.dev>]

On Thu,  3 Sep 2026 19:29:38 +0100 Kiryl Shutsemau <kirill@shutemov.name> wrote:

> 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.

Hi Kiryl,

The motivation makes sense to me. I will look into the patches.

Just wanted to check, the above xfs example, is that on an ARM host?

Thanks,
Usama

> 
> 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
> 
>