Re: [PATCH 0/2] mm, iomap: account dirty at page granularity for per-block dirty folios

[Kiryl Shutsemau]

On Tue, Oct 27, 2026 at 01:22:22PM +0100, David Hildenbrand (Arm) wrote:
> On 10/27/26 12:27, Salvatore Dipietro wrote:
> > Since iomap gained per-block dirty tracking, a large folio can be
> > partially dirty: struct iomap_folio_state records exactly which
> > blocks are dirty, and writeback only writes those ranges.  The mm
> > dirty accounting, however, still charges the whole folio even if a
> > single block is dirty (up to 512x over-accounting for a 2 MB folio
> > over 4 KB blocks).
> > 
> > balance_dirty_pages() then throttles writers against a number that
> > is unrelated to how much data actually needs writing back.  On
> > workloads that repeatedly dirty small ranges of large folios --
> > e.g. PostgreSQL, which writes mostly 8/16 KB pages -- dirty_thresh
> > is crossed and writers are throttled long before the real dirty data
> > justifies it.  On the benchmark below, run on v7.3-rc7, the
> > unpatched kernel charges ~159 TB of dirtied pages while writing
> > ~6 TB to the array, a factor of 27; with this series the dirtied
> > pages charged match the data actually written.
> > 
> > This series makes iomap charge and discharge the mm dirty counters
> > per dirty page for folios that carry per-block dirty state, so a
> > partially dirty large folio is accounted for the dirty part only:
> > 
> >   - Patch 1 adds the mm infrastructure: an AS_DIRTY_PER_BLOCK
> >     mapping flag, an @accounted argument to __folio_mark_dirty()
> >     and __filemap_dirty_folio() to skip the generic per-folio
> >     charge, and an address_space_operations::accounted_dirty_pages()
> >     callback used by the discharge and migration paths instead of
> >     folio_nr_pages().  No filesystem opts in, so it is a no-op on
> >     its own.
> > 
> >   - Patch 2 makes iomap charge per dirty page from the per-block
> >     dirty bitmap and wires iomap_accounted_dirty_pages() into every
> >     iomap-based filesystem.
> > 
> > This was benchmarked with PostgreSQL using the pgbench simple-update
> > workload on v7.3-rc7.  Average throughput rises 46.8% (139.8k ->
> > 205.2k tps) and run-to-run variability drops from a 23.60% to a
> > 3.98% coefficient of variation: the worst iteration improves from
> > 97.9k to 191.5k tps, while the best gains 8%, 199.4k -> 215.9k.
> > 
> > Ten iterations per kernel, same database and workload (tps):
> > 
> >   Kernel                     Avg     Min     Max      CV   iters   iters
> >                                                              1-5    6-10
> >   ______________________  ______  ______  ______  ______  ______  ______
> >   v7.3-rc7                139.8k   97.9k  199.4k  23.60%  164.4k  115.2k
> >   v7.3-rc7 + this series  205.2k  191.5k  215.9k   3.98%  200.1k  210.2k
> > 
> > On unpatched v7.3-rc7 the worst iteration lands ~51% below the best,
> > and throughput decays as the run progresses: iterations 6-10 average
> > 30% below iterations 1-5.  With this series the decay is gone -- the
> > second half is similar to the first -- and the coefficient of
> > variation stays at 3.98%.
> > 
> > The over-accounting has existed since commit 4ce02c679722 ("iomap: Add
> > per-block dirty state tracking to improve performance"), merged in
> > v6.6, where the ifs bitmap started recording partial dirty state while
> > the mm counters kept charging whole folios.
> > 
> > The throughput collapse bisects to the v6.17 readahead series, first
> > bad commit c4602f9fa77f ("mm/readahead: store folio order in struct
> > file_ra_state"), which lets readahead stay at the largest folio order
> > once it has ramped up, so a single dirty block charges far more than
> > is written.  Patch 2 therefore carries:
> 
> 
> How does this relate to
> 
> [RFC PATCH 0/5] mm: sub-folio dirty tracking for PTE-mapped mmap writes
> 
> https://lore.kernel.org/all/20260903182943.662461-1-kirill@xxxxxxxxxxxxx/
> 
> ?
> 
> I assume they try to solve different problems, but would more fine-grained folio
> dirtying also make your solution benefit?

Yeah, it solves different problem.

And I abandoned my patchset. The right way to do this is to make
page_mkwrite() per-page, not folio.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov