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

[David Hildenbrand (Arm)]

On 9/3/26 20:29, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
> 
> 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.

Just a note that with things like cont-pte we see the trend that we only have a
single logical dirty bit for the entire coalesced PTE. Similar to having only a
single dirty bit for a PMD-mapped THP.

-- 
Cheers,

David