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

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Sep 16, 2026 at 05:27:42PM +0100, Matthew Wilcox wrote:
> On Mon, Sep 07, 2026 at 11:15:15AM +0100, Kiryl Shutsemau wrote:
> > Boris pointed me to Matthew's proposal to remove ->dirty_folio:
> > 
> > https://lore.kernel.org/all/aoyWln-Gt-yvZQkE@casper.infradead.org
> 
> Thanks, Boris ;-)
> 
> > I agree that the current ->dirty_folio() makes little sense and that
> > dirtying the folio can be bundled into ->page_mkwrite(), as they are
> > matched 1-to-1.
> > 
> > My proposal makes the distinction between making the folio writable and
> > making it dirty meaningful. ->page_mkwrite() allocates whatever is needed
> > on the filesystem side to track dirty state and drive writeback for the
> > *folio*, while ->dirty_folio_range() marks part of the folio dirty.
> 
> Why do you think that's a meaningful distinction?  We create a writable
> PTE because we've taken a page fault for write.  There's probably a few
> naoseconds where the PTE is writable+clean before it becomes
> writable+dirty, but even then sometimes we do both pte_mkwrite() and
> pte_mkdirty() as an optimisation in the write fault path.

This is true for the PTE that the fault was for. But we don't necessarily
want to dirty the other 511 pages at the same time.

The basic idea is to make the whole folio writable at fault and shift
dirtying to be per-PTE on write to it.

> > We can still drop ->dirty_folio(). A filesystem can provide
> > ->dirty_folio_range() if it wants fine-grained (sub-folio) dirty
> > tracking.
> > 
> > A separate question is whether we want to avoid installing a writable PMD
> > entry for filesystems that want fine-grained dirty tracking. I have a
> > patch for this, but it deserves a separate discussion once we agree that we
> > want this for PTE-mapped folios first.
> > 
> > Any feedback?
> 
> It's very odd to be optimising for shared-writable-mmap.  This is a
> horrid model for I/O.  https://cs.brown.edu/people/acrotty/pubs/p13-crotty.pdf

Sure.

But not everybody got the memo[1] :P

I think it worth considering if we want to make large folio adoption
smoother.

[1] https://www.reddit.com/r/bcachefs/comments/1vepk4a/comment/p1sn4v1/

-- 
  Kiryl Shutsemau / Kirill A. Shutemov