Re: [PATCH v1 10/11] mm/memory: ignore dirty/accessed/soft-dirty bits in folio_pte_batch()

[David Hildenbrand]

On 23.01.24 14:42, Ryan Roberts wrote:
> On 23/01/2024 13:06, David Hildenbrand wrote:
> > On 23.01.24 13:25, Ryan Roberts wrote:
> > > On 22/01/2024 19:41, David Hildenbrand wrote:
> > > > Let's ignore these bits: they are irrelevant for fork, and will likely
> > > > be irrelevant for upcoming users such as page unmapping.
> > > >
> > > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > > > ---
> > > >    mm/memory.c | 10 ++++++++--
> > > >    1 file changed, 8 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index f563aec85b2a8..341b2be845b6e 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -953,24 +953,30 @@ static __always_inline void __copy_present_ptes(struct
> > > > vm_area_struct *dst_vma,
> > > >        set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
> > > >    }
> > > >    +static inline pte_t __pte_batch_clear_ignored(pte_t pte)
> > > > +{
> > > > +    return pte_clear_soft_dirty(pte_mkclean(pte_mkold(pte)));
> > > > +}
> > > > +
> > > >    /*
> > > >     * Detect a PTE batch: consecutive (present) PTEs that map consecutive
> > > >     * pages of the same folio.
> > > >     *
> > > >     * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN.
> > >
> > > nit: last char should be a comma (,) not a full stop (.)
> > >
> > > > + * the accessed bit, dirty bit and soft-dirty bit.
> > > >     */
> > > >    static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> > > >            pte_t *start_ptep, pte_t pte, int max_nr)
> > > >    {
> > > >        unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
> > > >        const pte_t *end_ptep = start_ptep + max_nr;
> > > > -    pte_t expected_pte = pte_next_pfn(pte);
> > > > +    pte_t expected_pte = __pte_batch_clear_ignored(pte_next_pfn(pte));
> > > >        pte_t *ptep = start_ptep + 1;
> > > >          VM_WARN_ON_FOLIO(!pte_present(pte), folio);
> > > >          while (ptep != end_ptep) {
> > > > -        pte = ptep_get(ptep);
> > > > +        pte = __pte_batch_clear_ignored(ptep_get(ptep));
> > > >              if (!pte_same(pte, expected_pte))
> > > >                break;
> > >
> > > I think you'll lose dirty information in the child for private mappings? If the
> > > first pte in a batch is clean, but a subsequent page is dirty, you will end up
> > > setting all the pages in the batch as clean in the child. Previous behavior
> > > would preserve dirty bit for private mappings.
> > >
> > > In my version (v3) that did arbitrary batching, I had some fun and games
> > > tracking dirty, write and uffd_wp:
> > > https://lore.kernel.org/linux-arm-kernel/20231204105440.61448-2-ryan.roberts@xxxxxxx/
> > >
> > > Also, I think you will currently either set soft dirty on all or none of the
> > > pages in the batch, depending on the value of the first. I previously convinced
> > > myself that the state was unimportant so always cleared it in the child to
> > > provide consistency.
> >
> > Good points regarding dirty and soft-dirty. I wanted to avoid passing flags to
> > folio_pte_batch(), but maybe that's just what we need to not change behavior.
>
> I think you could not bother with the enforce_uffd_wp - just always enforce
> uffd-wp. So that's one simplification vs mine. Then you just need an any_dirty

I think I'll just leave uffd-wp alone for now, corner case with 
fork/munmap that can be optimized later on top if really needed.

Regarding soft-dirty (which is set automatically much more often), I can 
certainly ignore the bit if !vma_soft_dirty_enabled(vma) [which is true 
in most of the cases]. So that's easy to handle. But likely, soft-dirty 
for the child is completely unexpressive and should always be cleared. 
Have to double check what the vmflag will be for the child process.

> flag following the same pattern as your any_writable. Then just set dirty on the
> whole batch in the child if any were dirty in the parent.

Regarding dirtying, I'm not 100% sure yet if we should just always dirty 
all ptes if any is dirty, or if we should preserve the state for private 
VMAs for now.

> Although now I'm wondering if there is a race here... What happens if a page in
> the parent becomes dirty after you have checked it but before you write protect
> it? Isn't that already a problem with the current non-batched version? Why do we
> even to preserve dirty in the child for private mappings?

I suspect, because the parent could zap the anon folio. If the folio is 
clean, but the PTE dirty, I suspect that we could lose data of the child 
if we were to evict that clean folio (swapout).

So I assume we simply copy the dirty PTE bit, so the system knows that 
that folio is actually dirty, because one PTE is dirty.

Touching only PTEs avoids having to mess with folio flags.

But that's just pure speculation. E.g., fs/proc/task_mmu.c does some 
slightly different accounting if a PTE is dirty. But usually, it checks 
if either the PTE or the folios is dirty.

I'll have to do some more digging.

--
Cheers,

David / dhildenb