Re: [PATCH v2 04/40] mm/rmap: introduce and use hugetlb_try_dup_anon_rmap()

[David Hildenbrand]

On 21.12.23 05:40, Matthew Wilcox wrote:
> On Wed, Dec 20, 2023 at 11:44:28PM +0100, David Hildenbrand wrote:
> > hugetlb rmap handling differs quite a lot from "ordinary" rmap code.
> > For example, hugetlb currently only supports entire mappings, and treats
> > any mapping as mapped using a single "logical PTE". Let's move it out
> > of the way so we can overhaul our "ordinary" rmap.
> > implementation/interface.
> >
> > So let's introduce and use hugetlb_try_dup_anon_rmap() to make all
> > hugetlb handling use dedicated hugetlb_* rmap functions.
> >
> > Add sanity checks that we end up with the right folios in the right
> > functions.
> >
> > Note that is_device_private_page() does not apply to hugetlb.
> >
> > Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> > Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

Thanks!

> > +static inline bool folio_needs_cow_for_dma(struct vm_area_struct *vma,
> > +					  struct folio *folio)
>
> I particularly like it that you introduced this.

And a later patch even removes page_needs_cow_for_dma() :)


A note that we have one remaining user of page_maybe_dma_pinned(). 
Instead of converting that code to folios, we should probably just 
remove that pte_is_pinned() handling completely: it's inconsistent (only 
checks PTEs) and cannot handle concurrent GUP-fast. It's a leftover from 
the COW issues we had before PageAnonExclusive. [I've had patch lying 
around to do that for a long time, but never sent it]

> > +static inline int hugetlb_try_dup_anon_rmap(struct folio *folio,
> > +		struct vm_area_struct *vma)
> > +{
> > +	VM_WARN_ON_FOLIO(!folio_test_hugetlb(folio), folio);
> > +	VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
> > +
> > +	if (PageAnonExclusive(&folio->page)) {
>
> I wonder if we need a folio_test_hugetlb_anon_exclusive() to make this
> a little more ergonomic?
>
> > +		if (unlikely(folio_needs_cow_for_dma(vma, folio)))
> > +			return -EBUSY;
> > +		ClearPageAnonExclusive(&folio->page);
>
> ... and set/clear variants.

I thought about that as well, and even going a step further and instead 
of having PageAnonExclusive checks outside rmap code, have something 
like the following instead:

hugetlb_test_anon_rmap_exclusive()
folio_test_anon_rmap_exclusive_[pte|pmd]()

I added that to my TODO list, because it results again in a bigger 
patchset (especially also in GUP).

--
Cheers,

David / dhildenb