Re: [PATCH 25/31] mm/gup: remove FOLL_SPLIT_PMD use of pmd_trans_unstable()

[Hugh Dickins]

On Wed, 24 May 2023, Yang Shi wrote:
> On Tue, May 23, 2023 at 9:26 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > On Mon, 22 May 2023, Yang Shi wrote:
> >
> > > For other unstable cases, it will return -ENOMEM instead of -EBUSY.
> >
> > I don't think so: the possibly-failing __pte_alloc() only gets called
> > in the pmd_none() case.
> 
> I mean what if pmd is not none for huge zero page. If it is not
> pmd_none pte_alloc() just returns 0,

Yes, I agree with you on that.

> then returns -ENOMEM instead of -EBUSY.

But disagree with you on that.

		return pte_alloc(mm, pmd) ? ERR_PTR(-ENOMEM) :
			follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);

Doesn't that say that if pte_alloc() returns 0, then follow_page_mask()
will call follow_page_pte() and return whatever that returns?

> Or it is impossible that pmd end up being pmd_huge_trans or
> !pmd_present? It should be very unlikely, for example, migration does
> skip huge zero page, but I'm not sure whether there is any corner case
> that I missed.

I'm assuming both are possible there (but not asserting that they are).

Hugh