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

[Yang Shi]

On Thu, May 25, 2023 at 2:16 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> 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?

Err... you are right. I misread the code. Anyway it returns -ENOMEM
instead of -EBUSY when pmd is none and pte alloc fails. Returning
-ENOMEM does make sense for this case. Is it worth some words in the
commit log for the slight behavior change?

>
> > 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