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Date:   Thu, 25 May 2023 14:16:34 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Yang Shi <shy828301@gmail.com>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 25/31] mm/gup: remove FOLL_SPLIT_PMD use of
 pmd_trans_unstable()
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On Wed, 24 May 2023, Yang Shi wrote:
> On Tue, May 23, 2023 at 9:26=E2=80=AFPM Hugh Dickins <hughd@google.com> w=
rote:
> > 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.
>=20
> 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.

=09=09return pte_alloc(mm, pmd) ? ERR_PTR(-ENOMEM) :
=09=09=09follow_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
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