linux-kernel - Re: [PATCH 25/31] mm/gup: remove FOLL_SPLIT_PMD use of pmd_trans

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Message-ID: <CAHbLzkobBuHGKdEXh02e54Vzd_1yyvuShKUYMxTGWtqGXrrVYQ@mail.gmail.com>
Date:   Thu, 25 May 2023 15:33:29 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 25/31] mm/gup: remove FOLL_SPLIT_PMD use of pmd_trans_unstable()

On Thu, May 25, 2023 at 2:16 PM Hugh Dickins <hughd@...gle.com> wrote:
>
> On Wed, 24 May 2023, Yang Shi wrote:
> > On Tue, May 23, 2023 at 9:26 PM Hugh Dickins <hughd@...gle.com> 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