linux-kernel - Re: [PATCH] mm/gup: fix gup_pud

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Message-ID: <20221207054018.GB23765@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date:   Tue, 6 Dec 2022 21:40:18 -0800
From:   Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     ssengar@...rosoft.com, akpm@...ux-foundation.org, jack@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jostarks@...rosoft.com, Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH] mm/gup: fix gup_pud_range() for dax

On Thu, Oct 20, 2022 at 04:18:37PM -0600, Yu Zhao wrote:
> On Thu, Oct 20, 2022 at 12:14 PM Saurabh Sengar
> <ssengar@...ux.microsoft.com> wrote:
> >
> > From: John Starks <jostarks@...rosoft.com>
> >
> > For dax pud, pud_huge() returns true on x86. So the function works as long
> > as hugetlb is configured. However, dax doesn't depend on hugetlb.
> > Commit 414fd080d125 ("mm/gup: fix gup_pmd_range() for dax") fixed
> > devmap-backed huge PMDs, but missed devmap-backed huge PUDs. Fix this as
> > well.
> >
> > Fixes: 414fd080d125 ("mm/gup: fix gup_pmd_range() for dax")
> > Signed-off-by: John Starks <jostarks@...rosoft.com>
> > Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> > ---
> >  mm/gup.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 05068d3d2557..9e07aa54a4cd 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2687,7 +2687,7 @@ static int gup_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr, unsigned lo
> >                 next = pud_addr_end(addr, end);
> >                 if (unlikely(!pud_present(pud)))
> >                         return 0;
> > -               if (unlikely(pud_huge(pud))) {
> > +               if (unlikely(pud_huge(pud) || pud_devmap(pud))) {
> 
> Perhaps s/pud_huge/pud_leaf/ ?
Looks good to me but I am not sure the wider impact of this, I will let maintainers
comment on it. Meanwhile I will send a V2 with more description of panic observed.
> 
> >                         if (!gup_huge_pud(pud, pudp, addr, next, flags,
> >                                           pages, nr))
> >                                 return 0;
> > --
> > 2.25.1
> >