linux-kernel - Re: [PATCH 04/16] huge tmpfs: revert shmem's use of transhuge_vma

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Date:   Mon, 2 Aug 2021 13:39:00 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Gladkov <legion@...nel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Matthew Auld <matthew.auld@...el.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 04/16] huge tmpfs: revert shmem's use of transhuge_vma_enabled()

On Sat, Jul 31, 2021 at 9:01 PM Hugh Dickins <hughd@...gle.com> wrote:
>
> On Fri, 30 Jul 2021, Yang Shi wrote:
> > On Fri, Jul 30, 2021 at 12:36 AM Hugh Dickins <hughd@...gle.com> wrote:
> > >
> > > 5.14 commit e6be37b2e7bd ("mm/huge_memory.c: add missing read-only THP
> > > checking in transparent_hugepage_enabled()") added transhuge_vma_enabled()
> > > as a wrapper for two very different checks: shmem_huge_enabled() prefers
> > > to show those two checks explicitly, as before.
> >
> > Basically I have no objection to separating them again. But IMHO they
> > seem not very different. Or just makes things easier for the following
> > patches?
>
> Well, it made it easier to apply the patch I'd prepared earlier,
> but that was not the point; and I thought it best to be upfront
> about the reversion, rather than hiding it in the movement.
>
> The end result of the two checks is the same (don't try for huge pages),
> and they have been grouped together because they occurred together in
> several places, and both rely on "vma".
>
> But one check is whether the app has marked that address range not to use
> THPs; and the other check is whether the process is running in a hierarchy
> that has been marked never to use THPs (which just uses vma to get to mm
> to get to mm->flags (whether current->mm would be more relevant is not an
> argument I want to get into, I'm not at all sure)).
>
> To me those are very different; and I'm particularly concerned to make
> MMF_DISABLE_THP references visible, since it did not exist when Kirill
> and I first implemented shmem huge pages, and I've tended to forget it:
> but consider it more in this series.

Yes, I agree one checks vma the other one checks mm, they are
different from this perspective. Anyway, as I said I have no objection
to this change. You could add Reviewed-by: Yang Shi
<shy828301@...il.com>

>
> Hugh
>
> >
> > >
> > > Signed-off-by: Hugh Dickins <hughd@...gle.com>
> > > ---
> > >  mm/shmem.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index ce3ccaac54d6..c6fa6f4f2db8 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -4003,7 +4003,8 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
> > >         loff_t i_size;
> > >         pgoff_t off;
> > >
> > > -       if (!transhuge_vma_enabled(vma, vma->vm_flags))
> > > +       if ((vma->vm_flags & VM_NOHUGEPAGE) ||
> > > +           test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> > >                 return false;
> > >         if (shmem_huge == SHMEM_HUGE_FORCE)
> > >                 return true;
> > > --
> > > 2.26.2