linux-kernel - Re: [PATCH v3 7/9] userfaultfd: add UFFDIO

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Message-ID: <20210201224034.GK260413@xz-x1>
Date:   Mon, 1 Feb 2021 17:40:34 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Koutný <mkoutny@...e.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>, Shaohua Li <shli@...com>,
        Shawn Anastasio <shawn@...stas.io>,
        Steven Rostedt <rostedt@...dmis.org>,
        Steven Price <steven.price@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Adam Ruprecht <ruprecht@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v3 7/9] userfaultfd: add UFFDIO_CONTINUE ioctl

On Mon, Feb 01, 2021 at 02:11:55PM -0800, Axel Rasmussen wrote:
> On Mon, Feb 1, 2021 at 11:21 AM Peter Xu <peterx@...hat.com> wrote:
> >
> > On Thu, Jan 28, 2021 at 02:48:17PM -0800, Axel Rasmussen wrote:
> > > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > > index f94a35296618..79e1f0155afa 100644
> > > --- a/include/linux/hugetlb.h
> > > +++ b/include/linux/hugetlb.h
> > > @@ -135,11 +135,14 @@ void hugetlb_show_meminfo(void);
> > >  unsigned long hugetlb_total_pages(void);
> > >  vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > >                       unsigned long address, unsigned int flags);
> > > +#ifdef CONFIG_USERFAULTFD
> >
> > I'm confused why this is needed.. hugetlb_mcopy_atomic_pte() should only be
> > called in userfaultfd.c, but if without uffd config set it won't compile
> > either:
> >
> >         obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
> 
> With this series as-is, but *without* the #ifdef CONFIG_USERFAULTFD
> here, we introduce a bunch of build warnings like this:
> 
> 
> 
> In file included from ./include/linux/migrate.h:8:0,
>                  from kernel/sched/sched.h:53,
>                  from kernel/sched/isolation.c:10:
> ./include/linux/hugetlb.h:143:12: warning: 'enum mcopy_atomic_mode'
> declared inside parameter list
>      struct page **pagep);
>             ^
> ./include/linux/hugetlb.h:143:12: warning: its scope is only this
> definition or declaration, which is probably not what you want
> 
> And similarly we get an error about the "mode" parameter having an
> incomplete type in hugetlb.c.
> 
> 
> 
> This is because enum mcopy_atomic_mode is defined in userfaultfd_k.h,
> and that entire header is wrapped in a #ifdef CONFIG_USERFAULTFD. So
> we either need to define enum mcopy_atomic_mode unconditionally, or we
> need to #ifdef CONFIG_USERFAULTFD the references to it also.
> 
> - I opted not to move it outside the #ifdef CONFIG_USERFAULTFD in
> userfaultfd_k.h (defining it unconditionally), because that seemed
> messy to me.
> - I opted not to define it unconditionally in hugetlb.h, because we'd
> have to move it to userfaultfd_k.h anyway when shmem or other users
> are introduced. I'm planning to send a series to add this a few days
> or so after this series is merged, so it seems churn-y to move it
> then.
> - It seemed optimal to not compile hugetlb_mcopy_atomic_pte anyway
> (even ignoring adding the continue ioctl), since as you point out
> userfaultfd is the only caller.
> 
> Hopefully this clarifies this and the next two comments. Let me know
> if you still feel strongly, I don't hate any of the alternatives, just
> wanted to clarify that I had considered them and thought this approach
> was best.

Then I'd suggest you use a standalone patch to put hugetlb_mcopy_atomic_pte()
into CONFIG_USERFAULTFD blocks, then propose your change with the minor mode.
Note that there're two hugetlb_mcopy_atomic_pte() defined in hugetlb.h.
Although I don't think it a problem since the other one is inlined - I think
you should still put that one into the same ifdef:

#ifdef CONFIG_USERFAULTFD
static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
						pte_t *dst_pte,
						struct vm_area_struct *dst_vma,
						unsigned long dst_addr,
						unsigned long src_addr,
						struct page **pagep)
{
	BUG();
	return 0;
}
#endif /* CONFIG_USERFAULTFD */

Let's also see whether Mike would have a preference on this.

Thanks,

-- 
Peter Xu