linux-kernel - Re: [PATCH v7 4/6] userfaultfd: add UFFDIO

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Date:   Tue, 23 Feb 2021 10:05:49 -0800
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Peter Xu <peterx@...hat.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>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v7 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl

On Tue, Feb 23, 2021 at 7:38 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Thu, Feb 18, 2021 at 04:48:22PM -0800, Axel Rasmussen wrote:
> > @@ -4645,8 +4646,18 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> >       spinlock_t *ptl;
> >       int ret;
> >       struct page *page;
> > +     int writable;
> >
> > -     if (!*pagep) {
> > +     mapping = dst_vma->vm_file->f_mapping;
> > +     idx = vma_hugecache_offset(h, dst_vma, dst_addr);
> > +
> > +     if (is_continue) {
> > +             ret = -EFAULT;
> > +             page = find_lock_page(mapping, idx);
> > +             *pagep = NULL;
>
> Why set *pagep to NULL?  Shouldn't it be NULL always?.. If that's the case,
> maybe WARN_ON_ONCE(*pagep) suite more.

Right, the caller should be passing in NULL in the
MCOPY_ATOMIC_CONTINUE case. Looking more closely at the caller
(__mcopy_atomic_hugetlb), it already has a BUG_ON(page), so at best
this assignment is redundant, and at worst it might actually cover up
a real bug (say the caller mistakenly *did* pass in some page, we'd
set it to NULL and the BUG_ON wouldn't trigger).

So, I'll just remove this - I don't think an additional WARN_ON_ONCE
is needed given the existing BUG_ON.

>
> Otherwise the patch looks good to me.

Shall I add a R-B? :)

Thanks for taking the time to review Peter!

>
> Thanks,
>
> --
> Peter Xu
>