linux-kernel - Re: [PATCH RFC 13/24] mm: merge parameters for change

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Date:   Thu, 24 Jan 2019 13:22:36 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH RFC 13/24] mm: merge parameters for change_protection()

On Mon, Jan 21, 2019 at 08:54:46AM -0500, Jerome Glisse wrote:
> On Mon, Jan 21, 2019 at 03:57:11PM +0800, Peter Xu wrote:
> > change_protection() was used by either the NUMA or mprotect() code,
> > there's one parameter for each of the callers (dirty_accountable and
> > prot_numa).  Further, these parameters are passed along the calls:
> > 
> >   - change_protection_range()
> >   - change_p4d_range()
> >   - change_pud_range()
> >   - change_pmd_range()
> >   - ...
> > 
> > Now we introduce a flag for change_protect() and all these helpers to
> > replace these parameters.  Then we can avoid passing multiple parameters
> > multiple times along the way.
> > 
> > More importantly, it'll greatly simplify the work if we want to
> > introduce any new parameters to change_protection().  In the follow up
> > patches, a new parameter for userfaultfd write protection will be
> > introduced.
> > 
> > No functional change at all.
> 
> There is one change i could spot and also something that looks wrong.
> 
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> 
> [...]
> 
> > @@ -428,8 +431,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
> >  	dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot);
> >  	vma_set_page_prot(vma);
> >  
> > -	change_protection(vma, start, end, vma->vm_page_prot,
> > -			  dirty_accountable, 0);
> > +	change_protection(vma, start, end, vma->vm_page_prot, MM_CP_DIRTY_ACCT);
> 
> Here you unconditionaly see the DIRTY_ACCT flag instead it should be
> something like:
> 
>     s/dirty_accountable/cp_flags
>     if (vma_wants_writenotify(vma, vma->vm_page_prot))
>         cp_flags = MM_CP_DIRTY_ACCT;
>     else
>         cp_flags = 0;
> 
>     change_protection(vma, start, end, vma->vm_page_prot, cp_flags);
> 
> Or any equivalent construct.

Oops, thanks for spotting this... it was definitely wrong.  I'll fix.

> 
> >  	/*
> >  	 * Private VM_LOCKED VMA becoming writable: trigger COW to avoid major
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 005291b9b62f..23d4bbd117ee 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -674,7 +674,7 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> >  		newprot = vm_get_page_prot(dst_vma->vm_flags);
> >  
> >  	change_protection(dst_vma, start, start + len, newprot,
> > -				!enable_wp, 0);
> > +			  enable_wp ? 0 : MM_CP_DIRTY_ACCT);
> 
> We had a discussion in the past on that, i have not look at other
> patches but this seems wrong to me. MM_CP_DIRTY_ACCT is an
> optimization to keep a pte with write permission if it is dirty
> while my understanding is that you want to set write flag for pte
> unconditionaly.
> 
> So maybe this patch that adds flag should be earlier in the serie
> so that you can add a flag to do that before introducing the UFD
> mwriteprotect_range() function.

I agree.  I'm going to move the UFFDIO_WRITEPROTECT patch to the last
so I'll rearrange this part too so these lines will be removed in my
next version.

Thanks!

-- 
Peter Xu