linux-kernel - Re: [RFC PATCH 4/6] mm: Add COW PTE fallback function

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Date:   Sat, 21 May 2022 12:15:08 +0800
From:   Chih-En Lin <shiyn.lin@...il.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        William Kucharski <william.kucharski@...cle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Arnd Bergmann <arnd@...db.de>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Colin Cross <ccross@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Mike Rapoport <rppt@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Daniel Axtens <dja@...ens.net>,
        Jonathan Marek <jonathan@...ek.ca>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        David Hildenbrand <david@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kaiyang Zhao <zhao776@...due.edu>,
        Huichun Feng <foxhoundsk.tw@...il.com>,
        Jim Huang <jserv.tw@...il.com>
Subject: Re: [RFC PATCH 4/6] mm: Add COW PTE fallback function

On Fri, May 20, 2022 at 02:21:54PM +0000, Christophe Leroy wrote:
> > +/* COW PTE fallback to normal PTE:
> > + * - two state here
> > + *   - After break child :   [parent, rss=1, ref=1, write=NO , owner=parent]
> > + *                        to [parent, rss=1, ref=1, write=YES, owner=NULL  ]
> > + *   - After break parent:   [child , rss=0, ref=1, write=NO , owner=NULL  ]
> > + *                        to [child , rss=1, ref=1, write=YES, owner=NULL  ]
> > + */
> > +void cow_pte_fallback(struct vm_area_struct *vma, pmd_t *pmd,
> > +               unsigned long addr)
> 
> There should be a prototype in a header somewhere for a non static function.
> 
> You are encouraged to run 'make mm/memory.o C=2' to check sparse reports.
> 

I will do all the above checking before sending the next version.

> > +{
> > +       struct mm_struct *mm = vma->vm_mm;
> > +       unsigned long start, end;
> > +       pmd_t new;
> > +
> > +       BUG_ON(pmd_write(*pmd));
> 
> You seem to add a lot of BUG_ONs(). Are they really necessary ? See 
> https://docs.kernel.org/process/deprecated.html?highlight=bug_on#bug-and-bug-on
> 
> You may also use VM_BUG_ON().
> 

Sure.
I added BUG_ON() when doing the debug.
I will consider again which one is necessary.
And change to use VM_BUG_ON().

Thanks.