linux-kernel - Re: [PATCH v7 3/3] mm: fix double page fault on arm64 if PTE

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Date:   Fri, 20 Sep 2019 20:00:30 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jia He <justin.he@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Suzuki Poulose <Suzuki.Poulose@....com>,
        Punit Agrawal <punitagrawal@...il.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Alex Van Brunt <avanbrunt@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>, hejianet@...il.com,
        Kaly Xin <Kaly.Xin@....com>, nd@....com
Subject: Re: [PATCH v7 3/3] mm: fix double page fault on arm64 if PTE_AF is
 cleared

On Fri, Sep 20, 2019 at 08:53:00AM -0700, Matthew Wilcox wrote:
> On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote:
> > -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
> > +static inline int cow_user_page(struct page *dst, struct page *src,
> > +				struct vm_fault *vmf)
> >  {
> 
> Can we talk about the return type here?
> 
> > +			} else {
> > +				/* Other thread has already handled the fault
> > +				 * and we don't need to do anything. If it's
> > +				 * not the case, the fault will be triggered
> > +				 * again on the same address.
> > +				 */
> > +				pte_unmap_unlock(vmf->pte, vmf->ptl);
> > +				return -1;
> ...
> > +	return 0;
> >  }
> 
> So -1 for "try again" and 0 for "succeeded".
> 
> > +		if (cow_user_page(new_page, old_page, vmf)) {
> 
> Then we use it like a bool.  But it's kind of backwards from a bool because
> false is success.
> 
> > +			/* COW failed, if the fault was solved by other,
> > +			 * it's fine. If not, userspace would re-fault on
> > +			 * the same address and we will handle the fault
> > +			 * from the second attempt.
> > +			 */
> > +			put_page(new_page);
> > +			if (old_page)
> > +				put_page(old_page);
> > +			return 0;
> 
> And we don't use the return value; in fact we invert it.
> 
> Would this make more sense:
> 
> static inline bool cow_user_page(struct page *dst, struct page *src,
> 					struct vm_fault *vmf)
> ...
> 				pte_unmap_unlock(vmf->pte, vmf->ptl);
> 				return false;
> ...
> 	return true;
> ...
> 		if (!cow_user_page(new_page, old_page, vmf)) {
> 
> That reads more sensibly for me.

I like this idea too.

-- 
 Kirill A. Shutemov