Re: [PATCH v7 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Mark Rutland" <mark.rutland@arm.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	linux-mm@kvack.org, "Punit Agrawal" <punitagrawal@gmail.com>,
	"Will Deacon" <will@kernel.org>,
	"Alex Van Brunt" <avanbrunt@nvidia.com>,
	"Jia He" <justin.he@arm.com>, "Marc Zyngier" <maz@kernel.org>,
	"Anshuman Khandual" <anshuman.khandual@arm.com>,
	"Kaly Xin" <Kaly.Xin@arm.com>,
	hejianet@gmail.com, "Ralph Campbell" <rcampbell@nvidia.com>,
	"Suzuki Poulose" <Suzuki.Poulose@arm.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	nd@arm.com, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, "James Morse" <james.morse@arm.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Robin Murphy" <robin.murphy@arm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v7 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared
Date: Fri, 20 Sep 2019 20:00:30 +0300	[thread overview]
Message-ID: <20190920170030.bpse5rlpjodbiv62@box> (raw)
In-Reply-To: <20190920155300.GC15392@bombadil.infradead.org>

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

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