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

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Message-ID: <20190920155300.GC15392@bombadil.infradead.org>
Date:   Fri, 20 Sep 2019 08:53:00 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jia He <justin.he@....com>
Cc:     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 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.  We could also go with returning a
vm_fault_t, but that would be more complex than needed today, I think.