linux-kernel - Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared memory types

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Message-ID: <YpPYkzbrQmy4FjrI@osiris>
Date:   Sun, 29 May 2022 22:33:23 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Will Deacon <will@...nel.org>,
        Matt Turner <mattst88@...il.com>, linux-s390@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared
 memory types

On Fri, May 27, 2022 at 03:39:36PM -0400, Peter Xu wrote:
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index e173b6187ad5..4608cc962ecf 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -433,6 +433,17 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  			goto out_up;
>  		goto out;
>  	}
> +
> +	/* The fault is fully completed (including releasing mmap lock) */
> +	if (fault & VM_FAULT_COMPLETED) {
> +		/*
> +		 * Gmap will need the mmap lock again, so retake it.  TODO:
> +		 * only conditionally take the lock when CONFIG_PGSTE set.
> +		 */
> +		mmap_read_lock(mm);
> +		goto out_gmap;
> +	}
> +
>  	if (unlikely(fault & VM_FAULT_ERROR))
>  		goto out_up;
>  

Guess the patch below on top of your patch is what we want.
Just for clarification: if gmap is not NULL then the process is a kvm
process. So, depending on the workload, this optimization makes sense.

diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 4608cc962ecf..e1d40ca341b7 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -436,12 +436,11 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 
 	/* The fault is fully completed (including releasing mmap lock) */
 	if (fault & VM_FAULT_COMPLETED) {
-		/*
-		 * Gmap will need the mmap lock again, so retake it.  TODO:
-		 * only conditionally take the lock when CONFIG_PGSTE set.
-		 */
-		mmap_read_lock(mm);
-		goto out_gmap;
+		if (gmap) {
+			mmap_read_lock(mm);
+			goto out_gmap;
+		}
+		goto out;
 	}
 
 	if (unlikely(fault & VM_FAULT_ERROR))