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

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Message-ID: <33fd4731-9765-d78b-bdc3-f8243c98e81f@linux.ibm.com>
Date:   Mon, 30 May 2022 11:35:10 +0200
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>, 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>,
        Brian Cain <bcain@...cinc.com>, Borislav Petkov <bp@...en8.de>,
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        Dave Hansen <dave.hansen@...ux.intel.com>,
        Richard Weinberger <richard@....at>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared
 memory types



Am 29.05.22 um 22:33 schrieb Heiko Carstens:
[...]
> 
> 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;

Yes, that makes sense. With that

Acked-by: Christian Borntraeger <borntraeger@...ux.ibm.com>