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

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Message-ID: <YpTqNKMTt8PoA41n@xz-m1.local>
Date:   Mon, 30 May 2022 12:00:52 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc:     Heiko Carstens <hca@...ux.ibm.com>, 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>,
        linux-alpha@...r.kernel.org, Alistair Popple <apopple@...dia.com>,
        Jonas Bonn <jonas@...thpole.se>,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
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        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared
 memory types

On Mon, May 30, 2022 at 11:52:54AM -0400, Peter Xu wrote:
> On Mon, May 30, 2022 at 11:35:10AM +0200, Christian Borntraeger wrote:
> > 
> > 
> > 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;

Hmm, right after I replied I found "goto out" could be problematic, since
all s390 callers of do_exception() will assume it an error condition (side
note: "goto out_gmap" contains one step to clear "fault" to 0).  I'll
replace this with "return 0" instead if it looks good to both of you.

I'll wait for a confirmation before reposting.  Thanks,

-- 
Peter Xu