linux-kernel - Re: [v3 2/3] mm: Defer TLB flush by keeping both src and dst folios at migration

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Date:   Wed, 8 Nov 2023 13:12:08 +0900
From:   Byungchul Park <byungchul@...com>
To:     Nadav Amit <namit@...are.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "kernel_team@...ynix.com" <kernel_team@...ynix.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "ying.huang@...el.com" <ying.huang@...el.com>,
        "xhao@...ux.alibaba.com" <xhao@...ux.alibaba.com>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "david@...hat.com" <david@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [v3 2/3] mm: Defer TLB flush by keeping both src and dst folios
 at migration

On Mon, Oct 30, 2023 at 09:51:30PM +0900, Byungchul Park wrote:
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 6c264d2f969c..75dc48b6e15f 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -3359,6 +3359,19 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> > > 	if (vmf->page)
> > > 		folio = page_folio(vmf->page);
> > > 
> > > +	/*
> > > +	 * This folio has its read copy to prevent inconsistency while
> > > +	 * deferring TLB flushes. However, the problem might arise if
> > > +	 * it's going to become writable.
> > > +	 *
> > > +	 * To prevent it, give up the deferring TLB flushes and perform
> > > +	 * TLB flush right away.
> > > +	 */
> > > +	if (folio && migrc_pending_folio(folio)) {
> > > +		migrc_unpend_folio(folio);
> > > +		migrc_try_flush_free_folios(NULL);
> > 
> > So many potential function calls… Probably they should have been combined
> > into one and at least migrc_pending_folio() should have been an inline
> > function in the header.
> 
> I will try to change it as you mention.
> 
> > > +	}
> > > +
> > 
> > What about mprotect? I thought David has changed it so it can set writable
> > PTEs.
> 
> I will check it out.

I found mprotect stuff is already performing TLB flushes needed for it.
So some redundant TLB flushes might happen by migrc but it's not that
harmful I think. Thanks.

	Byungchul