linux-kernel - Re: [PATCH v6 06/11] powerpc/mm/book3s64/hash: Use functions to track lockless pgtbl walks

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Message-ID: <31b7229b979da2b0bdec041724dd1698cf76298c.camel@linux.ibm.com>
Date:   Fri, 07 Feb 2020 00:49:56 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Steven Price <steven.price@....com>,
        Robin Murphy <robin.murphy@....com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Reza Arbab <arbab@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Allison Randal <allison@...utok.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michal Suchanek <msuchanek@...e.de>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v6 06/11] powerpc/mm/book3s64/hash: Use functions to
 track lockless pgtbl walks

On Thu, 2020-02-06 at 07:06 +0100, Christophe Leroy wrote:
> > -	/* Get PTE and page size from page tables */
> > +	/* Get PTE and page size from page tables :
> > +	 * Called in from DataAccess interrupt (data_access_common: 0x300),
> > +	 * interrupts are disabled here.
> > +	 */
> 
> Comments formatting is not in line with Linux kernel rules. Should be no 
> text on the first /* line.

My mistake. Will be corrected in v7.

> > +	__begin_lockless_pgtbl_walk(false);
> 
> I think it would be better to not use __begin_lockless_pgtbl_walk() 
> directly but keep it in a single place, and define something like 
> begin_lockless_pgtbl_walk_noirq() similar to begin_lockless_pgtbl_walk()

There are places where touching irq is decided by a boolean, like in
patch 8: http://patchwork.ozlabs.org/patch/1234130/

If I were to change this, I would have to place ifs and decide to call
either normal or *_noirq() versions in every function.

What you suggest?

> >   	ptep = find_linux_pte(pgdir, ea, &is_thp, &hugeshift);
> >   	if (ptep == NULL || !pte_present(*ptep)) {
> >   		DBG_LOW(" no PTE !\n");
> >   		rc = 1;
> > -		goto bail;
> > +		goto bail_pgtbl_walk;
> 
> What's the point in changing the name of this label ? There is only one 
> label, why polute the function with so huge name ?
> 
> For me, everyone understand what 'bail' means. Unneccessary changes 
> should be avoided. If you really really want to do it, it should be 
> another patch.
> 
> See kernel codying style, chapter 'naming':
> "LOCAL variable names should be short". This also applies to labels.
> 
> "C is a Spartan language, and so should your naming be. Unlike Modula-2 
> and Pascal programmers, C programmers do not use cute names like 
> ThisVariableIsATemporaryCounter. A C programmer would call that variable 
> tmp, which is much easier to write, and not the least more difficult to 
> understand."
> 

It's not label name changing. There are two possible bails in
hash_page_mm(): one for before __begin_lockless_pagetable_walk() and
other for after it. The new one also runs __end_lockless_pgtbl_walk()
before running what the previous did:

> > +bail_pgtbl_walk:
> > +	__end_lockless_pgtbl_walk(0, false);
> >   bail:
> >   	exception_exit(prev_state);
> >   	return rc;

As for the label name lengh, I see no problem changing it to something
like bail_ptw. 


> > @@ -1545,7 +1551,7 @@ static void hash_preload(struct mm_struct *mm, unsigned long ea,
> >   	unsigned long vsid;
> >   	pgd_t *pgdir;
> >   	pte_t *ptep;
> > -	unsigned long flags;
> > +	unsigned long irq_mask;
> >   	int rc, ssize, update_flags = 0;
> >   	unsigned long access = _PAGE_PRESENT | _PAGE_READ | (is_exec ? _PAGE_EXEC : 0);
> >   
> > @@ -1567,11 +1573,12 @@ static void hash_preload(struct mm_struct *mm, unsigned long ea,
> >   	vsid = get_user_vsid(&mm->context, ea, ssize);
> >   	if (!vsid)
> >   		return;
> > +
> 
> Is this new line related to the patch ?

Nope. I have added while reading code and it just went trough my pre-
sending revision. I can remove it, if it bothers.

> 
> >   	/*
> >   	 * Hash doesn't like irqs. Walking linux page table with irq disabled
> >   	 * saves us from holding multiple locks.
> >   	 */
> > -	local_irq_save(flags);
> > +	irq_mask = begin_lockless_pgtbl_walk();
> >   
> >   	/*
> >   	 * THP pages use update_mmu_cache_pmd. We don't do
> > @@ -1616,7 +1623,7 @@ static void hash_preload(struct mm_struct *mm, unsigned long ea,
> >   				   mm_ctx_user_psize(&mm->context),
> >   				   pte_val(*ptep));
> >   out_exit:
> > -	local_irq_restore(flags);
> > +	end_lockless_pgtbl_walk(irq_mask);
> >   }
> >   
> >   /*
> > @@ -1679,16 +1686,16 @@ u16 get_mm_addr_key(struct mm_struct *mm, unsigned long address)
> >   {
> >   	pte_t *ptep;
> >   	u16 pkey = 0;
> > -	unsigned long flags;
> > +	unsigned long irq_mask;
> >   
> >   	if (!mm || !mm->pgd)
> >   		return 0;
> >   
> > -	local_irq_save(flags);
> > +	irq_mask = begin_lockless_pgtbl_walk();
> >   	ptep = find_linux_pte(mm->pgd, address, NULL, NULL);
> >   	if (ptep)
> >   		pkey = pte_to_pkey_bits(pte_val(READ_ONCE(*ptep)));
> > -	local_irq_restore(flags);
> > +	end_lockless_pgtbl_walk(irq_mask);
> >   
> >   	return pkey;
> >   }
> > 
> 
> Christophe

Thanks for giving feedback,

Leonardo Bras

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