linux-kernel - Re: [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to

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Date:   Wed, 31 Mar 2021 09:09:30 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     <linux-mm@...ck.org>, <nouveau@...ts.freedesktop.org>,
        <bskeggs@...hat.com>, <akpm@...ux-foundation.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kvm-ppc@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <jhubbard@...dia.com>, <rcampbell@...dia.com>,
        <jglisse@...hat.com>, <hch@...radead.org>, <daniel@...ll.ch>,
        <willy@...radead.org>, "Christoph Hellwig" <hch@....de>
Subject: Re: [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap

On Wednesday, 31 March 2021 5:49:03 AM AEDT Jason Gunthorpe wrote:
> On Fri, Mar 26, 2021 at 11:08:00AM +1100, Alistair Popple wrote:
> 
> > +static bool try_to_munlock_one(struct page *page, struct vm_area_struct 
*vma,
> > +		     unsigned long address, void *arg)
> > +{
> 
> Is this function name right?

Perhaps. This is called from try_to_munlock() hence the name, but see below 
for some commentary on that naming.

> > +	struct page_vma_mapped_walk pvmw = {
> > +		.page = page,
> > +		.vma = vma,
> > +		.address = address,
> > +	};
> > +
> > +	/* munlock has nothing to gain from examining un-locked vmas */
> > +	if (!(vma->vm_flags & VM_LOCKED))
> > +		return true;
> > +
> > +	while (page_vma_mapped_walk(&pvmw)) {
> > +		/* PTE-mapped THP are never mlocked */
> > +		if (!PageTransCompound(page)) {
> > +			/*
> > +			 * Holding pte lock, we do *not* need
> > +			 * mmap_lock here
> > +			 */
> > +			mlock_vma_page(page);
> 
> Because the only action this function seems to take is to call
> *mlock*_vma_page()
> 
> > +		}
> > +		page_vma_mapped_walk_done(&pvmw);
> > +
> > +		/* found a mlocked page, no point continuing munlock check */
> > +		return false;
> > +	}
> > +
> > +	return true;
> > +}
> > +
> >  /**
> >   * try_to_munlock - try to munlock a page
> >   * @page: the page to be munlocked
> > @@ -1796,8 +1821,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags 
flags)
> >  void try_to_munlock(struct page *page)
> >  {
> 
> But this is also called try_to_munlock ??

As far as I can tell this has always been called try_to_munlock() even though 
it appears to do the opposite.

> /**
>  * try_to_munlock - try to munlock a page
>  * @page: the page to be munlocked
>  *
>  * Called from munlock code.  Checks all of the VMAs mapping the page
>  * to make sure nobody else has this page mlocked. The page will be
>  * returned with PG_mlocked cleared if no other vmas have it mlocked.
>  */

In other words it sets PG_mlocked if one or more vmas has it mlocked. So 
try_to_mlock() might be a better name, except that seems to have the potential 
for confusion as well because it's only called from the munlock code path and 
never for mlock.

> So what clears PG_mlocked on this call path?

See munlock_vma_page(). munlock works by clearing PG_mlocked, then calling 
try_to_munlock to check if any VMAs still need it locked in which case 
PG_mlocked gets set again. There are no other callers of try_to_munlock().

> Something needs attention here..

I think the code is correct, but perhaps the naming could be better. Would be 
interested hearing any thoughts on renaming try_to_munlock() to try_to_mlock() 
as the current name appears based on the context it is called from (munlock) 
rather than what it does (mlock).

 - Alistair

> Jason
>