Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb MADV_DONTNEED processing

From: Mike Kravetz <mike.kravetz@oracle.com>
To: Peter Xu <peterx@redhat.com>
Cc: Nadav Amit <nadav.amit@gmail.com>, Linux-MM <linux-mm@kvack.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Naoya Horiguchi <naoya.horiguchi@linux.dev>,
	David Hildenbrand <david@redhat.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	Mina Almasry <almasrymina@google.com>,
	Rik van Riel <riel@surriel.com>, Vlastimil Babka <vbabka@suse.cz>,
	Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Wei Chen <harperchen1110@gmail.com>,
	"# 5 . 10+" <stable@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb MADV_DONTNEED processing
Date: Mon, 7 Nov 2022 12:01:57 -0800	[thread overview]
Message-ID: <Y2lkNXC9cEYqPugY@monkey> (raw)
In-Reply-To: <Y2LD1Vxt3vbChUyD@x1n>

On 11/02/22 15:24, Peter Xu wrote:
> On Sun, Oct 30, 2022 at 06:44:10PM -0700, Mike Kravetz wrote:
> > On 10/30/22 11:52, Nadav Amit wrote:
> > > On Oct 30, 2022, at 11:43 AM, Peter Xu <peterx@redhat.com> wrote:
> > > 
> > > > The loop comes from 7e027b14d53e ("vm: simplify unmap_vmas() calling
> > > > convention", 2012-05-06), where zap_page_range() was used to replace a call
> > > > to unmap_vmas() because the patch wanted to eliminate the zap details
> > > > pointer for unmap_vmas(), which makes sense.
> > > > 
> > > > I didn't check the old code, but from what I can tell (and also as Mike
> > > > pointed out) I don't think zap_page_range() in the lastest code base is
> > > > ever used on multi-vma at all.  Otherwise the mmu notifier is already
> > > > broken - see mmu_notifier_range_init() where the vma pointer is also part
> > > > of the notification.
> > > > 
> > > > Perhaps we should just remove the loop?
> > > 
> > > There is already zap_page_range_single() that does exactly that. Just need
> > > to export it.
> > 
> > I was thinking that zap_page_range() should perform a notification call for
> > each vma within the loop.  Something like this?
> 
> I'm boldly guessing what Nadav suggested was using zap_page_range_single()
> and export it for MADV_DONTNEED.  Hopefully that's also the easiest for
> stable?

I started making this change, then noticed that zap_vma_ptes() just calls
zap_page_range_single().  And, it is already exported.  That may be a
better fit since exporting zap_page_range_single would require a wrapper
as I do not think we want to export struct zap_details as well.

In any case, we still need to add the adjust_range_if_pmd_sharing_possible()
call to zap_page_range_single.

> 
> For the long term, I really think we should just get rid of the loop..
> 

Yes.  It will look a little strange if adjust_range_if_pmd_sharing_possible is
added to zap_page_range_single but not zap_page_range.  And, to properly add
it to zap_page_range means rewriting the routine as I did here:
https://lore.kernel.org/linux-mm/20221102013100.455139-1-mike.kravetz@oracle.com/

-- 
Mike Kravetz