linux-kernel - Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb MADV

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Date:   Mon, 7 Nov 2022 12:01:57 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Nadav Amit <nadav.amit@...il.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Hildenbrand <david@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Chen <harperchen1110@...il.com>,
        "# 5 . 10+" <stable@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb
 MADV_DONTNEED processing

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@...hat.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