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

From: Peter Xu <peterx@redhat.com>
To: Nadav Amit <nadav.amit@gmail.com>,
	Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Kravetz <mike.kravetz@oracle.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>,
	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: Sun, 30 Oct 2022 14:43:51 -0400	[thread overview]
Message-ID: <Y17F50ktT9fZw4do@x1n> (raw)
In-Reply-To: <7048D2B5-5FA5-4F72-8FDC-A02411CFD71D@gmail.com>

On Sat, Oct 29, 2022 at 05:54:44PM -0700, Nadav Amit wrote:
> On Oct 29, 2022, at 5:15 PM, Mike Kravetz <mike.kravetz@oracle.com> wrote:
> 
> > zap_page_range is a bit confusing.  It appears that the passed range can
> > span multiple vmas.  Otherwise, there would be no do while loop.  Yet, there
> > is only one mmu_notifier_range_init call specifying the passed vma.
> > 
> > It appears all callers pass a range entirely within a single vma.
> > 
> > The modifications above would work for a range within a single vma.  However,
> > things would be more complicated if the range can indeed span multiple vmas.
> > For multiple vmas, we would need to check the first and last vmas for
> > pmd sharing.
> > 
> > Anyone know more about this seeming confusing behavior?  Perhaps, range
> > spanning multiple vmas was left over earlier code?
> 
> I don’t have personal knowledge, but I noticed that it does not make much
> sense, at least for MADV_DONTNEED. I tried to batch the TLB flushes across
> VMAs for madvise’s. [1]

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?

> 
> Need to get to it sometime.
> 
> [1] https://lore.kernel.org/lkml/20210926161259.238054-7-namit@vmware.com/
> 

-- 
Peter Xu