linux-kernel - Re: [PATCH v1 2/4] mm/hugetlb: fix two comments related to huge_pmd

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Message-ID: <8e732953-80f5-4e81-bd02-2949bda804d5@lucifer.local>
Date: Mon, 15 Dec 2025 09:54:31 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, Will Deacon <will@...nel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>, Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>, Muchun Song <muchun.song@...ux.dev>,
        Oscar Salvador <osalvador@...e.de>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Rik van Riel <riel@...riel.com>,
        Harry Yoo <harry.yoo@...cle.com>,
        Laurence Oberman <loberman@...hat.com>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>, Liu Shixin <liushixin2@...wei.com>
Subject: Re: [PATCH v1 2/4] mm/hugetlb: fix two comments related to
 huge_pmd_unshare()

On Thu, Dec 11, 2025 at 02:58:51AM +0100, David Hildenbrand (Red Hat) wrote:
> On 12/10/25 12:22, Lorenzo Stoakes wrote:
> > On Fri, Dec 05, 2025 at 10:35:56PM +0100, David Hildenbrand (Red Hat) wrote:
> > > Ever since we stopped using the page count to detect shared PMD
> > > page tables, these comments are outdated.
> > >
> > > The only reason we have to flush the TLB early is because once we drop
> > > the i_mmap_rwsem, the previously shared page table could get freed (to
> > > then get reallocated and used for other purpose). So we really have to
> > > flush the TLB before that could happen.
> > >
> > > So let's simplify the comments a bit.
> > >
> > > The "If we unshared PMDs, the TLB flush was not recorded in mmu_gather."
> > > part introduced as in commit a4a118f2eead ("hugetlbfs: flush TLBs
> > > correctly after huge_pmd_unshare") was confusing: sure it is recorded
> > > in the mmu_gather, otherwise tlb_flush_mmu_tlbonly() wouldn't do
> > > anything. So let's drop that comment while at it as well.
> > >
> > > We'll centralize these comments in a single helper as we rework the code
> > > next.
> > >
> > > Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count")
> > > Cc: Liu Shixin <liushixin2@...wei.com>
> > > Signed-off-by: David Hildenbrand (Red Hat) <david@...nel.org>
> >
> > LGTM, so:
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Thanks!
>
> >
> > > ---
> > >   mm/hugetlb.c | 24 ++++++++----------------
> > >   1 file changed, 8 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index 51273baec9e5d..3c77cdef12a32 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -5304,17 +5304,10 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > >   	tlb_end_vma(tlb, vma);
> > >
> > >   	/*
> > > -	 * If we unshared PMDs, the TLB flush was not recorded in mmu_gather. We
> > > -	 * could defer the flush until now, since by holding i_mmap_rwsem we
> > > -	 * guaranteed that the last reference would not be dropped. But we must
> > > -	 * do the flushing before we return, as otherwise i_mmap_rwsem will be
> > > -	 * dropped and the last reference to the shared PMDs page might be
> > > -	 * dropped as well.
> > > -	 *
> > > -	 * In theory we could defer the freeing of the PMD pages as well, but
> > > -	 * huge_pmd_unshare() relies on the exact page_count for the PMD page to
> > > -	 * detect sharing, so we cannot defer the release of the page either.
> >
> > Was it this comment that led you to question the page_count issue? :)
>
> Heh, no, I know about the changed handling already. I stumbled over the
> page_count() remaining usage while working on some cleanups I previously had
> as part of this series :)

Ah :)

>
> --
> Cheers
>
> David