From: Peter Xu <peterx@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrew Morton <akpm@linux-foundation.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Muchun Song <songmuchun@bytedance.com>,
Peter Feiner <pfeiner@google.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings
Date: Mon, 8 Aug 2022 18:08:01 -0400 [thread overview]
Message-ID: <YvGJQaYeT9Y8PlDX@xz-m1.local> (raw)
In-Reply-To: <YvFwU4e3WOSRseh6@xz-m1.local>
On Mon, Aug 08, 2022 at 04:21:39PM -0400, Peter Xu wrote:
> On Mon, Aug 08, 2022 at 06:25:21PM +0200, David Hildenbrand wrote:
> > >> Relying on VM_SHARED to detect MAP_PRIVATE vs. MAP_SHARED is
> > >> unfortunately wrong.
> > >>
> > >> If you're curious, take a look at f83a275dbc5c ("mm: account for
> > >> MAP_SHARED mappings using VM_MAYSHARE and not VM_SHARED in hugetlbfs")
> > >> and mmap() code.
> > >>
> > >> Long story short: if the file is read-only, we only have VM_MAYSHARE but
> > >> not VM_SHARED (and consequently also not VM_MAYWRITE).
> > >
> > > To ask in another way: if file is RO but mapped RW (mmap() will have
> > > VM_SHARED cleared but VM_MAYSHARE set), then if we check VM_MAYSHARE here
> > > won't we grant write bit errornously while we shouldn't? As the user
> > > doesn't really have write permission to the file.
> >
> > Thus the VM_WRITE check. :)
> >
> > I wonder if we should just do it cleanly and introduce the maybe_mkwrite
> > semantics here as well. Then there is no need for additional VM_WRITE
> > checks and hugetlb will work just like !hugetlb.
>
> Hmm yeah I think the VM_MAYSHARE check is correct, since we'll need to fail
> the cases where MAYSHARE && !SHARE - we used to silently let it pass.
Sorry I think this is a wrong statement I made.. IIUC we'll fail correctly
with/without the patch on any write to hugetlb RO regions.
Then I just don't see a difference on checking VM_SHARED or VM_MAYSHARE
here, it's just that VM_MAYSHARE check should work too like VM_SHARED so I
don't see a problem.
It also means I can't think of any valid case of having VM_WRITE when
reaching here, then the WARN_ON_ONCE() is okay but maybe also redundant.
Using maybe_mkwrite() seems misleading to me if FOLL_FORCE not ready for
hugetlbfs after all.
--
Peter Xu
next prev parent reply other threads:[~2022-08-08 22:08 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-05 11:03 [PATCH v1 0/2] mm/hugetlb: fix write-fault handling for shared mappings David Hildenbrand
2022-08-05 11:03 ` [PATCH v1 1/2] mm/hugetlb: fix hugetlb not supporting write-notify David Hildenbrand
2022-08-05 18:14 ` Peter Xu
2022-08-05 18:22 ` David Hildenbrand
2022-08-05 18:23 ` Mike Kravetz
2022-08-05 18:25 ` David Hildenbrand
2022-08-05 18:33 ` Mike Kravetz
2022-08-05 18:57 ` David Hildenbrand
2022-08-05 20:48 ` Mike Kravetz
2022-08-05 23:13 ` Peter Xu
2022-08-05 23:33 ` Mike Kravetz
2022-08-08 16:10 ` Peter Xu
2022-08-08 16:36 ` David Hildenbrand
2022-08-08 19:28 ` Peter Xu
2022-08-10 9:29 ` David Hildenbrand
2022-08-05 11:03 ` [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings David Hildenbrand
2022-08-05 18:12 ` Peter Xu
2022-08-05 18:20 ` David Hildenbrand
2022-08-08 16:05 ` Peter Xu
2022-08-08 16:25 ` David Hildenbrand
2022-08-08 20:21 ` Peter Xu
2022-08-08 22:08 ` Peter Xu [this message]
2022-08-10 9:37 ` David Hildenbrand
2022-08-10 9:45 ` David Hildenbrand
2022-08-10 19:29 ` Peter Xu
2022-08-10 19:40 ` David Hildenbrand
2022-08-10 19:52 ` Peter Xu
2022-08-10 23:55 ` Mike Kravetz
2022-08-11 8:48 ` David Hildenbrand
2022-08-05 23:08 ` Mike Kravetz
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