From: Michal Hocko <mhocko@kernel.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Yang Shi <yang.shi@linux.alibaba.com>,
willy@infradead.org, ldufour@linux.vnet.ibm.com,
akpm@linux-foundation.org, peterz@infradead.org,
mingo@redhat.com, acme@kernel.org,
alexander.shishkin@linux.intel.com, jolsa@redhat.com,
namhyung@kernel.org, tglx@linutronix.de, hpa@zytor.com,
linux-mm@kvack.org, x86@kernel.org
Subject: Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping
Date: Tue, 3 Jul 2018 13:34:53 +0200 [thread overview]
Message-ID: <20180703113453.GJ16767@dhcp22.suse.cz> (raw)
In-Reply-To: <20180703091911.hhxhnqpeqb2kn42x@kshutemo-mobl1>
On Tue 03-07-18 12:19:11, Kirill A. Shutemov wrote:
> On Tue, Jul 03, 2018 at 10:27:18AM +0200, Michal Hocko wrote:
> > On Tue 03-07-18 11:12:05, Kirill A. Shutemov wrote:
> > > On Mon, Jul 02, 2018 at 02:49:28PM +0200, Michal Hocko wrote:
> > > > On Mon 02-07-18 15:33:50, Kirill A. Shutemov wrote:
> > > > [...]
> > > > > I probably miss the explanation somewhere, but what's wrong with allowing
> > > > > other thread to re-populate the VMA?
> > > >
> > > > We have discussed that earlier and it boils down to how is racy access
> > > > to munmap supposed to behave. Right now we have either the original
> > > > content or SEGV. If we allow to simply madvise_dontneed before real
> > > > unmap we could get a new page as well. There might be (quite broken I
> > > > would say) user space code that would simply corrupt data silently that
> > > > way.
> > >
> > > Okay, so we add a lot of complexity to accommodate broken userspace that
> > > may or may not exist. Is it right? :)
> >
> > I would really love to do the most simple and obious thing
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 336bee8c4e25..86ffb179c3b5 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2811,6 +2811,8 @@ EXPORT_SYMBOL(vm_munmap);
> > SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len)
> > {
> > profile_munmap(addr);
> > + if (len > LARGE_NUMBER)
> > + do_madvise(addr, len, MADV_DONTNEED);
> > return vm_munmap(addr, len);
> > }
> >
> > but the argument that current semantic of good data or SEGV on
> > racing threads is no longer preserved sounds valid to me. Remember
> > optimizations shouldn't eat your data. How do we ensure that we won't
> > corrupt data silently?
>
> +linux-api
>
> Frankly, I don't see change in semantics here.
>
> Code that has race between munmap() and page fault would get intermittent
> SIGSEGV before and after the approach with simple MADV_DONTNEED.
prior to this patch you would either get an expected content (if you
win the race) or SEGV otherwise. With the above change you would get a
third state - a fresh new page (zero page) if you lost the race half
way. That sounds like a change of a long term semantic.
How much that matters is of course a question. Userspace is known to do
the most unexpected things you never even dreamed of.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2018-07-03 11:34 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-29 22:39 [RFC v3 PATCH 0/5] mm: zap pages with read mmap_sem in munmap for large mapping Yang Shi
2018-06-29 22:39 ` [RFC v3 PATCH 1/5] uprobes: make vma_has_uprobes non-static Yang Shi
2018-06-29 22:39 ` [RFC v3 PATCH 2/5] mm: introduce VM_DEAD flag Yang Shi
2018-07-02 13:40 ` Michal Hocko
2018-06-29 22:39 ` [RFC v3 PATCH 3/5] mm: refactor do_munmap() to extract the common part Yang Shi
2018-07-02 13:42 ` Michal Hocko
2018-07-02 16:59 ` Yang Shi
2018-07-02 17:58 ` Michal Hocko
2018-07-02 18:02 ` Yang Shi
2018-06-29 22:39 ` [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping Yang Shi
2018-06-30 1:28 ` Andrew Morton
2018-06-30 2:10 ` Yang Shi
2018-06-30 1:35 ` Andrew Morton
2018-06-30 2:28 ` Yang Shi
2018-06-30 3:15 ` Andrew Morton
2018-06-30 4:26 ` Yang Shi
2018-07-03 0:01 ` Yang Shi
2018-07-02 14:05 ` Michal Hocko
2018-07-02 20:48 ` Andrew Morton
2018-07-03 6:09 ` Michal Hocko
2018-07-03 16:53 ` Yang Shi
2018-07-03 18:22 ` Yang Shi
2018-07-04 8:13 ` Michal Hocko
2018-07-02 12:33 ` Kirill A. Shutemov
2018-07-02 12:49 ` Michal Hocko
2018-07-03 8:12 ` Kirill A. Shutemov
2018-07-03 8:27 ` Michal Hocko
2018-07-03 9:19 ` Kirill A. Shutemov
2018-07-03 11:34 ` Michal Hocko [this message]
2018-07-03 12:14 ` Kirill A. Shutemov
2018-07-03 17:00 ` Yang Shi
2018-07-02 17:19 ` Yang Shi
2018-07-03 8:07 ` Kirill A. Shutemov
2018-07-02 13:53 ` Michal Hocko
2018-07-02 17:07 ` Yang Shi
2018-06-29 22:39 ` [RFC v3 PATCH 5/5] x86: check VM_DEAD flag in page fault Yang Shi
2018-07-02 8:45 ` Laurent Dufour
2018-07-02 12:15 ` Michal Hocko
2018-07-02 12:26 ` Laurent Dufour
2018-07-02 12:45 ` Michal Hocko
2018-07-02 13:33 ` Laurent Dufour
2018-07-02 13:37 ` Michal Hocko
2018-07-02 17:24 ` Yang Shi
2018-07-02 17:57 ` Michal Hocko
2018-07-02 18:10 ` Yang Shi
2018-07-03 6:17 ` Michal Hocko
2018-07-03 16:50 ` Yang Shi
2018-07-02 13:39 ` [RFC v3 PATCH 0/5] mm: zap pages with read mmap_sem in munmap for large mapping Michal Hocko
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