From: Oleg Nesterov <oleg@redhat.com>
To: Mircea CIRJALIU - MELIU <mcirjaliu@bitdefender.com>
Cc: "Adalbert Lazăr" <alazar@bitdefender.com>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Alexander Graf" <graf@amazon.com>,
"Stefan Hajnoczi" <stefanha@redhat.com>,
"Jerome Glisse" <jglisse@redhat.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Mihai Donțu" <mdontu@bitdefender.com>,
"Andy Lutomirski" <luto@kernel.org>,
"Arnd Bergmann" <arnd@arndb.de>,
"Sargun Dhillon" <sargun@sargun.me>,
"Aleksa Sarai" <cyphar@cyphar.com>,
"Jann Horn" <jannh@google.com>,
"Kees Cook" <keescook@chromium.org>,
"Matthew Wilcox" <willy@infradead.org>,
"Christian Brauner" <christian.brauner@ubuntu.com>
Subject: Re: [RESEND RFC PATCH 4/5] mm/remote_mapping: use a pidfd to access memory belonging to unrelated process
Date: Thu, 10 Sep 2020 18:43:03 +0200 [thread overview]
Message-ID: <20200910164302.GA12976@redhat.com> (raw)
In-Reply-To: <AM7PR02MB608232256B97797EDE394552BB260@AM7PR02MB6082.eurprd02.prod.outlook.com>
On 09/09, Mircea CIRJALIU - MELIU wrote:
>
> > From: Oleg Nesterov <oleg@redhat.com>
> >
> > But why is it safe to drop ->mmap_sem without checking
> > FAULT_FLAG_ALLOW_RETRY/RETRY_NOWAIT ?
> >
> Dropping mmap_sem will have the same effects regardless of FAULT_FLAG_ALLOW_RETRY/RETRY_NOWAIT.
> Another thread can unmap the VMA from underneath us, or remap another one in its place.
> In the end, the VMA has to be revalidated when re-acquiring the mmap_sem.
> Or am I wrong?!
To simplify, lets forget about RETRY_NOWAIT/TRIED.
Again, I can be easily wrong. But iiuc, you simply can't drop mmap_sem
if FAULT_FLAG_ALLOW_RETRY is not set, the caller doesn't expect mmap_sem
can be unlocked.
OTOH, if FAULT_FLAG_ALLOW_RETRY is set and you drop mmap_sem, you can
only return VM_FAULT_RETRY to let the caller know it was dropped.
> > > + * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_sem must be
> > released
> > > + * before returning VM_FAULT_RETRY only if
> > FAULT_FLAG_RETRY_NOWAIT is
> > > + * not set.
> >
> > Well, iiuc FAULT_FLAG_ALLOW_RETRY means that ->fault() _can_ drop
> > mmap_sem and return VM_FAULT_RETRY (unless NOWAIT).
>
> That comment is just copied from elsewhere in the code.
> My interpretation was that the fault handler _should_ return with mmap_sem
> held or not depending on FAULT_FLAG_RETRY_NOWAIT.
Yes, this depends on FAULT_FLAG_RETRY_NOWAIT.
But your comment above looks as if he mmap_sem must be _always_ released
if FAULT_FLAG_ALLOW_RETRY && !FAULT_FLAG_RETRY_NOWAIT. This is not true.
Nevermind. If you ever resend this patch, please CC mm/ experts. I tried
to look at this code again and I feel it has much more problems, but as
I said this is not my area.
And I think you should split this patch, mirror_vm_fault() should come in
a separate patch to simplify the review.
Oleg.
next prev parent reply other threads:[~2020-09-10 16:43 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-04 11:31 [RESEND RFC PATCH 0/5] Remote mapping Adalbert Lazăr
2020-09-04 11:31 ` [RESEND RFC PATCH 1/5] mm: add atomic capability to zap_details Adalbert Lazăr
2020-09-04 11:31 ` [RESEND RFC PATCH 2/5] mm: let the VMA decide how zap_pte_range() acts on mapped pages Adalbert Lazăr
2020-09-04 11:31 ` [RESEND RFC PATCH 3/5] mm/mmu_notifier: remove lockdep map, allow mmu notifier to be used in nested scenarios Adalbert Lazăr
2020-09-04 12:03 ` Jason Gunthorpe
2020-09-04 11:31 ` [RESEND RFC PATCH 4/5] mm/remote_mapping: use a pidfd to access memory belonging to unrelated process Adalbert Lazăr
2020-09-04 17:55 ` Oleg Nesterov
2020-09-07 14:30 ` Oleg Nesterov
2020-09-07 15:16 ` Adalbert Lazăr
2020-09-09 8:32 ` Mircea CIRJALIU - MELIU
2020-09-10 16:43 ` Oleg Nesterov [this message]
2020-09-07 15:02 ` Christian Brauner
2020-09-07 16:04 ` Mircea CIRJALIU - MELIU
2020-09-04 11:31 ` [RESEND RFC PATCH 5/5] pidfd_mem: implemented remote memory mapping system call Adalbert Lazăr
2020-09-04 19:18 ` Florian Weimer
2020-09-07 14:55 ` Christian Brauner
2020-09-04 12:11 ` [RESEND RFC PATCH 0/5] Remote mapping Jason Gunthorpe
2020-09-04 13:24 ` Mircea CIRJALIU - MELIU
2020-09-04 13:39 ` Jason Gunthorpe
2020-09-04 14:18 ` Mircea CIRJALIU - MELIU
2020-09-04 14:39 ` Jason Gunthorpe
2020-09-04 15:40 ` Mircea CIRJALIU - MELIU
2020-09-04 16:11 ` Jason Gunthorpe
2020-09-04 19:41 ` Matthew Wilcox
2020-09-04 19:49 ` Jason Gunthorpe
2020-09-04 20:08 ` Paolo Bonzini
2020-12-01 18:01 ` Jason Gunthorpe
2020-09-04 19:19 ` Florian Weimer
2020-09-04 20:18 ` Paolo Bonzini
2020-09-07 8:33 ` Christian Brauner
2020-09-04 19:39 ` Andy Lutomirski
2020-09-04 20:09 ` Paolo Bonzini
2020-09-04 20:34 ` Andy Lutomirski
2020-09-04 21:58 ` Paolo Bonzini
2020-09-04 23:17 ` Andy Lutomirski
2020-09-05 18:27 ` Paolo Bonzini
2020-09-07 8:38 ` Christian Brauner
2020-09-07 12:41 ` Mircea CIRJALIU - MELIU
2020-09-07 7:05 ` Christoph Hellwig
2020-09-07 8:44 ` Paolo Bonzini
2020-09-07 10:25 ` Mircea CIRJALIU - MELIU
2020-09-07 15:05 ` Christian Brauner
2020-09-07 20:43 ` Andy Lutomirski
2020-09-09 11:38 ` Stefan Hajnoczi
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