From: Michel Lespinasse <walken@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Davidlohr Bueso <davidlohr@hp.com>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Rik van Riel <riel@redhat.com>,
Tim Chen <tim.c.chen@linux.intel.com>,
"Chandramouleeswaran, Aswin" <aswin@hp.com>,
linux-mm <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/3] mm,vdso: preallocate new vmas
Date: Tue, 22 Oct 2013 10:04:12 -0700 [thread overview]
Message-ID: <CANN689EyWqCsr2rDBtHohMYwXG81dny+iQBKSt4AWqjEWnUkow@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFzwRoM4w8mGqSeeVuDGhQgnnomu=vxoWC6dbHD9w-9A+Q@mail.gmail.com>
On Tue, Oct 22, 2013 at 9:20 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Oct 22, 2013 at 4:48 PM, <walken@google.com> wrote:
>> Generally the problems I see with mmap_sem are related to long latency
>> operations. Specifically, the mmap_sem write side is currently held
>> during the entire munmap operation, which iterates over user pages to
>> free them, and can take hundreds of milliseconds for large VMAs.
>
> So this would be the *perfect* place to just downgrade the semaphore
> from a write to a read.
It's not as simple as that, because we currently rely on mmap_sem
write side being held during page table teardown in order to exclude
things like follow_page() which may otherwise access page tables while
we are potentially freeing them.
I do think it's solvable, but it gets complicated fast. Hugh & I have
been talking about it; the approach I'm looking at would involve
unwiring the page tables first (under protection of the mmap_sem write
lock) and then iterating on the unwired page tables to free the data
pages, issue TLB shootdowns and free the actual page tables (we
probably don't need even the mmap_sem read side at that point). But,
that's nowhere like a 10 line change anymore at that point...
--
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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next prev parent reply other threads:[~2013-10-22 17:04 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-18 0:50 Davidlohr Bueso
2013-10-18 0:50 ` [PATCH 1/3] mm: add mlock_future_check helper Davidlohr Bueso
2013-10-23 9:30 ` walken
2013-10-18 0:50 ` [PATCH 2/3] mm/mlock: prepare params outside critical region Davidlohr Bueso
2013-10-23 9:33 ` walken
2013-10-23 9:46 ` Vlastimil Babka
2013-10-18 0:50 ` [PATCH 3/3] vdso: preallocate new vmas Davidlohr Bueso
2013-10-18 1:17 ` Linus Torvalds
2013-10-18 5:59 ` Richard Weinberger
2013-10-18 6:05 ` [PATCH 4/3] x86/vdso: Optimize setup_additional_pages() Ingo Molnar
2013-10-21 3:52 ` Davidlohr Bueso
2013-10-21 5:27 ` Ingo Molnar
2013-10-21 3:26 ` [PATCH 3/3] vdso: preallocate new vmas Davidlohr Bueso
2013-10-23 9:53 ` walken
2013-10-25 0:55 ` Davidlohr Bueso
2013-10-22 15:48 ` [PATCH 0/3] mm,vdso: " walken
2013-10-22 16:20 ` Linus Torvalds
2013-10-22 17:04 ` Michel Lespinasse [this message]
2013-10-22 17:54 ` Andy Lutomirski
2013-10-23 10:13 ` Michel Lespinasse
2013-10-23 21:42 ` Andy Lutomirski
2013-10-23 2:46 ` Davidlohr Bueso
2013-11-05 0:39 ` Davidlohr Bueso
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