From: Michal Hocko <mhocko@kernel.org>
To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
willy@infradead.org, ldufour@linux.vnet.ibm.com,
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, linux-kernel@vger.kernel.org
Subject: Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping
Date: Wed, 4 Jul 2018 10:13:47 +0200 [thread overview]
Message-ID: <20180704081347.GG22503@dhcp22.suse.cz> (raw)
In-Reply-To: <658e4c7b-d426-11ab-ef9a-018579cbf756@linux.alibaba.com>
On Tue 03-07-18 11:22:17, Yang Shi wrote:
>
>
> On 7/2/18 11:09 PM, Michal Hocko wrote:
> > On Mon 02-07-18 13:48:45, Andrew Morton wrote:
> > > On Mon, 2 Jul 2018 16:05:02 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > > On Fri 29-06-18 20:15:47, Andrew Morton wrote:
> > > > [...]
> > > > > Would one of your earlier designs have addressed all usecases? I
> > > > > expect the dumb unmap-a-little-bit-at-a-time approach would have?
> > > > It has been already pointed out that this will not work.
> > > I said "one of". There were others.
> > Well, I was aware only about two potential solutions. Either do the
> > heavy lifting under the shared lock and do the rest with the exlusive
> > one and this, drop the lock per parts. Maybe I have missed others?
> >
> > > > You simply
> > > > cannot drop the mmap_sem during unmap because another thread could
> > > > change the address space under your feet. So you need some form of
> > > > VM_DEAD and handle concurrent and conflicting address space operations.
> > > Unclear that this is a problem. If a thread does an unmap of a range
> > > of virtual address space, there's no guarantee that upon return some
> > > other thread has not already mapped new stuff into that address range.
> > > So what's changed?
> > Well, consider the following scenario:
> > Thread A = calling mmap(NULL, sizeA)
> > Thread B = calling munmap(addr, sizeB)
> >
> > They do not use any external synchronization and rely on the atomic
> > munmap. Thread B only munmaps range that it knows belongs to it (e.g.
> > called mmap in the past). It should be clear that ThreadA should not
> > get an address from the addr, sizeB range, right? In the most simple case
> > it will not happen. But let's say that the addr, sizeB range has
> > unmapped holes for what ever reasons. Now anytime munmap drops the
> > exclusive lock after handling one VMA, Thread A might find its sizeA
> > range and use it. ThreadB then might remove this new range as soon as it
> > gets its exclusive lock again.
>
> I'm a little bit confused here. If ThreadB already has unmapped that range,
> then ThreadA uses it. It sounds not like a problem since ThreadB should just
> go ahead to handle the next range when it gets its exclusive lock again,
> right? I don't think of why ThreadB would re-visit that range to remove it.
Not if the new range overlap with the follow up range that ThreadB does.
Example
B: munmap [XXXXX] [XXXXXX] [XXXXXXXXXX]
B: breaks the lock after processing the first vma.
A: mmap [XXXXXXXXXXXX]
B: munmap retakes the lock and revalidate from the last vm_end because
the old vma->vm_next might be gone
B: [XXX][XXXXX] [XXXXXXXXXX]
so you munmap part of the range. Sure you can plan some tricks and skip
over vmas that do not start above your last vma->vm_end or something
like that but I expect there are other can of worms hidden there.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2018-07-04 8:13 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 [this message]
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
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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