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Re: [Qemu-devel] [PATCH 0/2] core dump: re-purpose VM_ALWAYSDUMP to user
From: |
Jason Baron |
Subject: |
Re: [Qemu-devel] [PATCH 0/2] core dump: re-purpose VM_ALWAYSDUMP to user controlled VM_DONTDUMP |
Date: |
Wed, 7 Mar 2012 20:32:17 -0500 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, Mar 07, 2012 at 02:30:28PM -0800, Andrew Morton wrote:
> On Wed, 7 Mar 2012 12:00:46 -0500
> Jason Baron <address@hidden> wrote:
>
> > Hi,
> >
> > The motivation for this change was that I was looking at a way for a
> > qemu-kvm
> > process, to exclude the guest memory from its core dump, which can be quite
> > large. There are already a number of filter flags in
> > /proc/<pid>/coredump_filter, however, these allow one to specify 'types' of
> > kernel memory, not specific address ranges (which is needed in this case).
> >
> > Since there are no more vma flags available, the first patch eliminates the
> > need for the 'VM_ALWAYSDUMP' flag. The flag is used internally by the
> > kernel to
> > mark vdso and vsyscall pages. However, it is simple enough to check if a vma
> > covers a vdso or vsyscall page without the need for this flag.
>
> Gee, we ran out?
>
> That makes it pretty inevitable that we will grow the vma by four
> bytes. Once we have done that, your always_dump_vma() trickery becomes
> unneeded and undesirable, yes? If so, we may as well recognise reality
> and grow the vma now.
>
We could. The reason I didn't propose increasing them was because I saw
in the archives that there was resistance when this was tried before:
http://lkml.indiana.edu/hypermail/linux/kernel/1111.1/02053.html
Also, the current use of 'VM_ALWAYSDUMP' is for vdso and gate pages
exclusively. That means that VM_ALWAYSDUMP flag is 0 the majority of the
time, and thus it probably makes sense to re-purpose it, now that we are
tight on flags.
> > The second patch then replaces the 'VM_ALWAYSDUMP' flag with a new
> > 'VM_DONTDUMP' flag, which can be set by userspace using new madvise flags:
> > 'MADV_DONTDUMP', and unset via 'MADV_DUMP'. The core dump filters continue
> > to
> > work the same as before unless 'MADV_DONTDUMP' is set on the region.
> >
> > The qemu code which implements this features is at:
> > http://people.redhat.com/~jbaron/qemu-dump/qemu-dump.patch
> >
> > In my testing the qemu core dump shrunk from 383MB -> 13MB with this patch.
> >
> > I also believe that the 'MADV_DONTDUMP' flag might be useful for security
> > sensitive apps, which might want to select which areas are dumped.
> >
>
> Is there any way for userspace to query the state of the flag?
>
A place for that might be /proc/<pid>/maps, but I don't think any of
the other vm flags are exposed.
Thanks,
-Jason
- [Qemu-devel] [PATCH 0/2] core dump: re-purpose VM_ALWAYSDUMP to user controlled VM_DONTDUMP, Jason Baron, 2012/03/07
- Re: [Qemu-devel] [PATCH 0/2] core dump: re-purpose VM_ALWAYSDUMP to user controlled VM_DONTDUMP, Andrew Morton, 2012/03/07
- Re: [Qemu-devel] [PATCH 0/2] core dump: re-purpose VM_ALWAYSDUMP to user controlled VM_DONTDUMP,
Jason Baron <=
- [Qemu-devel] [PATCH 1/2] core dump: drop VM_ALWAYSDUMP flag, Jason Baron, 2012/03/07
- Re: [Qemu-devel] [PATCH 1/2] core dump: drop VM_ALWAYSDUMP flag, Roland McGrath, 2012/03/07
- Re: [Qemu-devel] [PATCH 1/2] core dump: drop VM_ALWAYSDUMP flag, Jason Baron, 2012/03/07
- Re: [Qemu-devel] [PATCH 1/2] core dump: drop VM_ALWAYSDUMP flag, Roland McGrath, 2012/03/07
- Re: [Qemu-devel] [PATCH 1/2] core dump: drop VM_ALWAYSDUMP flag, Jason Baron, 2012/03/07
- Re: [Qemu-devel] [PATCH 1/2] core dump: drop VM_ALWAYSDUMP flag, Roland McGrath, 2012/03/07
- Re: [Qemu-devel] [PATCH 1/2] core dump: drop VM_ALWAYSDUMP flag, Chris Metcalf, 2012/03/07
- Re: [Qemu-devel] [PATCH 1/2] core dump: drop VM_ALWAYSDUMP flag, Jason Baron, 2012/03/07
[Qemu-devel] [PATCH 2/2] core dump: add a new VM_DONTDUMP flag, Jason Baron, 2012/03/07