On Wed, Mar 07, 2012 at 02:30:28PM -0800, Andrew Morton wrote: > On Wed, 7 Mar 2012 12:00:46 -0500 > Jason Baron <jba...@redhat.com> 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