Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKEDfile segments

[Hugh Dickins]

On Tue, 3 Feb 2009, Lee Schermerhorn wrote:
> On Sat, 2009-01-31 at 12:35 +0000, Hugh Dickins wrote:
> > We need a way to communicate not-MAP_NORESERVE to shmem.c, and we don't
> > just need it in the explicit shmem_zero_setup() case, we also need it
> > for the (probably rare nowadays) case when mmap() is working on file
>           ^^^^^^^^^^^^^^^^^^^^^^^^
> > /dev/zero (drivers/char/mem.c mmap_zero()), rather than using MAP_ANON.
> 
> 
> This reminded me of something I'd seen recently looking
> at /proc/<pid>/[numa]_maps for <a large commercial database> on
> Linux/x86_64: 
>...
> 2adadf711000-2adadf721000 rwxp 00000000 00:0e 4072                       /dev/zero
> 2adadf721000-2adadf731000 rwxp 00000000 00:0e 4072                       /dev/zero
> 2adadf731000-2adadf741000 rwxp 00000000 00:0e 4072                       /dev/zero
> 
> <and so on, for another 90 lines until>
> 
> 7fffcdd36000-7fffcdd4e000 rwxp 7fffcdd36000 00:00 0                      [stack]
> ffffffffff600000-ffffffffffe00000 ---p 00000000 00:00 0                  [vdso]
> 
> For portability between Linux and various Unix-like systems that don't
> support MAP_ANON*, perhaps?
> 
> Anyway, from the addresses and permissions, these all look potentially
> mergeable.  The offset is preventing merging, right?  I guess that's one
> of the downsides of mapping /dev/zero rather than using MAP_ANONYMOUS?
> 
> Makes one wonder whether it would be worthwhile [not to mention
> possible] to rework mmap_zero() to mimic MAP_ANONYMOUS...

That's certainly an interesting observation, and thank you for sharing
it with us (hmm, I sound like a self-help group leader or something).

I don't really have anything to add to what Linus said (and hadn't
got around to realizing the significance of the "p" there before I
saw his reply).

Mmm, it's interesting, but I fear to add more hacks in there just
for this - I guess we could, but I'd rather not, unless it becomes
a serious issue.

Let's just tuck away the knowledge of this case for now.

Hugh
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