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From: "jyescas at google dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug dynamic-link/31076] Extra struct vm_area_struct with ---p
 created when PAGE_SIZE < max-page-size
Date: Tue, 28 Nov 2023 23:58:54 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31076

--- Comment #16 from Juan Yescas <jyescas at google dot com> ---
(In reply to Ryan Prichard from comment #14)
> > When we compile all the shared libraries in the system with -Wl,-z,max-=
page-size=3D65536 and use 4k page size kernel, for every PT_LOAD segment (e=
xcept the last) loaded in memory, there will be a vm_area_struct with ---p =
permissions.
>=20
> > In mobile devices we have seen an increased of 120K vm_area_structs due=
 -Wl,-z,max-page-size=3D65536. There used to be 40K vm_area_structs. This i=
s an increase of ~18MB (120K * 152).
>=20
> I would've expected the number of VMAs to nearly double, but it sounds li=
ke
> they quadrupled instead?


Sorry for the confusion, I was comparing different devices.

These are results where we can see the increase in extra vmas when the
alignment is max-page-size=3D65536.

VMA size =3D 200
Loaded ELF segments =3D 181806
PROT_NONE (---p) gaps =3D 124297    <- This is the # of extra vmas
Slab Memory ELF Usage: 34 MB
Slab Memory noneprot Usage: 23 MB

The script that was used is below:

```
#!/bin/sh

vma_size=3D$(cat /proc/slabinfo | grep vm_area_struct | awk '{ print $4 }')
echo "VMA size =3D $vma_size"

nr_elf_segs=3D$(cat /proc/*/maps | grep '\.so' | wc -l)
echo "Loaded ELF segments =3D $nr_elf_segs"

nr_noneprot=3D$(cat /proc/*/maps | grep -A1 '\.so' | grep '\---p' | wc -l)
echo "PROT_NONE (---p) gaps =3D $nr_noneprot"

slab_mem_mb=3D$(echo "$vma_size * $nr_elf_segs / 1024 / 1024" | bc)
echo "Slab Memory ELF Usage: $slab_mem_mb MB"

slab_mem_mb=3D$(echo "$vma_size * $nr_noneprot / 1024 / 1024" | bc)
echo "Slab Memory PROTNONE GAP Usage: $slab_mem_mb MB"
```

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