tree f83a66a7e5f9ce53e6d33eb048ed38048b3a086b
parent c5c1d19ebb6d675f3028906be34d447c14ffbce5
author Kalesh Singh <kaleshsingh@google.com> 1710808079 -0700
committer Kalesh Singh <kaleshsingh@google.com> 1714673213 -0700

bionic: loader: Drop readahead padding pages

These are padding pages are only needed to layout the ELF to be
compatible with max-page-size. They are zero-filled (holes) and
can be dropped from the page cache.

The madvise() here is a special case that also serves to hint to the
kernel what part of the segment is padding.

For example the kernel then shows these padding regions as PROT_NONE
VMAs (labeled [page size compat]) in /proc/*/maps.

Note: This doesn't use backing vm_area_structs, so doesn't consume
additional slab memory.

Before:

❯ cf-adb shell cat /proc/1/maps | grep -A1 'libbase.so$'
7f8d13600000-7f8d13614000 r--p 00000000 fe:09 21909460    /system/lib64/libbase.so
7f8d13614000-7f8d13638000 r-xp 00014000 fe:09 21909460    /system/lib64/libbase.so
7f8d13638000-7f8d1363c000 r--p 00038000 fe:09 21909460    /system/lib64/libbase.so
7f8d1363c000-7f8d1363d000 rw-p 0003c000 fe:09 21909460    /system/lib64/libbase.so

Segments appear extended in /proc/<pid>/maps

After:

❯ cf-adb shell cat /proc/1/maps | grep -A1 'libbase.so$'
7f3650043000-7f3650054000 r--p 00000000 fe:09 21906900    /system/lib64/libbase.so
7f3650054000-7f3650057000 ---p 00000000 00:00 0           [page size compat]
7f3650057000-7f3650079000 r-xp 00014000 fe:09 21906900    /system/lib64/libbase.so
7f3650079000-7f365007b000 ---p 00000000 00:00 0           [page size compat]
7f365007b000-7f365007c000 r--p 00038000 fe:09 21906900    /system/lib64/libbase.so
7f365007c000-7f365007f000 ---p 00000000 00:00 0           [page size compat]
7f365007f000-7f3650080000 rw-p 0003c000 fe:09 21906900    /system/lib64/libbase.so

Segments maintain PROT_NONE gaps ("[page size compat]") for app
compatiblity but these are not backed by actual slab VMA memory.

Bug: 330117029
Bug: 327600007
Bug: 330767927
Bug: 328266487
Bug: 329803029
Test: Manual - Launch Free Fire Chaos app
Change-Id: Ic50540e247b4294eb08f8cf70e74bd2bf6606684
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>