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>

1 files changed, 22 insertions, 4 deletions

diff --git a/linker/linker_phdr.cpp b/linker/linker_phdr.cpp
index 80f1ad9d4..fa712a10a 100644
--- a/linker/linker_phdr.cpp
+++ b/linker/linker_phdr.cpp
@@ -918,10 +918,28 @@ bool ElfReader::LoadSegments() {
     //   2) Break the COW backing, faulting in new anon pages for a region
     //      that will not be used.
 
-    // _seg_file_end = unextended seg_file_end
-    uint64_t _seg_file_end = seg_start + phdr->p_filesz;
-    if ((phdr->p_flags & PF_W) != 0 && page_offset(_seg_file_end) > 0) {
-      memset(reinterpret_cast<void*>(_seg_file_end), 0, kPageSize - page_offset(_seg_file_end));
+    uint64_t unextended_seg_file_end = seg_start + phdr->p_filesz;
+    if ((phdr->p_flags & PF_W) != 0 && page_offset(unextended_seg_file_end) > 0) {
+      memset(reinterpret_cast<void*>(unextended_seg_file_end), 0,
+             kPageSize - page_offset(unextended_seg_file_end));
+    }
+
+    // Pages may be brought in due to readahead.
+    // Drop the padding (zero) pages, to avoid reclaim work later.
+    //
+    // NOTE: The madvise() here is special, as it also serves to hint to the
+    // kernel the portion of the LOAD segment that is padding.
+    //
+    // See: [1] https://android-review.googlesource.com/c/kernel/common/+/3032411
+    //      [2] https://android-review.googlesource.com/c/kernel/common/+/3048835
+    uint64_t pad_start = page_end(unextended_seg_file_end);
+    uint64_t pad_end = page_end(seg_file_end);
+    CHECK(pad_start <= pad_end);
+    uint64_t pad_len = pad_end - pad_start;
+    if (page_size_migration_supported() && pad_len > 0 &&
+        madvise(reinterpret_cast<void*>(pad_start), pad_len, MADV_DONTNEED)) {
+      DL_WARN("\"%s\": madvise(0x%" PRIx64 ", 0x%" PRIx64 ", MADV_DONTNEED) failed: %m",
+              name_.c_str(), pad_start, pad_len);
     }
 
     seg_file_end = page_end(seg_file_end);