From 756eebe8198f33dd298b81a2a3bb041f97cfe1c3 Mon Sep 17 00:00:00 2001
From: Mark Salyzyn <salyzyn@google.com>
Date: Tue, 27 Nov 2018 16:14:35 -0800
Subject: [PATCH] fs_mgr: Change scratch margin to 3 X logical_block_size.

Found a device with a 3/4MB erase block, give a wider margin so that
flashing a single partition is unlikely to be blocked by scratch.

Add documentation to tell developers to flashall to clear scratch.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: Ic219283f4c42e457b98991bcd1752253e182eff3
---
 fs_mgr/README.overlayfs.md  |  5 +++++
 fs_mgr/fs_mgr_overlayfs.cpp | 18 +++++++++++++++---
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/fs_mgr/README.overlayfs.md b/fs_mgr/README.overlayfs.md
index d715d7bcb..2d42d6c6e 100644
--- a/fs_mgr/README.overlayfs.md
+++ b/fs_mgr/README.overlayfs.md
@@ -89,4 +89,9 @@ Caveats
   if higher than 4.6.
 - *adb enable-verity* will free up overlayfs and as a bonus the
   device will be reverted pristine to before any content was updated.
+- If dynamic partitions runs out of space, resizing a logical
+  partition larger may fail because of the scratch partition.
+  If this happens, either fastboot flashall or adb enable-verity can
+  be used to clear scratch storage to permit the flash.
+  Then reinstate the overrides and continue.
 - File bugs or submit fixes for review.
diff --git a/fs_mgr/fs_mgr_overlayfs.cpp b/fs_mgr/fs_mgr_overlayfs.cpp
index 20652ad86..57e984d18 100644
--- a/fs_mgr/fs_mgr_overlayfs.cpp
+++ b/fs_mgr/fs_mgr_overlayfs.cpp
@@ -675,14 +675,26 @@ bool fs_mgr_overlayfs_setup_scratch(const fstab* fstab, bool* change) {
         }
         changed = true;
     }
-    // Take half of free space, minimum 512MB or free space - 256KB margin.
+    // Take half of free space, minimum 512MB or maximum free - margin.
     static constexpr auto kMinimumSize = uint64_t(512 * 1024 * 1024);
-    static constexpr auto kMarginSize = uint64_t(256 * 1024);
     if (partition->size() < kMinimumSize) {
         auto partition_size =
                 builder->AllocatableSpace() - builder->UsedSpace() + partition->size();
         if ((partition_size > kMinimumSize) || !partition->size()) {
-            partition_size = std::max(std::min(kMinimumSize, partition_size - kMarginSize),
+            // Leave some space for free space jitter of a few erase
+            // blocks, in case they are needed for any individual updates
+            // to any other partition that needs to be flashed while
+            // overlayfs is in force.  Of course if margin_size is not
+            // enough could normally get a flash failure, so
+            // ResizePartition() will delete the scratch partition in
+            // order to fulfill.  Deleting scratch will destroy all of
+            // the adb remount overrides :-( .
+            auto margin_size = uint64_t(3 * 256 * 1024);
+            BlockDeviceInfo info;
+            if (builder->GetBlockDeviceInfo(partition_name, &info)) {
+                margin_size = 3 * info.logical_block_size;
+            }
+            partition_size = std::max(std::min(kMinimumSize, partition_size - margin_size),
                                       partition_size / 2);
             if (partition_size > partition->size()) {
                 if (!builder->ResizePartition(partition, partition_size)) {