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diff --git a/dexopt_chroot_setup/README.md b/dexopt_chroot_setup/README.md
index 71401248d6..b94cde2307 100644
--- a/dexopt_chroot_setup/README.md
+++ b/dexopt_chroot_setup/README.md
@@ -8,3 +8,66 @@ seamless updates.
 It requires elevated permissions that are not available to system_server, such
 as mounting filesystems. It publishes a binder interface that is internal to ART
 Service's Java code.
+
+### Pre-reboot Dexopt file structure
+
+#### Overview
+
+```
+/mnt/pre_reboot_dexopt
+|-- chroot
+|   |-- system_ext
+|   |-- vendor
+|   |-- product
+|   |-- data
+|   |-- mnt
+|   |   |-- expand
+|   |   `-- artd_tmp
+|   |-- dev
+|   |-- proc
+|   |-- sys
+|   |-- metadata
+|   |-- apex
+|   `-- linkerconfig
+`-- mount_tmp
+```
+
+#### `/mnt/pre_reboot_dexopt`
+
+The root directory for Pre-reboot Dexopt, prepared by `init`.
+
+#### `/mnt/pre_reboot_dexopt/chroot`
+
+The root directory of the chroot environment for Pre-reboot Dexopt. It is the
+mount point of the `system` image. Created by `dexopt_chroot_setup`, and only
+exists for the duration of the Pre-reboot Dexopt.
+
+#### `/mnt/pre_reboot_dexopt/chroot/{system_ext,vendor,product}`
+
+Mount points of other readonly images.
+
+#### `/mnt/pre_reboot_dexopt/chroot/{data,mnt/expand,dev,proc,sys,metadata}`
+
+Same as the corresponding directories outside of chroot. These are read-write
+mounts.
+
+#### `/mnt/pre_reboot_dexopt/chroot/mnt/artd_tmp`
+
+An empty directory for storing temporary files during Pre-reboot Dexopt, managed
+by `artd`.
+
+#### `/mnt/pre_reboot_dexopt/chroot/apex`
+
+For holding the apex mount points used in the chroot environment, managed by
+`apexd`. Note that this is not the same as `/apex` outside of chroot.
+
+#### `/mnt/pre_reboot_dexopt/chroot/linkerconfig`
+
+For holding the linker config used in the chroot environment, managed by
+`linkerconfig`. Note that this is not the same as `/linkerconfig` outside of
+chroot.
+
+#### `/mnt/pre_reboot_dexopt/mount_tmp`
+
+An ephemeral directory used as a temporary mount point for bind-mounting
+directories "slave+shared".