I had a user of my FUSE based filesystem report corruption while using an app which used mmap for file IO on a Raspberry Pi 3B. Files would be corrupted only if greater than 4GB. I replicated the behavior with a simple test app[0] on a RPi2 using the latest Raspbian (Linux raspberrypi 4.14.98-v7+ #1200 SMP Tue Feb 12 20:27:48 GMT 2019 armv7l GNU/Linux) and i686 Debian 9 in a VM (Linux debian 4.9.0-8-686 #1 SMP Debian 4.9.144-3.1 (2019-02-19) i686 GNU/Linux). What appears to happen is that after the offset reaches (st_size & 0xFFFFFFFF) bytes the FUSE server no longer receives write requests for modified pages. If the file size is exactly 4GB it won't ever receive a write. Reads still come in for all addresses and there are no errors on the client's side. Easiest way to reproduce is probably the following: $ sudo apt install mergerfs $ mkdir /tmp/mergerfs $ sudo mergerfs -d -o allow_other,default_permissions ~/ /tmp/mergerfs $ # in another shell $ cd /tmp $ wget https://gist.githubusercontent.com/trapexit/82187c6a74a3df66753caa74df8c8232/raw/47bb6f1085ca847c15c7c31a57f822c3b72ed982/mmap-test.c $ gcc -D_FILE_OFFSET_BITS=64 -o mmap-test mmap-test.c $ cd mergerfs $ ../mmap-test 8 offset: 0x0 offset: 0x20000 offset: 0x40000 offset: 0x60000 offset: 0x80000 offset: 0xa0000 offset: 0xc0000 offset: 0xe0000 $ hexdump test.img 0000000 ffff ffff ffff ffff ffff ffff ffff ffff * 0020000 0000 0000 0000 0000 0000 0000 0000 0000 * 100020000 In the mergerfs terminal you'll see libfuse debug info which should include reads (and initially some writes). [0] https://gist.githubusercontent.com/trapexit/82187c6a74a3df66753caa74df8c8232/raw/47bb6f1085ca847c15c7c31a57f822c3b72ed982/mmap-test.c
