Linux: How to enable Execute in place (XIP) for RAMFS/TMPFS

Question

I'm working on an embedded system where the rootfs is constructed in a tmpfs partition by the init process. After the rootfs is complete, it will do a pivot-root and start spawning processes located in the rootfs.

But it seems like XIP is not working for our tmpfs, and all the applications is therefore loaded into ram twice (in the tmpfs and again into ram when loaded).

Can this really be true?

I found an old discussion thread at https://ez.analog.com/thread/45262 which describe the same issue as I'm seeing.

How can I achieve XIP for a file-system located in memory?

Answer 1

What you are attempting to do should be indeed possible (though I haven't tried it myself). The problem is simply you are not going about it the correct way. If you use the block RAM device ("brd") you can create a block device that is actually RAM presented as a block device. To enable this on your kernel (sorry you do not say which kernel you have so I will just go with the kernel 4.14), you need to enable CONFIG_BLK_DEV_RAM as well as CONFIG_BLK_DEV_RAM_DAX in your kernel configuration. They are both under "Device Drivers" -> "Block Devices". Then you create such a RAM backed block device and then create for example an ext2 or ext4 or XFS file system on it and then prepare your rootfs into that file system and then pivot-root into it. Now you are executing in a RAM backed file system which has XIP (now replaced by DAX) functionality thus executing applications should now at least in theory work correctly without creating a copy of the data and simply running it out of the RAM pages of the block RAM device.

Please do beware that such approach has limitation such as for example that kernel modules themselves will still be copied into RAM, get_user_pages() may not work, O_DIRECT may not work, and neither might RDMA, sendfile() and splice().

Some relevant things to look at include:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/block/Kconfig?h=v3.19#n359

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/block/Kconfig?h=v3.19#n396

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/blockdev/ramdisk.txt?h=v3.19

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/xip.txt?h=v3.19

Note XIP was replaced by DAX since 4.0 kernel so there see:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/dax.txt?h=4.14

Also note that support for DAX was removed from block RAM driver with kernel 4.15 so you will no longer be able to do this once you move to kernel 4.15 and later... See commit 7a862fbbdec665190c5ef298c0c6ec9f3915cf45 for the reasoning behind removing the functionality.

I hope this is enough to set you on the right track and sorry about the bad news that the functionality has been removed since 4.15 kernel...