On Fri, Dec 28, 2018 at 11:18:18AM +0000, Peter Maydell wrote: > In general inodes and offsets start from 0 and work up -- > so almost all of the time they don't actually overflow. > The problem with ext4 directory hash "offsets" is that they > overflow all the time and immediately, so instead of "works > unless you have a weird edge case" like all the other filesystems,h > it's "never works". Actually, XFS uses the inode number to encode the location of the inode (it doesn't have a fixed inode table, so it's effectively the block number shifted left by 3 or 4 bits, with the low bits indicating the slot in the 4k block). It has a hack to provide backwards compatibility for 32-bit API's, but there is a similar, "oh, we're on a non-paleolithic CPU, let's use the full 64-bits" sort of logic that ext4 has. > The problem is that there is no 32-bit API in some cases > (unless I have misunderstood the kernel code) -- not all > host architectures implement compat syscalls or allow them > to be called from 64-bit processes or implement all the older > syscall variants that had smaller offets. If there was a guaranteed > "this syscall always exists and always gives me 32-bit offsets" > we could use it. Are there going to be cases where a process or a thread will sometimes want the 64-bit interface, and sometimes want the 32-bit interface? Or is it always going to be one or the other? I wonder if we could simply add a new flag to the process personality(2) flags. > Yes, that has been suggested, but it seemed a bit dubious > to bake in knowledge of ext4's internal implementation details. > Can we rely on this as an ABI promise that will always work > for all versions of all file systems going forwards? Yeah, that seems dubious because I'm pretty sure there are other file systems that may have their own 32/64-bit quirks. - Ted
