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Ts'o" To: Peter Maydell CC: Andreas Dilger , Florian Weimer , linux-fsdevel , Linux API , Ext4 Developers List , , , Arnd Bergmann , , , lkml - Kernel Mailing List , QEMU Developers , , Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation Message-ID: <20181229021157.GG5864@mit.edu> Mail-Followup-To: "Theodore Y. 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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