Messages in this thread | | | From | "Andrew Tipton" <> | Subject | Tmpfs and loop device don't get along | Date | Fri, 1 Feb 2002 16:19:50 -0500 |
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If I have an image (initrd.img for example) located on a tmpfs filesystem, when I attempt to mount it:
% mount -o loop /initrd.img /mnt ioctl: LOOP_SET_FD: Invalid argument
After deciding that it was not due to lack of loop devices (/dev/loop0..7), an unreadable file, and other common userspace causes, I dug through the kernel sources. When calling LOOP_SET_FD, the single argument is the filedescriptor of the file (/initrd.img). The loop_set_fd function (in drivers/block/loop.c) first gets the inode entry for the file, and then the address_space_operations struct for that inode. If there is no readpage function in that struct, loop_set_fd fails with EINVAL. The reason for this check is "if we can't read - sorry" -- apparently testing to see if it's possible to read from the file/filesystem?
In the tmpfs source (mm/shmem.c), the address_space_operations struct for this filesystem is defined. The struct (shmem_aops) is defined statically, and the only member is writepage. readpage is missing, causing a guaranteed failure in loop_set_fd.
Something smells wrong here (why does loop_set_fd care about address_space_operations, and not file_operations? it only needs access to the file....), but I don't know enough about the internal workings of the page cache/vfs/etc to do much about it.
What is the best way to fix this short-term? I could bypass the readpage check in loop_set_fd, or I could add a dummy readpage entry to the address_space_operations struct for tmpfs. Would either of these have serious repercussions?
I'm not subscribed to the linux-kernel mailing list, so I would appreciate it if you could CC all replies to me as well.
Thanks, Andrew Tipton Cadre5, LLC
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