Re: [PATCH v2] Introduce a version6 of autofs interface, to fix design error.
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On Sun, Apr 29, 2012 at 12:45 AM, Michael Tokarev <mjt@xxxxxxxxxx> wrote: > On 29.04.2012 11:19, Linus Torvalds wrote: >> >> Gaah, it should, but it won't. >> >> I bet my original patch worked fine, because the pipe has only one >> inode and pipe structure. But it has *two* 'struct file's associated >> with it, and autofs only ever sees the writing side, and never gets to >> mark the reading side O_DIRECT. So yeah, the reading side won't do the >> proper packetized read. > > Can't we go - in kernel - from one struct file to pipe structure to > another file structure and set O_DIRECT there? Autofs kernel code > checks if the file descriptor is a pipe, so it should be possible... Nope. We don't have any back-pointers to the other "struct file". They have the inode - and the "struct pipe_inode_info" in common, but there's no way to find the other "struct file" from that. And sure, a pipe in Linux could work with a single read-write "struct file" that is used both for the reading and writing, but that's not how the "pipe()" system call is defined - it gets two separate file descriptors, one read-only, one write-only. So that's what autofs and systemd hands the kernel - the write side of the pipe. And we can't find the read-side one. But I think I have another approach. We can make the *writer* be the only one that cares about the packetized nature, and if it's a packet write, it will set a PIPE_BUF_PACKET flag in the pipe buffer. The reader can react to that. I think that actually has the potential to make the code a bit prettier too.. I'll cook it up. Linus -- To unsubscribe from this list: send the line "unsubscribe autofs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html