Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Mar 19, 2013 at 10:10:32PM +0000, Al Viro wrote:

> OK, it's going to be an interesting series - aforementioned tentative patch
> was badly incomplete ;-/

The interesting question is how far do we want to lift that.  ->aio_write()
part is trivial - see vfs.git#experimental; the trouble begins with
->splice_write().  For *everything* except default_file_splice_write(),
lifting into the caller (do_splice_from()) is the right thing to do.

default_file_splice_write(), however, it trickier; there we end up calling
vfs_write() (via an ugly callchain).  And _that_ is a real bitch.  Granted,
vfs_write() is somewhat an overkill there (we'd already done rw_verify_area()
and access_ok() is pointless due to set_fs() we do around vfs_write()
there) and we'd already lifted it up to do_sync_write().  But if we lift
it any further, we'll need to deal with ->write() callers in the tree.
Current situation:

fs/coredump.c:662:      return access_ok(VERIFY_READ, addr, nr) && file->f_op->write(file, addr, nr, &file->f_pos) == nr;
arch/powerpc/platforms/cell/spufs/coredump.c:63:        written = file->f_op->write(file, addr, nr, &file->f_pos);

for these guys we might actually want to lift all way up to do_coredump()

drivers/staging/comedi/drivers/serial2002.c:91: result = f->f_op->write(f, buf, count, &f->f_pos);
fs/autofs4/waitq.c:73:         (wr = file->f_op->write(file,data,bytes,&file->f_pos)) > 0) {

not regular files, unless I'm seriously misreading the code.

kernel/acct.c:553:      file->f_op->write(file, (char *)&ac,

BTW, this is probably where we want to deal with your acct deadlock.

fs/compat.c:1103:               fn = (io_fn_t)file->f_op->write;
fs/read_write.c:435:                    ret = file->f_op->write(file, buf, count, pos);
fs/read_write.c:732:            fn = (io_fn_t)file->f_op->write;

syscalls - the question here is whether we lift it up to vfs_write/vfs_writev/
compat_writev, or actually take it further.

fs/cachefiles/rdwr.c:967:                       ret = file->f_op->write(

cachefiles_write_page(); no fucking idea what locks might be held by caller
and potentially that's a rather nasty source of PITA

fs/coda/file.c:84:      ret = host_file->f_op->write(host_file, buf, count, ppos);

coda writing to file in cache on local fs.  Potentially a nasty bugger, since
it's hard to lift any further - the caller has no idea that the thing is
on CODA, let alone what happens to hold the local cache.

drivers/block/loop.c:234:       bw = file->f_op->write(file, buf, len, &pos);

do_bio_filebacked(), with some ugliness between that and callsite.  Note,
BTW, that we have a pair of possible vfs_fsync() calls in there; how do those
interact with freeze?

This does *not* touch the current callers of vfs_write()/vfs_writev(); any of
those called while holding ->i_mutex on a directory (or mnt_want_write(), for
that matter) is a deadlock right now.

And we'd better start thinking about how we'll backport that crap - deadlock
in e.g. xfs ->splice_write() had been there since last summer ;-/