On Wed, Feb 07, 2007 at 03:56:15PM +0000, Hugh Dickins wrote:

On Thu, 8 Feb 2007, David Chinner wrote:

quoted

On Wed, Feb 07, 2007 at 01:00:28PM +0000, Hugh Dickins wrote:

quoted

I'm worried about concurrent truncation.  Isn't it the case that
i_mutex is held when prepare_write and commit_write are normally
called?  But not here when page_mkwrite is called.

I'm not holding i_mutex. I assumed that it was probably safe to do
because we are likely to be reading the page off disk just before we
call mkwrite and that has to be synchronised with truncate in some
manner....

"assumed"..."probably"..."likely"..."just before"..."in some manner"
doesn't sound very safe, does it :-?

You're right, it doesn't sound safe, does it? Why do you think I
posted the patches for comment?

But page_mkwrite is something new: so far, it's up to the implementor
(the filesystem) to work out how to guard against truncation.

Ok. I can do that now I know I have to.

quoted

So, do I need to grab the i_mutex here? Is that safe to do that in
the middle of a page fault?

It's certainly easier to think about if you don't grab i_mutex there:
sys_msync used to take i_mutex within down_read of mmap_sem, but we
were quite happy to get rid of that, because usually it's down_read
of mmap_sem within i_mutex (page fault when writing from userspace
to file).  I can't at this moment put my finger on an actual deadlock
if you take i_mutex in page_mkwrite, but it feels wrong: hmm, if you
add another thread waiting to down_write the mmap_sem, I think you
would be able to deadlock.

Right, so i_mutex is out. That needs to be commented in big flashing
neon lights somewhere in the code.

You don't need to lock out all truncation, but you do need to lock
out truncation of the page in question.  Instead of your i_size
checks, check page->mapping isn't NULL after the lock_page?

Yes, that can be done, but we still need to know if part of
the page is beyond EOF for when we call block_commit_write()
and mark buffers dirty. Hence we need to check the inode size.

I guess if we block the truncate with the page lock, then the
inode size is not going to change until we unlock the page.
If the inode size has already been changed but the page not yet
removed from the mapping we'll be beyond EOF.

So it seems to me that we can get away with not using the i_mutex
in the generic code here.

But aside from the truncation issue, if prepare_write and commit_write
are always called with i_mutex held at present, I'm doubtful whether
you can provide a generic default page_mkwrite which calls them without.
Which would take us back to grabbing i_mutex within page_mkwrite.  Ugh.

The only thing that is asserted as a requirement for
block_prepare_write is that the page is locked. Apart fromteh page
truncation issue, it is safe to do this at least on XFS because it
has internal locks that ensure sanity even when the i_mutex is not
held.  If a particular filesystem has different locking
requirements, they can be met in the filesystem wrapper function
(e.g.  xfs_vm_page_mkwrite()) which calls block_page_mkwrite().

quoted

If we do race with a truncate and the
page is now beyond EOF, what am I supposed to return?

Something negative.  Nothing presently reports the error code in
question, it just does SIGBUS; but it would be better for the
truncation case to avoid -ENOMEM and -ENOSPC, which could easily
have meanings here.  I don't see a good choice, so maybe -EINVAL.

Ok.

quoted

I'm fishing for what I'm supposed to be doing here because there's
zero implementations of this callout in the kernel and the comments
in the code explaining the interface constraints are
non-existant....

Well, you seem to be the first to implement it.  Hmm, that's not true,
David was: what magic saved him from addressing the truncation issue?

No idea. His code is not in mainline....

Don't be surprised if it turns out page_mkwrite needs more thought.

I'll add a patch to the series adding some comments on the restrictions
placed on implementers of this function.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>