Re: [PATCH v2]Btrfs: pwrite blocked when writing from the mmaped buffer of the same page

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Mitch Harder's message of 2011-02-25 13:43:37 -0500:
> > The loop is more-or-less following this process (from within the
> > "while (iov_iter_count(&i) > 0) {}" loop):
> >
> > (1) Reserve some space with btrfs_delalloc_reserve_space()
> > (2) Prepare the reserved space with prepare_pages()
> > (3) Call btrfs_copy_from_user() to copy to the prepared space.
> > -------------> From btrfs_copy_from_user()
> > (4) ........Try to copy with copied = iov_iter_copy_from_user_atomic()
> > (5) ........The above operation results with copied == 0. Break and
> > return with a return value of 0 bytes copied.
> > (6) There is no special handling for copied == 0 in the "while
> > (iov_iter_count(&i) > 0) {}" loop, so it loops back around, reserves
> > some more space, and tries again.
> >
> > If I look back at how the code was set up before the patch at the head
> > of this thread was applied (Btrfs: pwrite blocked when writing from
> > the mmaped buffer of the same page), the btrfs_copy_from_user()
> > function had some handling for "copied == 0" that would change the
> > scope of the amount to write, and loop back to try the write again.
> >
> > I attempted to construct a patch that just reverted the handling for
> > "copied == 0" in btrfs_copy_from_user(), however, that just resulted
> > in my computer locking up when it reached the point where it was
> > previously beginning to allocate disk space.
> >
> > So, I apologize for not having a patch to address the issue I'm
> > seeing, but I hope I've added some insight.
> >
> 
> Some clarification on my previous message...
> 
> After looking at my ftrace log more closely, I can see where Btrfs is
> trying to release the allocated pages.  However, the calculation for
> the number of dirty_pages is equal to 1 when "copied == 0".
> 
> So I'm seeing at least two problems:
> (1)  It keeps looping when "copied == 0".
> (2)  One dirty page is not being released on every loop even though
> "copied == 0" (at least this problem keeps it from being an infinite
> loop by eventually exhausting reserveable space on the disk).

Very nice, I think you're exactly right.  I should be able to reproduce this
now, thanks!

-chris