Re: [PATCH v2 5/5] xfs: revalidate imap properly before writeback delalloc conversion

[Christoph Hellwig <hch@infradead.org>]

On Fri, Jan 18, 2019 at 01:39:15PM -0500, Brian Foster wrote:
> This doesn't really seem all that different to me. Rather than firm up
> the range in the caller, we turn XFS_BMAPI_DELALLOC into something that
> seemingly behaves a bit more like CONVERT_ONLY.

The differences are:

 (a) we save one lookup in the extent tree
 (b) we have a less fragile API

> A few notes/thoughts:
> 
> 1. What's the purpose of the nimaps == 0 check in
> xfs_iomap_write_allocate()? If we got to this point with the page lock
> held, shouldn't we always expect to find something backing the current
> page?

It protects against the case where we migrate a COW fork mapping to the
data fork, which is not protected by the page lock.  But I guess the
check warrants a comment and an assert.

> 2. If so, then it also seems that the whole "eof:" thing in
> xfs_map_blocks() should never happen for data forks. If that's the case,
> the use of the eof label there seems gratuitous.

Let me try with asserts enabled.

> 
> 3. If the starting bno param to xfs_bmapi_write() lands in a hole (due
> to racing with a hole punch for example) and it finds some subsequent
> delalloc extent to convert in the requested range, the arithmetic in
> xfs_bmapi_trim_map() actually fabricates physical blocks for the hole
> portion of the range relative to the startblock of the converted
> delalloc extent. I don't think that causes a problem with the writeback
> code because the fabricated blocks are before the page offset, but those
> semantics are insane. I think you need to do something like fix up
> xfs_bmapi_write() to return the actual converted mapping and make sure
> xfs_iomap_write_allocate() handles that it might start beyond
> map_start_fsb.

Sounds good.  To be honest I think the whole idea of converting the
mapping before the requested offset is a rather bad idea to start
with, just increasin our window that exposes stale data.  But to fix
this properly we need to create everything as unwritten first, and
I didn't have time to get back to that series.