Re: [patch] call truncate_inode_pages in the DIO fallback to bufferedI/O path

[Zach Brown]

> Why is this a problem?  It's just like someone did a write(), and we'll
> invalidate the pagecache on the next direct-io operation.

This was noticed as a distro regression as they moved from the kernels
that used to invalidate the entire address space on direct io ops to
more modern ones that only invalidate the region being written.

You can end up with significant memory pressure after this change with a
large enough working set on disk.

> eek.  truncate_inode_pages() will throw away dirty data.  Very dangerous,
> much chin-scratching needed.

Yeah, I failed to tell Jeff that it should be calling
filemap_fdatawrite() first to get things into writeback.  (And
presumably not truncating if that returns an error.)
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