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

[Jeff Moyer <jmoyer@redhat.com>]

==> Regarding Re: [patch] call truncate_inode_pages in the DIO fallback to buffered I/O path; Zach Brown <zach.brown@oracle.com> adds:

>> 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.

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

Right.

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

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

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

Ahh, that explains it.  The strange thing is that my test validates the
file afterwards, and I was seeing correct data.

I'll repost after another round of testing.

Thanks!

Jeff
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