Re: [PATCH 1/3] fs: Introduce new flag FALLOC_FL_COLLAPSE_RANGE

From: Dave Chinner
Date: Wed Jul 31 2013 - 20:55:04 EST

On Wed, Jul 31, 2013 at 08:46:45PM -0400, Theodore Ts'o wrote:
> On Thu, Aug 01, 2013 at 10:23:41AM +1000, Dave Chinner wrote:
> > On Wed, Jul 31, 2013 at 06:01:54PM -0400, Theodore Ts'o wrote:
> > > Have you considered what happens if you have a 10 megabyte file, of
> > > which the first 5 megs are mmap'ed into a userspace process.
> > >
> > > Now suppose you call COLLAPASE_RANGE on a one megabyte range starting
> > > at offset 1024k from the beginning of the file.
> > >
> > > Does the right thing happen to the mmap'ed region in memory?
> >
> > Implementation detail. like a hole punch, it needs to invalidate the
> > range that it is operating over so mmap()d regions are refaulted
> > after the operation is done.
> It's not just the range that it's operating on, but also the region
> beyond the range that's been collapsed out.

Yes, that's part of "the range that it is operating over".

> A quick eyeball of the patch didn't seem to show any code that handled
> this, which is why I asked the question.

Right, but really it's the least of the problems I've noticed - the
XFS code is fundamentally broken in many ways - once I've finished
commenting on it, I'll have a quick look to see if the ext4 code has
the same fundamental flaws....



Dave Chinner
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