linux-kernel - Re: [PATCH 3/14] tmpfs: take control of its truncate

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Date:	Wed, 1 Jun 2011 09:58:18 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Christoph Hellwig <hch@...radead.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/14] tmpfs: take control of its truncate_range

Thanks a lot for looking at these.

On Tue, 31 May 2011, Christoph Hellwig wrote:
> > Note that drivers/gpu/drm/i915/i915_gem.c i915_gem_object_truncate()
> > calls the tmpfs ->truncate_range directly: update that in a separate
> > patch later, for now just let it duplicate the truncate_inode_pages().
> > Because i915 handles unmap_mapping_range() itself at a different stage,
> > we have chosen not to bundle that into ->truncate_range.
> 
> In your next series that makes it call the readpae replacement directly
> it might be nice to also call directly into shmem for hole punching.

(i915 isn't really doing hole-punching there, I think it just found it
a useful interface to remove the page-and-swapcache without touching
i_size.  Parentheses because it makes no difference to your point.)

Okay, I'd better do a v2 (probably not before the weekend), and change
that around to go explicitly to shmem there as well: I'd rather settle
the interfaces to other subsystems in this series, than mix it with the
implementation in the next series.

When I say "shmem", I am including the !SHMEM-was-TINY_SHMEM case too,
which goes to ramfs.  Currently i915 has been configured to disable that
possibility, though we insisted on it originally: there may or may not be
good reason for disabling it - may just be a side-effect of the rather
twisted unintuitive SHMEM/TMPFS dependencies.

> 
> > I notice that ext4 is now joining ocfs2 and xfs in supporting fallocate
> > FALLOC_FL_PUNCH_HOLE: perhaps they should support truncate_range, and
> > tmpfs should support fallocate?  But worry about that another time...
> 
> No, truncate_range and the madvice interface are pretty sad hacks that
> should never have been added in the first place.  Adding
> FALLOC_FL_PUNCH_HOLE support for shmem on the other hand might make
> some sense.

Fine, I'll add tmpfs PUNCH_HOLE later on.  And wire up madvise MADV_REMOVE
to fallocate PUNCH_HOLE, yes?

Would you like me to remove the ->truncate_range method from
inode_operations completely?  I can do that now, hack directly to tmpfs
in the interim, in the same way as for i915.

Hugh
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