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Add interface for file hole punching #168

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@dechamps dechamps commented Sep 3, 2012

This adds an interface to "punch holes" (deallocate space) in VFS files. The interface is identical to the Solaris VOP_SPACE interface. This interface is necessary for TRIM support on file vdevs (see openzfs/zfs#924).

This is implemented using Linux fallocate(FALLOC_FL_PUNCH_HOLE), which was introduced in 2.6.38. For a brief time before 2.6.38 this was done using the truncate_range inode operation, which was quickly deprecated. This patch only supports FALLOC_FL_PUNCH_HOLE.

@behlendorf: like the previous patch, you can either wait for ZFS TRIM support to be finished or you can merge this now. Your call.

This adds an interface to "punch holes" (deallocate space) in VFS
files. The interface is identical to the Solaris VOP_SPACE interface.
This interface is necessary for TRIM support on file vdevs.

This is implemented using Linux fallocate(FALLOC_FL_PUNCH_HOLE), which
was introduced in 2.6.38. For a brief time before 2.6.38 this was done
using the truncate_range inode operation, which was quickly deprecated.
This patch only supports FALLOC_FL_PUNCH_HOLE.
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@dechamps I'm going to hold of merging this until -rc12 since I want to get something known stable tagged soonish.

Currently, VOP_SPACE() returns a negative error code in some cases. It
shouldn't. This patch fixes that.
@dechamps
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Don't merge this just yet, I'll be adding truncate_range support shortly. After that's done just squash everything together.

Reverts a remarkably dumb part of
0c0cb0d.

"Yo Dawg, we heard you like configure checks so we put a #define in
your #define so you can #ifdef while you #ifdef"
@dechamps
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Okay, this is ready for merging now. Just squash everything together and you're good to go.

This adds support for the truncate_range() inode operation to
VOP_SPACE() for file hole punching. This API is deprecated and removed
in 3.5, so it's only useful for old kernels.
On tmpfs, the truncate_range() inode operation translates to
shmem_truncate_range(). Unfortunately, this function expects the end
offset to be inclusive and aligned to the end of a page. If it is not,
the kernel will stop with a BUG_ON().

This patch fixes the issue by adapting to the constraints set forth by
shmem_truncate_range().
@dechamps
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dechamps commented Oct 5, 2012

@behlendorf: now would probably be a good time to merge this.

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Yup. I was working on pulling it in yesterday I'll get it finished today.

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