Re: [PATCH 3/5] vfs: add a zero-initialization mode to fallocate

Darrick J. Wong
2021-09-20T18:51:27Z

On Mon, Sep 20, 2021 at 10:52:09AM -0700, Eric Biggers wrote:
> On Fri, Sep 17, 2021 at 06:31:01PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> >
> > Add a new mode to fallocate to zero-initialize all the storage backing a
> > file.
> >
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> > fs/open.c | 5 +++++
> > include/linux/falloc.h | 1 +
> > include/uapi/linux/falloc.h | 9 +++++++++
> > 3 files changed, 15 insertions(+)
> >
> >
> > diff --git a/fs/open.c b/fs/open.c
> > index daa324606a41..230220b8f67a 100644
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -256,6 +256,11 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> > (mode & ~FALLOC_FL_INSERT_RANGE))
> > return -EINVAL;
> >
> > + /* Zeroinit should only be used by itself and keep size must be set. */
> > + if ((mode & FALLOC_FL_ZEROINIT_RANGE) &&
> > + (mode != (FALLOC_FL_ZEROINIT_RANGE | FALLOC_FL_KEEP_SIZE)))
> > + return -EINVAL;
> > +
> > /* Unshare range should only be used with allocate mode. */
> > if ((mode & FALLOC_FL_UNSHARE_RANGE) &&
> > (mode & ~(FALLOC_FL_UNSHARE_RANGE | FALLOC_FL_KEEP_SIZE)))
> > diff --git a/include/linux/falloc.h b/include/linux/falloc.h
> > index f3f0b97b1675..4597b416667b 100644
> > --- a/include/linux/falloc.h
> > +++ b/include/linux/falloc.h
> > @@ -29,6 +29,7 @@ struct space_resv {
> > FALLOC_FL_PUNCH_HOLE | \
> > FALLOC_FL_COLLAPSE_RANGE | \
> > FALLOC_FL_ZERO_RANGE | \
> > + FALLOC_FL_ZEROINIT_RANGE | \
> > FALLOC_FL_INSERT_RANGE | \
> > FALLOC_FL_UNSHARE_RANGE)
> >
> > diff --git a/include/uapi/linux/falloc.h b/include/uapi/linux/falloc.h
> > index 51398fa57f6c..8144403b6102 100644
> > --- a/include/uapi/linux/falloc.h
> > +++ b/include/uapi/linux/falloc.h
> > @@ -77,4 +77,13 @@
> > */
> > #define FALLOC_FL_UNSHARE_RANGE 0x40
> >
> > +/*
> > + * FALLOC_FL_ZEROINIT_RANGE is used to reinitialize storage backing a file by
> > + * writing zeros to it. Subsequent read and writes should not fail due to any
> > + * previous media errors. Blocks must be not be shared or require copy on
> > + * write. Holes and unwritten extents are left untouched. This mode must be
> > + * used with FALLOC_FL_KEEP_SIZE.
> > + */
> > +#define FALLOC_FL_ZEROINIT_RANGE 0x80
> > +
>
> How does this differ from ZERO_RANGE? Especially when the above says that
> ZEROINIT_RANGE leaves unwritten extents untouched. That implies that unwritten
> extents are still "allowed". If that's the case, why not just use ZERO_RANGE?

(Note: I changed the second to last sentence to read "Holes are ignored,
and inline data regions are not supported.")

ZERO_RANGE only guarantees that a subsequent read returns all zeroes,
which means that implementations are allowed to play games with the file
mappings to make that happen with as little work as possible.

ZEROINIT_RANGE implies that the filesystem doesn't change the mappings
at all and actually writes/resets the mapped storage, though I didn't
want to exclude the possibility that the filesystem could change the
mapping as a last resort to guarantee that "subsequent read and writes
should not fail" if the media write fails.

I /could/ encode all that in the definition, but that feels like
overspecifying the implementation.

--D

> - Eric