Re: [RFC v6 04/10] iomap: Add iomap_get_folio helper

[Dave Chinner <david@fromorbit.com>]

On Sun, Jan 08, 2023 at 08:40:28PM +0100, Andreas Gruenbacher wrote:
> Add an iomap_get_folio() helper that gets a folio reference based on
> an iomap iterator and an offset into the address space.  Use it in
> iomap_write_begin().
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap/buffered-io.c | 39 ++++++++++++++++++++++++++++++---------
>  include/linux/iomap.h  |  1 +
>  2 files changed, 31 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d4b444e44861..de4a8e5f721a 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -457,6 +457,33 @@ bool iomap_is_partially_uptodate(struct folio *folio, size_t from, size_t count)
>  }
>  EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);
>  
> +/**
> + * iomap_get_folio - get a folio reference for writing
> + * @iter: iteration structure
> + * @pos: start offset of write
> + *
> + * Returns a locked reference to the folio at @pos, or an error pointer if the
> + * folio could not be obtained.
> + */
> +struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos)
> +{
> +	unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
> +	struct folio *folio;
> +
> +	if (iter->flags & IOMAP_NOWAIT)
> +		fgp |= FGP_NOWAIT;
> +
> +	folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
> +			fgp, mapping_gfp_mask(iter->inode->i_mapping));
> +	if (folio)
> +		return folio;
> +
> +	if (iter->flags & IOMAP_NOWAIT)
> +		return ERR_PTR(-EAGAIN);
> +	return ERR_PTR(-ENOMEM);
> +}
> +EXPORT_SYMBOL_GPL(iomap_get_folio);

Hmmmm.

This is where things start to get complex. I have sent a patch to
fix a problem with iomap_zero_range() failing to zero cached dirty
pages over UNWRITTEN extents, and that requires making FGP_CREAT
optional. This is an iomap bug, and needs to be fixed in the core
iomap code:

https://lore.kernel.org/linux-xfs/20221201005214.3836105-1-david@fromorbit.com/

Essentially, we need to pass fgp flags to iomap_write_begin() need
so the callers can supply a 0 or FGP_CREAT appropriately. This
allows iomap_write_begin() to act only on pre-cached pages rather
than always instantiating a new page if one does not exist in cache.

This allows that iomap_write_begin() to return a NULL folio
successfully, and this is perfectly OK for callers that pass in fgp
= 0 as they are expected to handle a NULL folio return indicating
there was no cached data over the range...

Exposing the folio allocation as an external interface makes bug
fixes like this rather messy - it's taking a core abstraction (iomap
hides all the folio and page cache manipulations from the
filesystem) and punching a big hole in it by requiring filesystems
to actually allocation page cache folios on behalf of the iomap
core.

Given that I recently got major push-back for fixing an XFS-only bug
by walking the page cache directly instead of abstracting it via the
iomap core, punching an even bigger hole in the abstraction layer to
fix a GFS2-only problem is just as bad....

-Dave.
-- 
Dave Chinner
david@fromorbit.com