Re: [PATCH v6 00/19] Change readahead API

[John Hubbard]

On 2/17/20 10:45 AM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> 
> This series adds a readahead address_space operation to eventually
> replace the readpages operation.  The key difference is that
> pages are added to the page cache as they are allocated (and
> then looked up by the filesystem) instead of passing them on a
> list to the readpages operation and having the filesystem add
> them to the page cache.  It's a net reduction in code for each
> implementation, more efficient than walking a list, and solves
> the direct-write vs buffered-read problem reported by yu kuai at
> https://lore.kernel.org/linux-fsdevel/20200116063601.39201-1-yukuai3@xxxxxxxxxx/
> 
> The only unconverted filesystems are those which use fscache.
> Their conversion is pending Dave Howells' rewrite which will make the
> conversion substantially easier.

Hi Matthew,

I see that Dave Chinner is reviewing this series, but I'm trying out his recent
advice about code reviews [1], and so I'm not going to read his comments first.
So you may see some duplication or contradictions this time around.


[1] Somewhere in this thread, "[LSF/MM/BPF TOPIC] FS Maintainers Don't Scale": 
https://lore.kernel.org/r/20200131052520.GC6869@magnolia


thanks,
-- 
John Hubbard
NVIDIA

> 
> v6:
>  - Name the private members of readahead_control with a leading underscore
>    (suggested by Christoph Hellwig)
>  - Fix whitespace in rst file
>  - Remove misleading comment in btrfs patch
>  - Add readahead_next() API and use it in iomap
>  - Add iomap_readahead kerneldoc.
>  - Fix the mpage_readahead kerneldoc
>  - Make various readahead functions return void
>  - Keep readahead_index() and readahead_offset() pointing to the start of
>    this batch through the body.  No current user requires this, but it's
>    less surprising.
>  - Add kerneldoc for page_cache_readahead_limit
>  - Make page_idx an unsigned long, and rename it to just 'i'
>  - Get rid of page_offset local variable
>  - Add patch to call memalloc_nofs_save() before allocating pages (suggested
>    by Michal Hocko)
>  - Resplit a lot of patches for more logical progression and easier review
>    (suggested by John Hubbard)
>  - Added sign-offs where received, and I deemed still relevant
> 
> v5 switched to passing a readahead_control struct (mirroring the
> writepages_control struct passed to writepages).  This has a number of
> advantages:
>  - It fixes a number of bugs in various implementations, eg forgetting to
>    increment 'start', an off-by-one error in 'nr_pages' or treating 'start'
>    as a byte offset instead of a page offset.
>  - It allows us to change the arguments without changing all the
>    implementations of ->readahead which just call mpage_readahead() or
>    iomap_readahead()
>  - Figuring out which pages haven't been attempted by the implementation
>    is more natural this way.
>  - There's less code in each implementation.
> 
> Matthew Wilcox (Oracle) (19):
>   mm: Return void from various readahead functions
>   mm: Ignore return value of ->readpages
>   mm: Use readahead_control to pass arguments
>   mm: Rearrange readahead loop
>   mm: Remove 'page_offset' from readahead loop
>   mm: rename readahead loop variable to 'i'
>   mm: Put readahead pages in cache earlier
>   mm: Add readahead address space operation
>   mm: Add page_cache_readahead_limit
>   fs: Convert mpage_readpages to mpage_readahead
>   btrfs: Convert from readpages to readahead
>   erofs: Convert uncompressed files from readpages to readahead
>   erofs: Convert compressed files from readpages to readahead
>   ext4: Convert from readpages to readahead
>   f2fs: Convert from readpages to readahead
>   fuse: Convert from readpages to readahead
>   iomap: Restructure iomap_readpages_actor
>   iomap: Convert from readpages to readahead
>   mm: Use memalloc_nofs_save in readahead path
> 
>  Documentation/filesystems/locking.rst |   6 +-
>  Documentation/filesystems/vfs.rst     |  13 ++
>  drivers/staging/exfat/exfat_super.c   |   7 +-
>  fs/block_dev.c                        |   7 +-
>  fs/btrfs/extent_io.c                  |  46 ++-----
>  fs/btrfs/extent_io.h                  |   3 +-
>  fs/btrfs/inode.c                      |  16 +--
>  fs/erofs/data.c                       |  39 ++----
>  fs/erofs/zdata.c                      |  29 ++--
>  fs/ext2/inode.c                       |  10 +-
>  fs/ext4/ext4.h                        |   3 +-
>  fs/ext4/inode.c                       |  23 ++--
>  fs/ext4/readpage.c                    |  22 ++-
>  fs/ext4/verity.c                      |  35 +----
>  fs/f2fs/data.c                        |  50 +++----
>  fs/f2fs/f2fs.h                        |   5 +-
>  fs/f2fs/verity.c                      |  35 +----
>  fs/fat/inode.c                        |   7 +-
>  fs/fuse/file.c                        |  46 +++----
>  fs/gfs2/aops.c                        |  23 ++--
>  fs/hpfs/file.c                        |   7 +-
>  fs/iomap/buffered-io.c                | 118 +++++++----------
>  fs/iomap/trace.h                      |   2 +-
>  fs/isofs/inode.c                      |   7 +-
>  fs/jfs/inode.c                        |   7 +-
>  fs/mpage.c                            |  38 ++----
>  fs/nilfs2/inode.c                     |  15 +--
>  fs/ocfs2/aops.c                       |  34 ++---
>  fs/omfs/file.c                        |   7 +-
>  fs/qnx6/inode.c                       |   7 +-
>  fs/reiserfs/inode.c                   |   8 +-
>  fs/udf/inode.c                        |   7 +-
>  fs/xfs/xfs_aops.c                     |  13 +-
>  fs/zonefs/super.c                     |   7 +-
>  include/linux/fs.h                    |   2 +
>  include/linux/iomap.h                 |   3 +-
>  include/linux/mpage.h                 |   4 +-
>  include/linux/pagemap.h               |  90 +++++++++++++
>  include/trace/events/erofs.h          |   6 +-
>  include/trace/events/f2fs.h           |   6 +-
>  mm/internal.h                         |   8 +-
>  mm/migrate.c                          |   2 +-
>  mm/readahead.c                        | 184 +++++++++++++++++---------
>  43 files changed, 474 insertions(+), 533 deletions(-)
> 
> 
> base-commit: 11a48a5a18c63fd7621bb050228cebf13566e4d8
>