Re: [PATCH v6 08/19] mm: Add readahead address space operation

Dave Chinner
2020-02-18T06:21:55Z

On Mon, Feb 17, 2020 at 10:45:54AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> This replaces ->readpages with a saner interface:
> - Return void instead of an ignored error code.
> - Pages are already in the page cache when ->readahead is called.

Might read better as:

- Page cache is already populates with locked pages when
->readahead is called.

> - Implementation looks up the pages in the page cache instead of
> having them passed in a linked list.

Add:

- cleanup of unused readahead handled by ->readahead caller, not
the method implementation.

>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> Documentation/filesystems/locking.rst | 6 +++++-
> Documentation/filesystems/vfs.rst | 13 +++++++++++++
> include/linux/fs.h | 2 ++
> include/linux/pagemap.h | 18 ++++++++++++++++++
> mm/readahead.c | 8 +++++++-
> 5 files changed, 45 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> index 5057e4d9dcd1..0ebc4491025a 100644
> --- a/Documentation/filesystems/locking.rst
> +++ b/Documentation/filesystems/locking.rst
> @@ -239,6 +239,7 @@ prototypes::
> int (*readpage)(struct file *, struct page *);
> int (*writepages)(struct address_space *, struct writeback_control *);
> int (*set_page_dirty)(struct page *page);
> + void (*readahead)(struct readahead_control *);
> int (*readpages)(struct file *filp, struct address_space *mapping,
> struct list_head *pages, unsigned nr_pages);
> int (*write_begin)(struct file *, struct address_space *mapping,
> @@ -271,7 +272,8 @@ writepage: yes, unlocks (see below)
> readpage: yes, unlocks
> writepages:
> set_page_dirty no
> -readpages:
> +readahead: yes, unlocks
> +readpages: no
> write_begin: locks the page exclusive
> write_end: yes, unlocks exclusive
> bmap:
> @@ -295,6 +297,8 @@ the request handler (/dev/loop).
> ->readpage() unlocks the page, either synchronously or via I/O
> completion.
>
> +->readahead() unlocks the pages like ->readpage().
> +

"... the pages that I/O is attempted on ..."

> ->readpages() populates the pagecache with the passed pages and starts
> I/O against them. They come unlocked upon I/O completion.
>
> diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
> index 7d4d09dd5e6d..81ab30fbe45c 100644
> --- a/Documentation/filesystems/vfs.rst
> +++ b/Documentation/filesystems/vfs.rst
> @@ -706,6 +706,7 @@ cache in your filesystem. The following members are defined:
> int (*readpage)(struct file *, struct page *);
> int (*writepages)(struct address_space *, struct writeback_control *);
> int (*set_page_dirty)(struct page *page);
> + void (*readahead)(struct readahead_control *);
> int (*readpages)(struct file *filp, struct address_space *mapping,
> struct list_head *pages, unsigned nr_pages);
> int (*write_begin)(struct file *, struct address_space *mapping,
> @@ -781,12 +782,24 @@ cache in your filesystem. The following members are defined:
> If defined, it should set the PageDirty flag, and the
> PAGECACHE_TAG_DIRTY tag in the radix tree.
>
> +``readahead``
> + Called by the VM to read pages associated with the address_space
> + object. The pages are consecutive in the page cache and are
> + locked. The implementation should decrement the page refcount
> + after starting I/O on each page. Usually the page will be
> + unlocked by the I/O completion handler. If the function does
> + not attempt I/O on some pages, the caller will decrement the page
> + refcount and unlock the pages for you. Set PageUptodate if the
> + I/O completes successfully. Setting PageError on any page will
> + be ignored; simply unlock the page if an I/O error occurs.
> +
> ``readpages``
> called by the VM to read pages associated with the address_space
> object. This is essentially just a vector version of readpage.
> Instead of just one page, several pages are requested.
> readpages is only used for read-ahead, so read errors are
> ignored. If anything goes wrong, feel free to give up.
> + This interface is deprecated; implement readahead instead.

What is the removal schedule for the deprecated interface?

> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 3613154e79e4..bd4291f78f41 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -665,6 +665,24 @@ static inline void readahead_next(struct readahead_control *rac)
> #define readahead_for_each(rac, page) \
> for (; (page = readahead_page(rac)); readahead_next(rac))
>
> +/* The byte offset into the file of this readahead block */
> +static inline loff_t readahead_offset(struct readahead_control *rac)
> +{
> + return (loff_t)rac->_start * PAGE_SIZE;
> +}

Urk. Didn't an early page use "offset" for the page index? That
was was "mm: Remove 'page_offset' from readahead loop" did, right?

That's just going to cause confusion to have different units for
readahead "offsets"....

> +
> +/* The number of bytes in this readahead block */
> +static inline loff_t readahead_length(struct readahead_control *rac)
> +{
> + return (loff_t)rac->_nr_pages * PAGE_SIZE;
> +}
> +
> +/* The index of the first page in this readahead block */
> +static inline unsigned int readahead_index(struct readahead_control *rac)
> +{
> + return rac->_start;
> +}

Based on this, I suspect the earlier patch should use "index" rather
than "offset" when walking the page cache indexes...

> +
> /* The number of pages in this readahead block */
> static inline unsigned int readahead_count(struct readahead_control *rac)
> {
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 9e430daae42f..975ff5e387be 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -121,7 +121,13 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages)
>
> blk_start_plug(&plug);
>
> - if (aops->readpages) {
> + if (aops->readahead) {
> + aops->readahead(rac);
> + readahead_for_each(rac, page) {
> + unlock_page(page);
> + put_page(page);
> + }

This needs a comment to explain the unwinding that needs to be done
here. I'm not going to remember in a year's time that this is just
for the pages that weren't submitted by ->readahead....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com