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Message-ID: <5D7CE6BD-FABD-4901-AEF0-E0F10FC00EB1@nvidia.com>
Date: Thu, 20 Feb 2020 10:00:30 -0500
From: Zi Yan <ziy@...dia.com>
To: Matthew Wilcox <willy@...radead.org>
CC: <linux-fsdevel@...r.kernel.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, <linux-btrfs@...r.kernel.org>,
<linux-erofs@...ts.ozlabs.org>, <linux-ext4@...r.kernel.org>,
<linux-f2fs-devel@...ts.sourceforge.net>,
<cluster-devel@...hat.com>, <ocfs2-devel@....oracle.com>,
<linux-xfs@...r.kernel.org>
Subject: Re: [PATCH v7 10/24] mm: Add readahead address space operation
On 19 Feb 2020, at 16:00, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
>
> This replaces ->readpages with a saner interface:
> - Return void instead of an ignored error code.
> - Page cache is already populated with locked pages when ->readahead
> is called.
> - New arguments can be passed to the implementation without changing
> all the filesystems that use a common helper function like
> mpage_readahead().
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
> Documentation/filesystems/locking.rst | 6 +++++-
> Documentation/filesystems/vfs.rst | 15 +++++++++++++++
> include/linux/fs.h | 2 ++
> include/linux/pagemap.h | 18 ++++++++++++++++++
> mm/readahead.c | 12 ++++++++++--
> 5 files changed, 50 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> index 5057e4d9dcd1..0af2e0e11461 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 that I/O is attempted on like ->readpage().
> +
> ->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..ed17771c212b 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 +782,26 @@ 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 filesystem decides
> + to stop attempting I/O before reaching the end of the readahead
> + window, it can simply return. The caller will decrement the page
> + refcount and unlock the remaining 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 and will be removed by the end of
> + 2020; implement readahead instead.
>
> ``write_begin``
> Called by the generic buffered write code to ask the filesystem
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3cd4fe6b845e..d4e2d2964346 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -292,6 +292,7 @@ enum positive_aop_returns {
> struct page;
> struct address_space;
> struct writeback_control;
> +struct readahead_control;
>
> /*
> * Write life time hint values.
> @@ -375,6 +376,7 @@ struct address_space_operations {
> */
> int (*readpages)(struct file *filp, struct address_space *mapping,
> struct list_head *pages, unsigned nr_pages);
> + void (*readahead)(struct readahead_control *);
>
> int (*write_begin)(struct file *, struct address_space *mapping,
> loff_t pos, unsigned len, unsigned flags,
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 4989d330fada..b3008605fd1b 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -669,6 +669,24 @@ static inline struct page *readahead_page(struct readahead_control *rac)
> return page;
> }
>
> +/* The byte offset into the file of this readahead block */
> +static inline loff_t readahead_pos(struct readahead_control *rac)
> +{
> + return (loff_t)rac->_index * PAGE_SIZE;
> +}
> +
> +/* 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->_index;
> +}
rac->_index is pgoff_t, so readahead_index() should return the same type, right?
BTW, pgoff_t is unsigned long.
> +
> /* 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 aaa209559ba2..07cdfbf00f4b 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -124,7 +124,14 @@ 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);
> + /* Clean up the remaining pages */
> + while ((page = readahead_page(rac))) {
> + unlock_page(page);
> + put_page(page);
> + }
> + } else if (aops->readpages) {
> aops->readpages(rac->file, rac->mapping, pages,
> readahead_count(rac));
> /* Clean up the remaining pages */
> @@ -234,7 +241,8 @@ void force_page_cache_readahead(struct address_space *mapping,
> struct file_ra_state *ra = &filp->f_ra;
> unsigned long max_pages;
>
> - if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
> + if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages &&
> + !mapping->a_ops->readahead))
> return;
>
> /*
> --
> 2.25.0
--
Best Regards,
Yan Zi
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