Re: [PATCH v5 04/13] mm: Add readahead address space operation

John Hubbard
2020-02-14T05:36:34Z

On 2/10/20 5:03 PM, 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.
> - Implementation looks up the pages in the page cache instead of
> having them passed in a linked list.
>
> 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 | 54 +++++++++++++++++++++++++++
> mm/readahead.c | 48 ++++++++++++++----------
> 5 files changed, 102 insertions(+), 21 deletions(-)
>

A minor question below, but either way you can add:

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

> 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().
> +
> ->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..cabee16b7406 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.
>
> ``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 ccb14b6a16b5..13efafaf7e1f 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -630,6 +630,60 @@ static inline int add_to_page_cache(struct page *page,
> return error;
> }
>
> +/*
> + * Readahead is of a block of consecutive pages.
> + */
> +struct readahead_control {
> + struct file *file;
> + struct address_space *mapping;
> +/* private: use the readahead_* accessors instead */
> + pgoff_t start;
> + unsigned int nr_pages;
> + unsigned int batch_count;
> +};
> +
> +static inline struct page *readahead_page(struct readahead_control *rac)
> +{
> + struct page *page;
> +
> + if (!rac->nr_pages)
> + return NULL;
> +
> + page = xa_load(&rac->mapping->i_pages, rac->start);

Is it worth asserting that the page was found:

VM_BUG_ON_PAGE(!page || xa_is_value(page), page);

? Or is that overkill here?

> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> + rac->batch_count = hpage_nr_pages(page);
> + rac->start += rac->batch_count;

The above was surprising, until I saw the other thread with Dave and you.
I was reviewing this patchset in order to have a chance at understanding the
follow-on patchset ("Large pages in the page cache"), and it seems like that
feature has a solid head start here. :)

thanks,
--
John Hubbard
NVIDIA

> +
> + return page;
> +}
> +
> +#define readahead_for_each(rac, page) \
> + for (; (page = readahead_page(rac)); rac->nr_pages -= rac->batch_count)
> +
> +/* 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;
> +}
> +
> +/* 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;
> +}
> +
> +/* The number of pages in this readahead block */
> +static inline unsigned int readahead_count(struct readahead_control *rac)
> +{
> + return rac->nr_pages;
> +}
> +
> static inline unsigned long dir_pages(struct inode *inode)
> {
> return (unsigned long)(inode->i_size + PAGE_SIZE - 1) >>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 96c6ca68a174..933b32e0c90a 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,25 +113,30 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
>
> EXPORT_SYMBOL(read_cache_pages);
>
> -static void read_pages(struct address_space *mapping, struct file *filp,
> - struct list_head *pages, pgoff_t start,
> - unsigned int nr_pages)
> +static void read_pages(struct readahead_control *rac, struct list_head *pages)
> {
> + struct page *page;
> struct blk_plug plug;
> + const struct address_space_operations *aops = rac->mapping->a_ops;
> +
> + if (rac->nr_pages == 0)
> + return;
>
> blk_start_plug(&plug);
>
> - if (mapping->a_ops->readpages) {
> - mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
> + if (aops->readahead) {
> + aops->readahead(rac);
> + readahead_for_each(rac, page) {
> + unlock_page(page);
> + put_page(page);
> + }
> + } else if (aops->readpages) {
> + aops->readpages(rac->file, rac->mapping, pages, rac->nr_pages);
> /* Clean up the remaining pages */
> put_pages_list(pages);
> } else {
> - struct page *page;
> - unsigned long index;
> -
> - xa_for_each_range(&mapping->i_pages, index, page, start,
> - start + nr_pages - 1) {
> - mapping->a_ops->readpage(filp, page);
> + readahead_for_each(rac, page) {
> + aops->readpage(rac->file, page);
> put_page(page);
> }
> }
> @@ -156,10 +161,15 @@ unsigned long __do_page_cache_readahead(struct address_space *mapping,
> LIST_HEAD(page_pool);
> int page_idx;
> pgoff_t page_offset = start;
> - unsigned long nr_pages = 0;
> loff_t isize = i_size_read(inode);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> bool use_list = mapping->a_ops->readpages;
> + struct readahead_control rac = {
> + .mapping = mapping,
> + .file = filp,
> + .start = start,
> + .nr_pages = 0,
> + };
>
> if (isize == 0)
> goto out;
> @@ -206,15 +216,14 @@ unsigned long __do_page_cache_readahead(struct address_space *mapping,
>
> if (page_idx == nr_to_read - lookahead_size)
> SetPageReadahead(page);
> - nr_pages++;
> + rac.nr_pages++;
> page_offset++;
> continue;
> skip:
> - if (nr_pages)
> - read_pages(mapping, filp, &page_pool, start, nr_pages);
> - nr_pages = 0;
> + read_pages(&rac, &page_pool);
> + rac.nr_pages = 0;
> page_offset++;
> - start = page_offset;
> + rac.start = page_offset;
> }
>
> /*
> @@ -222,11 +231,10 @@ unsigned long __do_page_cache_readahead(struct address_space *mapping,
> * uptodate then the caller will launch readpage again, and
> * will then handle the error.
> */
> - if (nr_pages)
> - read_pages(mapping, filp, &page_pool, start, nr_pages);
> + read_pages(&rac, &page_pool);
> BUG_ON(!list_empty(&page_pool));
> out:
> - return nr_pages;
> + return rac.nr_pages;
> }
>
> /*
>