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

Matthew Wilcox
2020-02-11T01:04:33Z

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(-)

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

/*
--
2.25.0