On Mon, Feb 17, 2020 at 10:45:54AM -0800, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" <[email protected]> > > 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) <[email protected]> > --- > 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 [email protected]
