Re: [PATCH][RFC] fast file mapping for loop

From: Peter Zijlstra
Date: Thu Jan 10 2008 - 04:37:23 EST


On Thu, 2008-01-10 at 08:37 +0000, Christoph Hellwig wrote:

> Peter, any chance you could chime in here?

I have this patch to add swap_out/_in methods. I expect we can loosen
the requirement for swapcache pages and change the name a little.

previously posted here:
http://lkml.org/lkml/2007/5/4/143

---
Subject: mm: add support for non block device backed swap files

New addres_space_operations methods are added:
int swapfile(struct address_space *, int);
int swap_out(struct file *, struct page *, struct writeback_control *);
int swap_in(struct file *, struct page *);

When during sys_swapon() the swapfile() method is found and returns no error
the swapper_space.a_ops will proxy to sis->swap_file->f_mapping->a_ops, and
make use of swap_{out,in}() to write/read swapcache pages.

The swapfile method will be used to communicate to the address_space that the
VM relies on it, and the address_space should take adequate measures (like
reserving memory for mempools or the like).

This new interface can be used to obviate the need for ->bmap in the swapfile
code. A filesystem would need to load (and maybe even allocate) the full block
map for a file into memory and pin it there on ->swapfile(,1) so that
->swap_{out,in}() have instant access to it. It can be released on
->swapfile(,0).

The reason to provide ->swap_{out,in}() over using {write,read}page() is to
1) make a distinction between swapcache and pagecache pages, and
2) to provide a struct file * for credential context (normally not needed
in the context of writepage, as the page content is normally dirtied
using either of the following interfaces:
write_{begin,end}()
{prepare,commit}_write()
page_mkwrite()
which do have the file context.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
Documentation/filesystems/Locking | 19 ++++++++++++
Documentation/filesystems/vfs.txt | 17 +++++++++++
include/linux/buffer_head.h | 2 -
include/linux/fs.h | 8 +++++
include/linux/swap.h | 3 +
mm/Kconfig | 3 +
mm/page_io.c | 58 ++++++++++++++++++++++++++++++++++++++
mm/swap_state.c | 5 +++
mm/swapfile.c | 22 +++++++++++++-
9 files changed, 135 insertions(+), 2 deletions(-)

Index: linux-2.6/include/linux/swap.h
===================================================================
--- linux-2.6.orig/include/linux/swap.h
+++ linux-2.6/include/linux/swap.h
@@ -164,6 +164,7 @@ enum {
SWP_USED = (1 << 0), /* is slot in swap_info[] used? */
SWP_WRITEOK = (1 << 1), /* ok to write to this swap? */
SWP_ACTIVE = (SWP_USED | SWP_WRITEOK),
+ SWP_FILE = (1 << 2), /* file swap area */
/* add others here before... */
SWP_SCANNING = (1 << 8), /* refcount in scan_swap_map */
};
@@ -261,6 +262,8 @@ extern void swap_unplug_io_fn(struct bac
/* linux/mm/page_io.c */
extern int swap_readpage(struct file *, struct page *);
extern int swap_writepage(struct page *page, struct writeback_control *wbc);
+extern void swap_sync_page(struct page *page);
+extern int swap_set_page_dirty(struct page *page);
extern void end_swap_bio_read(struct bio *bio, int err);

/* linux/mm/swap_state.c */
Index: linux-2.6/mm/page_io.c
===================================================================
--- linux-2.6.orig/mm/page_io.c
+++ linux-2.6/mm/page_io.c
@@ -17,6 +17,7 @@
#include <linux/bio.h>
#include <linux/swapops.h>
#include <linux/writeback.h>
+#include <linux/buffer_head.h>
#include <asm/pgtable.h>

static struct bio *get_swap_bio(gfp_t gfp_flags, pgoff_t index,
@@ -102,6 +103,18 @@ int swap_writepage(struct page *page, st
unlock_page(page);
goto out;
}
+#ifdef CONFIG_SWAP_FILE
+ {
+ struct swap_info_struct *sis = page_swap_info(page);
+ if (sis->flags & SWP_FILE) {
+ ret = sis->swap_file->f_mapping->
+ a_ops->swap_out(sis->swap_file, page, wbc);
+ if (!ret)
+ count_vm_event(PSWPOUT);
+ return ret;
+ }
+ }
+#endif
bio = get_swap_bio(GFP_NOIO, page_private(page), page,
end_swap_bio_write);
if (bio == NULL) {
@@ -120,6 +133,39 @@ out:
return ret;
}

+#ifdef CONFIG_SWAP_FILE
+void swap_sync_page(struct page *page)
+{
+ struct swap_info_struct *sis = page_swap_info(page);
+
+ if (sis->flags & SWP_FILE) {
+ const struct address_space_operations * a_ops =
+ sis->swap_file->f_mapping->a_ops;
+ if (a_ops->sync_page)
+ a_ops->sync_page(page);
+ } else
+ block_sync_page(page);
+}
+
+int swap_set_page_dirty(struct page *page)
+{
+ struct swap_info_struct *sis = page_swap_info(page);
+
+ if (sis->flags & SWP_FILE) {
+ const struct address_space_operations * a_ops =
+ sis->swap_file->f_mapping->a_ops;
+ int (*spd)(struct page *) = a_ops->set_page_dirty;
+#ifdef CONFIG_BLOCK
+ if (!spd)
+ spd = __set_page_dirty_buffers;
+#endif
+ return (*spd)(page);
+ }
+
+ return __set_page_dirty_nobuffers(page);
+}
+#endif
+
int swap_readpage(struct file *file, struct page *page)
{
struct bio *bio;
@@ -127,6 +173,18 @@ int swap_readpage(struct file *file, str

BUG_ON(!PageLocked(page));
ClearPageUptodate(page);
+#ifdef CONFIG_SWAP_FILE
+ {
+ struct swap_info_struct *sis = page_swap_info(page);
+ if (sis->flags & SWP_FILE) {
+ ret = sis->swap_file->f_mapping->
+ a_ops->swap_in(sis->swap_file, page);
+ if (!ret)
+ count_vm_event(PSWPIN);
+ return ret;
+ }
+ }
+#endif
bio = get_swap_bio(GFP_KERNEL, page_private(page), page,
end_swap_bio_read);
if (bio == NULL) {
Index: linux-2.6/mm/swap_state.c
===================================================================
--- linux-2.6.orig/mm/swap_state.c
+++ linux-2.6/mm/swap_state.c
@@ -27,8 +27,13 @@
*/
static const struct address_space_operations swap_aops = {
.writepage = swap_writepage,
+#ifdef CONFIG_SWAP_FILE
+ .sync_page = swap_sync_page,
+ .set_page_dirty = swap_set_page_dirty,
+#else
.sync_page = block_sync_page,
.set_page_dirty = __set_page_dirty_nobuffers,
+#endif
.migratepage = migrate_page,
};

Index: linux-2.6/mm/swapfile.c
===================================================================
--- linux-2.6.orig/mm/swapfile.c
+++ linux-2.6/mm/swapfile.c
@@ -1015,6 +1015,13 @@ static void destroy_swap_extents(struct
list_del(&se->list);
kfree(se);
}
+#ifdef CONFIG_SWAP_FILE
+ if (sis->flags & SWP_FILE) {
+ sis->flags &= ~SWP_FILE;
+ sis->swap_file->f_mapping->a_ops->
+ swapfile(sis->swap_file->f_mapping, 0);
+ }
+#endif
}

/*
@@ -1107,6 +1114,19 @@ static int setup_swap_extents(struct swa
goto done;
}

+#ifdef CONFIG_SWAP_FILE
+ if (sis->swap_file->f_mapping->a_ops->swapfile) {
+ ret = sis->swap_file->f_mapping->a_ops->
+ swapfile(sis->swap_file->f_mapping, 1);
+ if (!ret) {
+ sis->flags |= SWP_FILE;
+ ret = add_swap_extent(sis, 0, sis->max, 0);
+ *span = sis->pages;
+ }
+ goto done;
+ }
+#endif
+
blkbits = inode->i_blkbits;
blocks_per_page = PAGE_SIZE >> blkbits;

@@ -1671,7 +1691,7 @@ asmlinkage long sys_swapon(const char __

mutex_lock(&swapon_mutex);
spin_lock(&swap_lock);
- p->flags = SWP_ACTIVE;
+ p->flags |= SWP_WRITEOK;
nr_swap_pages += nr_good_pages;
total_swap_pages += nr_good_pages;

Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h
+++ linux-2.6/include/linux/fs.h
@@ -479,6 +479,14 @@ struct address_space_operations {
int (*migratepage) (struct address_space *,
struct page *, struct page *);
int (*launder_page) (struct page *);
+
+ /*
+ * swapfile support
+ */
+ int (*swapfile)(struct address_space *, int);
+ int (*swap_out)(struct file *file, struct page *page,
+ struct writeback_control *wbc);
+ int (*swap_in)(struct file *file, struct page *page);
};

/*
Index: linux-2.6/Documentation/filesystems/Locking
===================================================================
--- linux-2.6.orig/Documentation/filesystems/Locking
+++ linux-2.6/Documentation/filesystems/Locking
@@ -171,6 +171,9 @@ prototypes:
int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
loff_t offset, unsigned long nr_segs);
int (*launder_page) (struct page *);
+ int (*swapfile) (struct address_space *, int);
+ int (*swap_out) (struct file *, struct page *, struct writeback_control *);
+ int (*swap_in) (struct file *, struct page *);

locking rules:
All except set_page_dirty may block
@@ -192,6 +195,9 @@ invalidatepage: no yes
releasepage: no yes
direct_IO: no
launder_page: no yes
+swapfile no
+swap_out no yes, unlocks
+swap_in no yes, unlocks

->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
may be called from the request handler (/dev/loop).
@@ -291,6 +297,19 @@ cleaned, or an error value if not. Note
getting mapped back in and redirtied, it needs to be kept locked
across the entire operation.

+ ->swapfile() will be called with a non zero argument on address spaces
+backing non block device backed swapfiles. A return value of zero indicates
+success. In which case this address space can be used for backing swapspace.
+The swapspace operations will be proxied to the address space operations.
+Swapoff will call this method with a zero argument to release the address
+space.
+
+ ->swap_out() when swapfile() returned success, this method is used to
+write the swap page.
+
+ ->swap_in() when swapfile() returned success, this method is used to
+read the swap page.
+
Note: currently almost all instances of address_space methods are
using BKL for internal serialization and that's one of the worst sources
of contention. Normally they are calling library functions (in fs/buffer.c)
Index: linux-2.6/mm/Kconfig
===================================================================
--- linux-2.6.orig/mm/Kconfig
+++ linux-2.6/mm/Kconfig
@@ -185,6 +185,9 @@ config BOUNCE
def_bool y
depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)

+config SWAP_FILE
+ def_bool n
+
config NR_QUICK
int
depends on QUICKLIST
Index: linux-2.6/include/linux/buffer_head.h
===================================================================
--- linux-2.6.orig/include/linux/buffer_head.h
+++ linux-2.6/include/linux/buffer_head.h
@@ -335,7 +335,7 @@ static inline void invalidate_inode_buff
static inline int remove_inode_buffers(struct inode *inode) { return 1; }
static inline int sync_mapping_buffers(struct address_space *mapping) { return 0; }
static inline void invalidate_bdev(struct block_device *bdev) {}
-
+static inline void block_sync_page(struct page *) { }

#endif /* CONFIG_BLOCK */
#endif /* _LINUX_BUFFER_HEAD_H */
Index: linux-2.6/Documentation/filesystems/vfs.txt
===================================================================
--- linux-2.6.orig/Documentation/filesystems/vfs.txt
+++ linux-2.6/Documentation/filesystems/vfs.txt
@@ -542,6 +542,10 @@ struct address_space_operations {
/* migrate the contents of a page to the specified target */
int (*migratepage) (struct page *, struct page *);
int (*launder_page) (struct page *);
+ int (*swapfile)(struct address_space *, int);
+ int (*swap_out)(struct file *file, struct page *page,
+ struct writeback_control *wbc);
+ int (*swap_in)(struct file *file, struct page *page);
};

writepage: called by the VM to write a dirty page to backing store.
@@ -727,6 +731,19 @@ struct address_space_operations {
prevent redirtying the page, it is kept locked during the whole
operation.

+ swapfile: Called with a non-zero argument when swapon is used on a file. A
+ return value of zero indicates success. In which case this
+ address_space can be used to back swapspace. The swapspace operations
+ will be proxied to this address space's ->swap_{out,in} methods.
+ Swapoff will call this method with a zero argument to release the
+ address space.
+
+ swap_out: Called to write a swapcache page to a backing store, similar to
+ writepage.
+
+ swap_in: Called to read a swapcache page from a backing store, similar to
+ readpage.
+
The File Object
===============



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