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SubjectRe: [PATCH] VFS: Pagecache usage optimization on pagesize !=blocksize environment
Thank you for your comment.

At 16:19 08/05/21, Andrew Morton wrote:
>That's not a terribly good benchmark, IMO. It's too complex.
>
>To work out the best-case for a change like this I'd suggest a
>microbenchmark which does something such as seeking all around a file
>doing single-byte reads.
>

I will try microbenchmark later.

>See, the code which you have here is assuming that if PagePrivate is
>set, then the thing which is at page.private is a ring of buffer_heads.
>
>But this code (do_generic_file_read) doesn't know that! Take a look at
>afs, nfs, perhaps other filesystems, grep for set_page_private().
>
>Only the address_space implementation (ie: the filesystem) knows
>whether page.private holds buffer_heads and only the
>address_space_operations functions are allowed to call into library
>functions which treat page.private as a buffer_head ring.
>
>Now, your code _may_ not crash, because perhaps there is no filesystem
>which puts something else into page.private which also uses
>do_generic_file_read(). But it's still wrong.
>
>I guess a suitable fix might be to implement the above using a new
>address_space_operations callback:
>
> if (PagePrivate(page) && aops->is_partially_uptodate) {
> if (aops->is_partially_uptodate(page, desc, offset))
> <OK, we can copy the data>
>
>then implement a generic_file_is_partially_uptodate() in fs/buffer.c
>and wire that up in the filesystems.

I modified my patch based on your comment.

I added is_partially_uptodate method to address_space_operations, and
I registered block_is_partially_uptodate in fs/buffer.c to ext2,ext3,ext4's aops.
Thanks.

Signed-off-by :Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>

diff -Nrup linux-2.6.26-rc3.org/fs/buffer.c linux-2.6.26-rc3/fs/buffer.c
--- linux-2.6.26-rc3.org/fs/buffer.c 2008-05-19 11:35:10.000000000 +0900
+++ linux-2.6.26-rc3/fs/buffer.c 2008-05-22 13:23:29.000000000 +0900
@@ -2084,6 +2084,51 @@ int generic_write_end(struct file *file,
EXPORT_SYMBOL(generic_write_end);

/*
+ * block_is_partially_uptodate checks whether buffers within a page are
+ * uptodate or not.
+ *
+ * Returns true if all buffers which correspond to a file portion
+ * we want to read are uptodate.
+ */
+int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
+ unsigned long from)
+{
+ struct inode *inode = page->mapping->host;
+ unsigned long block_start, block_end, blocksize;
+ unsigned long to;
+ struct buffer_head *bh, *head;
+ int ret = 1;
+
+ if (!page_has_buffers(page))
+ return 0;
+
+ blocksize = 1 << inode->i_blkbits;
+ to = from + desc->count;
+ if (to > PAGE_CACHE_SIZE)
+ to = PAGE_CACHE_SIZE;
+ if (from < blocksize && to > PAGE_CACHE_SIZE - blocksize)
+ return 0;
+
+ head = page_buffers(page);
+
+ for (bh = head, block_start = 0; bh != head || !block_start;
+ block_start = block_end, bh = bh->b_this_page) {
+ block_end = block_start + blocksize;
+ if (block_end <= from || block_start >= to)
+ continue;
+ else {
+ if (!buffer_uptodate(bh)) {
+ ret = 0;
+ break;
+ }
+ if (block_end >= to)
+ break;
+ }
+ }
+ return ret;
+}
+
+/*
* Generic "read page" function for block devices that have the normal
* get_block functionality. This is most of the block device filesystems.
* Reads the page asynchronously --- the unlock_buffer() and
diff -Nrup linux-2.6.26-rc3.org/fs/ext2/inode.c linux-2.6.26-rc3/fs/ext2/inode.c
--- linux-2.6.26-rc3.org/fs/ext2/inode.c 2008-05-19 11:35:10.000000000 +0900
+++ linux-2.6.26-rc3/fs/ext2/inode.c 2008-05-22 12:39:46.000000000 +0900
@@ -791,6 +791,7 @@ const struct address_space_operations ex
.direct_IO = ext2_direct_IO,
.writepages = ext2_writepages,
.migratepage = buffer_migrate_page,
+ .is_partially_uptodate = block_is_partially_uptodate,
};

const struct address_space_operations ext2_aops_xip = {
diff -Nrup linux-2.6.26-rc3.org/fs/ext3/inode.c linux-2.6.26-rc3/fs/ext3/inode.c
--- linux-2.6.26-rc3.org/fs/ext3/inode.c 2008-05-19 11:35:10.000000000 +0900
+++ linux-2.6.26-rc3/fs/ext3/inode.c 2008-05-22 13:10:40.000000000 +0900
@@ -1767,44 +1767,47 @@ static int ext3_journalled_set_page_dirt
}

static const struct address_space_operations ext3_ordered_aops = {
- .readpage = ext3_readpage,
- .readpages = ext3_readpages,
- .writepage = ext3_ordered_writepage,
- .sync_page = block_sync_page,
- .write_begin = ext3_write_begin,
- .write_end = ext3_ordered_write_end,
- .bmap = ext3_bmap,
- .invalidatepage = ext3_invalidatepage,
- .releasepage = ext3_releasepage,
- .direct_IO = ext3_direct_IO,
- .migratepage = buffer_migrate_page,
+ .readpage = ext3_readpage,
+ .readpages = ext3_readpages,
+ .writepage = ext3_ordered_writepage,
+ .sync_page = block_sync_page,
+ .write_begin = ext3_write_begin,
+ .write_end = ext3_ordered_write_end,
+ .bmap = ext3_bmap,
+ .invalidatepage = ext3_invalidatepage,
+ .releasepage = ext3_releasepage,
+ .direct_IO = ext3_direct_IO,
+ .migratepage = buffer_migrate_page,
+ .is_partially_uptodate = block_is_partially_uptodate,
};

static const struct address_space_operations ext3_writeback_aops = {
- .readpage = ext3_readpage,
- .readpages = ext3_readpages,
- .writepage = ext3_writeback_writepage,
- .sync_page = block_sync_page,
- .write_begin = ext3_write_begin,
- .write_end = ext3_writeback_write_end,
- .bmap = ext3_bmap,
- .invalidatepage = ext3_invalidatepage,
- .releasepage = ext3_releasepage,
- .direct_IO = ext3_direct_IO,
- .migratepage = buffer_migrate_page,
+ .readpage = ext3_readpage,
+ .readpages = ext3_readpages,
+ .writepage = ext3_writeback_writepage,
+ .sync_page = block_sync_page,
+ .write_begin = ext3_write_begin,
+ .write_end = ext3_writeback_write_end,
+ .bmap = ext3_bmap,
+ .invalidatepage = ext3_invalidatepage,
+ .releasepage = ext3_releasepage,
+ .direct_IO = ext3_direct_IO,
+ .migratepage = buffer_migrate_page,
+ .is_partially_uptodate = block_is_partially_uptodate,
};

static const struct address_space_operations ext3_journalled_aops = {
- .readpage = ext3_readpage,
- .readpages = ext3_readpages,
- .writepage = ext3_journalled_writepage,
- .sync_page = block_sync_page,
- .write_begin = ext3_write_begin,
- .write_end = ext3_journalled_write_end,
- .set_page_dirty = ext3_journalled_set_page_dirty,
- .bmap = ext3_bmap,
- .invalidatepage = ext3_invalidatepage,
- .releasepage = ext3_releasepage,
+ .readpage = ext3_readpage,
+ .readpages = ext3_readpages,
+ .writepage = ext3_journalled_writepage,
+ .sync_page = block_sync_page,
+ .write_begin = ext3_write_begin,
+ .write_end = ext3_journalled_write_end,
+ .set_page_dirty = ext3_journalled_set_page_dirty,
+ .bmap = ext3_bmap,
+ .invalidatepage = ext3_invalidatepage,
+ .releasepage = ext3_releasepage,
+ .is_partially_uptodate = block_is_partially_uptodate,
};

void ext3_set_aops(struct inode *inode)
diff -Nrup linux-2.6.26-rc3.org/fs/ext4/inode.c linux-2.6.26-rc3/fs/ext4/inode.c
--- linux-2.6.26-rc3.org/fs/ext4/inode.c 2008-05-19 11:35:10.000000000 +0900
+++ linux-2.6.26-rc3/fs/ext4/inode.c 2008-05-22 13:13:17.000000000 +0900
@@ -1817,44 +1817,47 @@ static int ext4_journalled_set_page_dirt
}

static const struct address_space_operations ext4_ordered_aops = {
- .readpage = ext4_readpage,
- .readpages = ext4_readpages,
- .writepage = ext4_ordered_writepage,
- .sync_page = block_sync_page,
- .write_begin = ext4_write_begin,
- .write_end = ext4_ordered_write_end,
- .bmap = ext4_bmap,
- .invalidatepage = ext4_invalidatepage,
- .releasepage = ext4_releasepage,
- .direct_IO = ext4_direct_IO,
- .migratepage = buffer_migrate_page,
+ .readpage = ext4_readpage,
+ .readpages = ext4_readpages,
+ .writepage = ext4_ordered_writepage,
+ .sync_page = block_sync_page,
+ .write_begin = ext4_write_begin,
+ .write_end = ext4_ordered_write_end,
+ .bmap = ext4_bmap,
+ .invalidatepage = ext4_invalidatepage,
+ .releasepage = ext4_releasepage,
+ .direct_IO = ext4_direct_IO,
+ .migratepage = buffer_migrate_page,
+ .is_partially_uptodate = block_is_partially_uptodate,
};

static const struct address_space_operations ext4_writeback_aops = {
- .readpage = ext4_readpage,
- .readpages = ext4_readpages,
- .writepage = ext4_writeback_writepage,
- .sync_page = block_sync_page,
- .write_begin = ext4_write_begin,
- .write_end = ext4_writeback_write_end,
- .bmap = ext4_bmap,
- .invalidatepage = ext4_invalidatepage,
- .releasepage = ext4_releasepage,
- .direct_IO = ext4_direct_IO,
- .migratepage = buffer_migrate_page,
+ .readpage = ext4_readpage,
+ .readpages = ext4_readpages,
+ .writepage = ext4_writeback_writepage,
+ .sync_page = block_sync_page,
+ .write_begin = ext4_write_begin,
+ .write_end = ext4_writeback_write_end,
+ .bmap = ext4_bmap,
+ .invalidatepage = ext4_invalidatepage,
+ .releasepage = ext4_releasepage,
+ .direct_IO = ext4_direct_IO,
+ .migratepage = buffer_migrate_page,
+ .is_partially_uptodate = block_is_partially_uptodate,
};

static const struct address_space_operations ext4_journalled_aops = {
- .readpage = ext4_readpage,
- .readpages = ext4_readpages,
- .writepage = ext4_journalled_writepage,
- .sync_page = block_sync_page,
- .write_begin = ext4_write_begin,
- .write_end = ext4_journalled_write_end,
- .set_page_dirty = ext4_journalled_set_page_dirty,
- .bmap = ext4_bmap,
- .invalidatepage = ext4_invalidatepage,
- .releasepage = ext4_releasepage,
+ .readpage = ext4_readpage,
+ .readpages = ext4_readpages,
+ .writepage = ext4_journalled_writepage,
+ .sync_page = block_sync_page,
+ .write_begin = ext4_write_begin,
+ .write_end = ext4_journalled_write_end,
+ .set_page_dirty = ext4_journalled_set_page_dirty,
+ .bmap = ext4_bmap,
+ .invalidatepage = ext4_invalidatepage,
+ .releasepage = ext4_releasepage,
+ .is_partially_uptodate = block_is_partially_uptodate,
};

void ext4_set_aops(struct inode *inode)
diff -Nrup linux-2.6.26-rc3.org/include/linux/buffer_head.h linux-2.6.26-rc3/include/linux/buffer_head.h
--- linux-2.6.26-rc3.org/include/linux/buffer_head.h 2008-05-19 11:35:11.000000000 +0900
+++ linux-2.6.26-rc3/include/linux/buffer_head.h 2008-05-22 11:22:26.000000000 +0900
@@ -205,6 +205,8 @@ void block_invalidatepage(struct page *p
int block_write_full_page(struct page *page, get_block_t *get_block,
struct writeback_control *wbc);
int block_read_full_page(struct page*, get_block_t*);
+int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
+ unsigned long from);
int block_write_begin(struct file *, struct address_space *,
loff_t, unsigned, unsigned,
struct page **, void **, get_block_t*);
diff -Nrup linux-2.6.26-rc3.org/include/linux/fs.h linux-2.6.26-rc3/include/linux/fs.h
--- linux-2.6.26-rc3.org/include/linux/fs.h 2008-05-19 11:35:11.000000000 +0900
+++ linux-2.6.26-rc3/include/linux/fs.h 2008-05-22 15:13:39.000000000 +0900
@@ -439,6 +439,27 @@ static inline size_t iov_iter_count(stru
return i->count;
}

+/*
+ * "descriptor" for what we're up to with a read.
+ * This allows us to use the same read code yet
+ * have multiple different users of the data that
+ * we read from a file.
+ *
+ * The simplest case just copies the data to user
+ * mode.
+ */
+typedef struct {
+ size_t written;
+ size_t count;
+ union {
+ char __user *buf;
+ void *data;
+ } arg;
+ int error;
+} read_descriptor_t;
+
+typedef int (*read_actor_t)(read_descriptor_t *, struct page *,
+ unsigned long, unsigned long);

struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
@@ -480,6 +501,8 @@ struct address_space_operations {
int (*migratepage) (struct address_space *,
struct page *, struct page *);
int (*launder_page) (struct page *);
+ int (*is_partially_uptodate) (struct page *, read_descriptor_t *,
+ unsigned long);
};

/*
@@ -1186,27 +1209,6 @@ struct block_device_operations {
struct module *owner;
};

-/*
- * "descriptor" for what we're up to with a read.
- * This allows us to use the same read code yet
- * have multiple different users of the data that
- * we read from a file.
- *
- * The simplest case just copies the data to user
- * mode.
- */
-typedef struct {
- size_t written;
- size_t count;
- union {
- char __user * buf;
- void *data;
- } arg;
- int error;
-} read_descriptor_t;
-
-typedef int (*read_actor_t)(read_descriptor_t *, struct page *, unsigned long, unsigned long);
-
/* These macros are for out of kernel modules to test that
* the kernel supports the unlocked_ioctl and compat_ioctl
* fields in struct file_operations. */
diff -Nrup linux-2.6.26-rc3.org/mm/filemap.c linux-2.6.26-rc3/mm/filemap.c
--- linux-2.6.26-rc3.org/mm/filemap.c 2008-05-19 11:35:11.000000000 +0900
+++ linux-2.6.26-rc3/mm/filemap.c 2008-05-22 12:34:58.000000000 +0900
@@ -932,8 +932,17 @@ find_page:
ra, filp, page,
index, last_index - index);
}
- if (!PageUptodate(page))
- goto page_not_up_to_date;
+ if (!PageUptodate(page)) {
+ if (inode->i_blkbits == PAGE_CACHE_SHIFT ||
+ !mapping->a_ops->is_partially_uptodate)
+ goto page_not_up_to_date;
+ if (TestSetPageLocked(page))
+ goto page_not_up_to_date;
+ if (!mapping->a_ops->is_partially_uptodate(page,
+ desc, offset))
+ goto page_not_up_to_date_locked;
+ unlock_page(page);
+ }
page_ok:
/*
* i_size must be checked after we know the page is Uptodate.
@@ -1003,6 +1012,7 @@ page_not_up_to_date:
if (lock_page_killable(page))
goto readpage_eio;

+page_not_up_to_date_locked:
/* Did it get truncated before we got the lock? */
if (!page->mapping) {
unlock_page(page);


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