Date:	Wed, 13 Oct 2010 18:14:08 +0200
From:	"Amir G." <amir73il@...rs.sourceforge.net>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH RFC 0/3] Block reservation for ext3

On Wed, Oct 13, 2010 at 10:49 AM, Amir G.
<amir73il@...rs.sourceforge.net> wrote:
> On Wed, Oct 13, 2010 at 1:14 AM, Jan Kara <jack@...e.cz> wrote:
>>
>> On Mon 11-10-10 14:59:45, Andrew Morton wrote:
>> > On Mon, 11 Oct 2010 16:28:13 +0200 Jan Kara <jack@...e.cz> wrote:
>> >
>> > >   Doing allocation at mmap time does not really work - on each mmap we
>> > > would have to map blocks for the whole file which would make mmap really
>> > > expensive operation. Doing it at page-fault as you suggest in (2a) works
>> > > (that's the second plausible option IMO) but the increased fragmentation
>> > > and thus loss of performance is rather noticeable. I don't have current
>> > > numbers but when I tried that last year Berkeley DB was like two or three
>> > > times slower.
>> >
>> > ouch.
>> >
>> > Can we fix the layout problem?  Are reservation windows of no use here?
>>  Reservation windows do not work for this load. The reason is that the
>> page-fault order is completely random so we just spend time creating and
>> removing tiny reservation windows because the next page fault doing
>> allocation is scarcely close enough to fall into the small window.
>>  The logic in ext3_find_goal() ends up picking blocks close together for
>> blocks belonging to the same indirect block if we are lucky but they
>> definitely won't be sequentially ordered. For Berkeley DB the situation is
>> made worse by the fact that there are several database files and their
>> blocks end up interleaved.
>>  So we could improve the layout but we'd have to tweak the reservation
>> logic and allocator and it's not completely clear to me how.
>>  One thing to note is that currently, ext3 *is* in fact doing delayed
>> allocation for writes via mmap. We just never called it like that and never
>> bothered to do proper space estimation...
>>
>> > > > 3) Keep a global counter of sparse blocks which are mapped at mmap()
>> > > > time, and update it as blocks are allocated, or when the region is
>> > > > freed at munmap() time.
>> > >   Here again I see the problem that mapping all file blocks at mmap time
>> > > is rather expensive and so does not seem viable to me. Also the
>> > > overestimation of needed blocks could be rather huge.
>> >
>> > When I did ext2 delayed allocation back in, err, 2001 I had
>> > considerable trouble working out how many blocks to actually reserve
>> > for a file block, because it also had to reserve the indirect blocks.
>> > One file block allocation can result in reserving four disk blocks!
>> > And iirc it was not possible with existing in-core data structures to
>> > work out whether all four blocks needed reserving until the actual
>> > block allocation had occurred.  So I ended up reserving the worst-case
>> > number of indirects, based upon the file offset.  If the disk ran out
>> > of "space" I'd do a forced writeback to empty all the reservations and
>> > would then take a look to see if the disk was _really_ out of space.
>> >
>> > Is all of this an issue with this work?  If so, what approach did you
>> > take?
>>  Yeah, I've spotted exactly the same problem. How I decided to solve it in
>> the end is that in memory we keep track of each indirect block that has
>> delay-allocated buffer under it. This allows us to reserve space for each
>> indirect block at most once (I didn't bother with making the accounting
>> precise for double or triple indirect blocks so when I need to reserve
>> space for indirect block, I reserve the whole path just to be sure). This
>> pushes the error in estimation to rather acceptable range for reasonably
>> common workloads - the error can still be 50% for workloads which use just
>> one data block in each indirect block but even in this case the absolute
>> number of blocks falsely reserved is small.
>>  The cost is of course increased complexity of the code, the memory
>> spent for tracking those indirect blocks (32 bytes per indirect block), and
>> some time for lookups in the RB-tree of the structures. At least the nice
>> thing is that when there are no delay-allocated blocks, there isn't any
>> overhead (tree is empty).
>>
>
> How about allocating *only* the indirect blocks on page fault.
> IMHO it seems like a fair mixture of high quota accuracy, low
> complexity of the accounting code and low file fragmentation (only
> indirect may be a bit further away from data).
>
> In my snapshot patches I use the @create arg to get_blocks_handle() to
> pass commands just like "allocate only indirect blocks".
> The patch is rather simple. I can prepare it for ext3 if you like.
>
> Amir.
>

Here is the indirect allocation patch.
The following debugfs dump shows the difference between a 1G file
allocated by dd (indirect interleaved with data) and by mmap (indirect
sequential before data).
(I did not test performance and I ignored SIGBUS at the end of mmap
file allocation)

debugfs:  stat dd.1G
Inode: 15   Type: regular    Mode:  0644   Flags: 0x0
Generation: 4035748347    Version: 0x00000000
User:     0   Group:     0   Size: 1073741824
File ACL: 0    Directory ACL: 0
Links: 1   Blockcount: 2099208
Fragment:  Address: 0    Number: 0    Size: 0
ctime: 0x4cb5d1a9 -- Wed Oct 13 17:35:05 2010
atime: 0x4cb5d199 -- Wed Oct 13 17:34:49 2010
mtime: 0x4cb5d1a9 -- Wed Oct 13 17:35:05 2010
Size of extra inode fields: 4
BLOCKS:
(0-11):16384-16395, (IND):16396, (12-134):16397-16519,
(135-641):17037-17543, (642-1035):17792-18185, (DIND):18186,
(IND):18187, (10
36-1279):18188-18431, (1280-1284):1311235-1311239,
(1285-2059):1334197-1334971, (IND):1334972,
(2060-3083):1334973-1335996, (IND):13
35997, (3084-4107):1335998-1337021, (IND):1337022,
(4108-5131):1337023-1338046, (IND):1338047,
(5132-6155):1338048-1339071, (IND):13
39072, (6156-7179):1339073-1340096, (IND):1340097,
(7180-8203):1340098-1341121, (IND):1341122,
(8204-9227):1341123-1342146, (IND):13
42147, (9228-10251):1342148-1343171, (IND):1343172,
(10252-10566):1343173-1343487, (10567-11275):1344008-1344716,
(IND):1344717, (11
276-12299):1344718-1345741, (IND):1345742,
(12300-13323):1345743-1346766, (IND):1346767,
(13324-14347):1346768-1347791, (IND):134779
2, (14348-15371):1347793-1348816, (IND):1348817,
(15372-16395):1348818-1349841, (IND):1349842,
(16396-17419):1349843-1350866, (IND):
...
debugfs:
debugfs:  stat mmap.1G
Inode: 14   Type: regular    Mode:  0644   Flags: 0x0
Generation: 1442185090    Version: 0x00000000
User:     0   Group:     0   Size: 1073741824
File ACL: 0    Directory ACL: 0
Links: 1   Blockcount: 1968016
Fragment:  Address: 0    Number: 0    Size: 0
ctime: 0x4cb5d044 -- Wed Oct 13 17:29:08 2010
atime: 0x4cb5bf81 -- Wed Oct 13 16:17:37 2010
mtime: 0x4cb5d025 -- Wed Oct 13 17:28:37 2010
Size of extra inode fields: 4
BLOCKS:
(DIND):14336, (IND):14337, (16384-16395):14360-14371, (IND):14338,
(16396-16481):14372-14457, (16482-17153):14600-15271, (17154-1741
9):16520-16785, (IND):14339, (17420-17670):16786-17036,
(17671-17675):1081859-1081863, (17676-18443):1086089-1086856,
(IND):14340, (
18444-19467):1086857-1087880, (IND):14341,
(19468-20491):1087881-1088904, (IND):14342,
(20492-21515):1088905-1089928, (IND):14343, (
21516-22539):1089929-1090952, (IND):14344,
(22540-23563):1090953-1091976, (IND):14345,
(23564-24587):1091977-1093000, (IND):14346, (
24588-25611):1093001-1094024, (IND):14347,
(25612-26635):1094025-1095048, (IND):14348,
(26636-27659):1095049-1096072, (IND):14349, (
27660-28683):1096073-1097096, (IND):14472,
(28684-29707):1097097-1098120, (IND):15496,
(29708-30731):1098121-1099144, (IND):17544, (
30732-31755):1099145-1100168, (IND):17545,
(31756-32779):1100169-1101192, (IND):17546,
(32780-33803):1101193-1102216, (IND):17547, (
...
debugfs:



Allocate file indirect blocks on page_mkwrite().

This is a sample patch to be merged with Jan Kara's ext3 delayed allocation
patches. Some of the code was taken from Jan's patches for testing only.
This patch is for kernel 2.6.35.6.

On page_mkwrite(), we allocate indirect blocks if needed and leave the data
blocks unallocated. Jan's patches take care of reserving space for the data.

Signed-off-by: Amir Goldstein <amir73il@...rs.sf.net>

--------------------------------------------------------------------------------
diff -Nuarp a/fs/buffer.c b/fs/buffer.c
--- a/fs/buffer.c	2010-10-13 17:42:06.472252298 +0200
+++ b/fs/buffer.c	2010-10-13 17:42:14.772244244 +0200
@@ -1687,8 +1687,9 @@ static int __block_write_full_page(struc
 			if (buffer_new(bh)) {
 				/* blockdev mappings never come here */
 				clear_buffer_new(bh);
-				unmap_underlying_metadata(bh->b_bdev,
-							bh->b_blocknr);
+				if (buffer_mapped(bh))
+					unmap_underlying_metadata(bh->b_bdev,
+								bh->b_blocknr);
 			}
 		}
 		bh = bh->b_this_page;
@@ -1873,7 +1874,8 @@ static int __block_prepare_write(struct
 			if (err)
 				break;
 			if (buffer_new(bh)) {
-				unmap_underlying_metadata(bh->b_bdev,
+				if (buffer_mapped(bh))
+					unmap_underlying_metadata(bh->b_bdev,
 							bh->b_blocknr);
 				if (PageUptodate(page)) {
 					clear_buffer_new(bh);
@@ -2592,7 +2594,7 @@ int nobh_write_begin_newtrunc(struct fil
 			goto failed;
 		if (!buffer_mapped(bh))
 			is_mapped_to_disk = 0;
-		if (buffer_new(bh))
+		if (buffer_new(bh) && buffer_mapped(bh))
 			unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
 		if (PageUptodate(page)) {
 			set_buffer_uptodate(bh);
diff -Nuarp a/fs/ext3/file.c b/fs/ext3/file.c
--- a/fs/ext3/file.c	2010-10-13 17:41:52.962541253 +0200
+++ b/fs/ext3/file.c	2010-10-13 17:42:25.083163556 +0200
@@ -52,6 +52,23 @@ static int ext3_release_file (struct ino
 	return 0;
 }

+static const struct vm_operations_struct ext3_file_vm_ops = {
+	.fault          = filemap_fault,
+	.page_mkwrite   = ext3_page_mkwrite,
+};
+
+static int ext3_file_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct address_space *mapping = file->f_mapping;
+
+	if (!mapping->a_ops->readpage)
+		return -ENOEXEC;
+	file_accessed(file);
+	vma->vm_ops = &ext3_file_vm_ops;
+	vma->vm_flags |= VM_CAN_NONLINEAR;
+	return 0;
+}
+
 const struct file_operations ext3_file_operations = {
 	.llseek		= generic_file_llseek,
 	.read		= do_sync_read,
@@ -62,7 +79,7 @@ const struct file_operations ext3_file_o
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	= ext3_compat_ioctl,
 #endif
-	.mmap		= generic_file_mmap,
+	.mmap		= ext3_file_mmap,
 	.open		= dquot_file_open,
 	.release	= ext3_release_file,
 	.fsync		= ext3_sync_file,
diff -Nuarp a/fs/ext3/inode.c b/fs/ext3/inode.c
--- a/fs/ext3/inode.c	2010-10-13 17:41:42.722231144 +0200
+++ b/fs/ext3/inode.c	2010-10-13 17:41:12.973180756 +0200
@@ -38,6 +38,7 @@
 #include <linux/bio.h>
 #include <linux/fiemap.h>
 #include <linux/namei.h>
+#include <linux/mount.h>
 #include "xattr.h"
 #include "acl.h"

@@ -562,10 +563,17 @@ static int ext3_alloc_blocks(handle_t *h
 			count--;
 		}

-		if (count > 0)
+		if (index == indirect_blks)
 			break;
 	}

+	if (blks == 0) {
+		/* blks == 0 when allocating only indirect blocks */
+		new_blocks[index] = 0;
+		*err = 0;
+		return 0;
+	}
+
 	/* save the new block number for the first direct block */
 	new_blocks[index] = current_block;

@@ -676,7 +684,9 @@ failed:
 	for (i = 0; i <indirect_blks; i++)
 		ext3_free_blocks(handle, inode, new_blocks[i], 1);

-	ext3_free_blocks(handle, inode, new_blocks[i], num);
+	if (num > 0)
+		/* num == 0 when allocating only indirect blocks */
+		ext3_free_blocks(handle, inode, new_blocks[i], num);

 	return err;
 }
@@ -735,7 +745,8 @@ static int ext3_splice_branch(handle_t *
 	 * in i_block_alloc_info, to assist find the proper goal block for next
 	 * allocation
 	 */
-	if (block_i) {
+	if (block_i && blks > 0) {
+		/* blks == 0 when allocating only indirect blocks */
 		block_i->last_alloc_logical_block = block + blks - 1;
 		block_i->last_alloc_physical_block =
 				le32_to_cpu(where[num].key) + blks - 1;
@@ -778,7 +789,9 @@ err_out:
 		ext3_journal_forget(handle, where[i].bh);
 		ext3_free_blocks(handle,inode,le32_to_cpu(where[i-1].key),1);
 	}
-	ext3_free_blocks(handle, inode, le32_to_cpu(where[num].key), blks);
+	if (blks > 0)
+		/* blks == 0 when allocating only indirect blocks */
+		ext3_free_blocks(handle, inode, le32_to_cpu(where[num].key), blks);

 	return err;
 }
@@ -905,6 +918,11 @@ int ext3_get_blocks_handle(handle_t *han

 	/* the number of blocks need to allocate for [d,t]indirect blocks */
 	indirect_blks = (chain + depth) - partial - 1;
+	if (indirect_blks + maxblocks == 0) {
+		/* maxblocks == 0 when allocating only indirect blocks */
+		mutex_unlock(&ei->truncate_mutex);
+		goto cleanup;
+	}

 	/*
 	 * Next look up the indirect map to count the totoal number of
@@ -929,7 +947,8 @@ int ext3_get_blocks_handle(handle_t *han
 		err = ext3_splice_branch(handle, inode, iblock,
 					partial, indirect_blks, count);
 	mutex_unlock(&ei->truncate_mutex);
-	if (err)
+	if (err || count == 0)
+		/* count == 0 when allocating only indirect blocks */
 		goto cleanup;

 	set_buffer_new(bh_result);
@@ -981,6 +1000,9 @@ static int ext3_get_block(struct inode *
 		started = 1;
 	}

+	if (create < 0)
+		/* create < 0 when allocating only indirect blocks */
+		max_blocks = 0;
 	ret = ext3_get_blocks_handle(handle, inode, iblock,
 					max_blocks, bh_result, create);
 	if (ret > 0) {
@@ -1827,6 +1849,43 @@ out:
 }

 /*
+ * Reserve block writes instead of allocation. Called only on buffer heads
+ * attached to a page (and thus for 1 block).
+ */
+static int ext3_da_get_block(struct inode *inode, sector_t iblock,
+		struct buffer_head *bh, int create)
+{
+	int ret;
+
+	/* Buffer has already blocks reserved? */
+	if (buffer_delay(bh))
+		return 0;
+
+	/* passing -1 to allocate only indirect blocks */
+	ret = ext3_get_block(inode, iblock, bh, -1);
+	if (ret < 0)
+		return ret;
+	if (ret > 0 || !create)
+		return 0;
+	set_buffer_delay(bh);
+	set_buffer_new(bh);
+	return 0;
+}
+
+int ext3_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	int retry = 0;
+	int ret;
+	struct super_block *sb = vma->vm_file->f_path.mnt->mnt_sb;
+
+	do {
+		ret = block_page_mkwrite(vma, vmf, ext3_da_get_block);
+	} while (ret == VM_FAULT_SIGBUS &&
+			ext3_should_retry_alloc(sb, &retry));
+	return ret;
+}
+
+/*
  * Pages can be marked dirty completely asynchronously from ext3's journalling
  * activity.  By filemap_sync_pte(), try_to_unmap_one(), etc.  We cannot do
  * much here because ->set_page_dirty is called under VFS locks.  The page is
diff -Nuarp a/include/linux/ext3_fs.h b/include/linux/ext3_fs.h
--- a/include/linux/ext3_fs.h	2010-10-13 17:49:26.892439258 +0200
+++ b/include/linux/ext3_fs.h	2010-10-13 17:49:10.662493115 +0200
@@ -909,6 +909,7 @@ extern void ext3_get_inode_flags(struct
 extern void ext3_set_aops(struct inode *inode);
 extern int ext3_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 		       u64 start, u64 len);
+extern int ext3_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf);

 /* ioctl.c */
 extern long ext3_ioctl(struct file *, unsigned int, unsigned long);
--
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