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Subject[RFC][PATCH] problem of cont_prepare_write()
From
Date
Hi,

If I do the following operation on fatfs, and my box under heavy load,

open("testfile", O_CREAT | O_TRUNC | O_RDWR, 0664);
lseek(fd, 500*1024*1024 - 1, SEEK_SET);
write(fd, "\0", 1);

In cont_prepare_write(), kernel fills the hole by zero cleared page.

fs/buffer.c:cont_prepare_write:2210,
while(page->index > (pgpos = *bytes>>PAGE_CACHE_SHIFT)) {

[...]

status = __block_prepare_write(inode, new_page, zerofrom,
PAGE_CACHE_SIZE, get_block);
if (status)
goto out_unmap;
kaddr = kmap_atomic(new_page, KM_USER0);
memset(kaddr+zerofrom, 0, PAGE_CACHE_SIZE-zerofrom);
flush_dcache_page(new_page);
kunmap_atomic(kaddr, KM_USER0);
__block_commit_write(inode, new_page,
zerofrom, PAGE_CACHE_SIZE);
unlock_page(new_page);
page_cache_release(new_page);
}

But until ->commit_write(), kernel doesn't update the ->i_size. Then,
if kernel writes out that hole page before updates of ->i_size, dirty
flag of buffer_head is cleared in __block_write_full_page(). So hole
page was not writed to disk.

fs/buffer.c:__block_write_full_page:1773,
if (block > last_block) {
/*
* mapped buffers outside i_size will occur, because
* this page can be outside i_size when there is a
* truncate in progress.
*/
/*
* The buffer was zeroed by block_write_full_page()
*/
clear_buffer_dirty(bh);
set_buffer_uptodate(bh);

This became cause of data corruption.

I thought that it is not good to update ->i_size before ->commit_write().
So, I wrote the following patch. Anyone has good idea for solving this
problem?



Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
---

fs/buffer.c | 4 ++--
include/linux/fs.h | 4 ++++
mm/memory.c | 2 ++
3 files changed, 8 insertions(+), 2 deletions(-)

diff -puN mm/memory.c~cont_prepare_write-fix mm/memory.c
--- linux-2.6.10-rc2/mm/memory.c~cont_prepare_write-fix 2004-11-22 18:59:19.000000000 +0900
+++ linux-2.6.10-rc2-hirofumi/mm/memory.c 2004-11-22 18:59:19.000000000 +0900
@@ -1246,9 +1246,11 @@ int vmtruncate(struct inode * inode, lof
*/
if (IS_SWAPFILE(inode))
goto out_busy;
+ inode->i_flags |= S_TRUNCATING;
i_size_write(inode, offset);
unmap_mapping_range(mapping, offset + PAGE_SIZE - 1, 0, 1);
truncate_inode_pages(mapping, offset);
+ inode->i_flags &= ~S_TRUNCATING;
goto out_truncate;

do_expand:
diff -puN fs/buffer.c~cont_prepare_write-fix fs/buffer.c
--- linux-2.6.10-rc2/fs/buffer.c~cont_prepare_write-fix 2004-11-22 18:59:19.000000000 +0900
+++ linux-2.6.10-rc2-hirofumi/fs/buffer.c 2004-11-22 18:59:19.000000000 +0900
@@ -1763,7 +1763,7 @@ static int __block_write_full_page(struc
* handle any aliases from the underlying blockdev's mapping.
*/
do {
- if (block > last_block) {
+ if (IS_TRUNCATING(inode) && block > last_block) {
/*
* mapped buffers outside i_size will occur, because
* this page can be outside i_size when there is a
@@ -2628,7 +2628,7 @@ int block_write_full_page(struct page *p

/* Is the page fully outside i_size? (truncate in progress) */
offset = i_size & (PAGE_CACHE_SIZE-1);
- if (page->index >= end_index+1 || !offset) {
+ if (IS_TRUNCATING(inode) && (page->index >= end_index+1 || !offset)) {
/*
* The page may have dirty, unmapped buffers. For example,
* they may have been added in ext3_writepage(). Make them
diff -puN include/linux/fs.h~cont_prepare_write-fix include/linux/fs.h
--- linux-2.6.10-rc2/include/linux/fs.h~cont_prepare_write-fix 2004-11-22 18:59:19.000000000 +0900
+++ linux-2.6.10-rc2-hirofumi/include/linux/fs.h 2004-11-22 18:59:19.000000000 +0900
@@ -151,6 +151,8 @@ extern int dir_notify_enable;
#define S_NOCMTIME 128 /* Do not update file c/mtime */
#define S_SWAPFILE 256 /* Do not truncate: swapon got its bmaps */

+#define S_TRUNCATING (1<<31) /* Hack: inode is truncating the data now */
+
/*
* Note that nosuid etc flags are inode-specific: setting some file-system
* flags just means all the inodes inherit those flags by default. It might be
@@ -185,6 +187,8 @@ extern int dir_notify_enable;
#define IS_NOCMTIME(inode) ((inode)->i_flags & S_NOCMTIME)
#define IS_SWAPFILE(inode) ((inode)->i_flags & S_SWAPFILE)

+#define IS_TRUNCATING(inode) ((inode)->i_flags & S_TRUNCATING)
+
/* the read-only stuff doesn't really belong here, but any other place is
probably as bad and I don't want to create yet another include file. */

_
--
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
-
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