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SubjectRe: [PATCH] fs-writeback: Flush plug before next iteration in wb_writeback()
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在 2022/4/15 14:39, Christoph Hellwig 写道:
Hi, Christoph
> This basically removes plugging entirely, so we might as well stop
> adding the plug if we can't solve it any other way. But it seems
> like that fake progress needs to be fixed instead.
>
Maybe there is a more ideal solution:
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index e524c0a1749c..9723f77841f8 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1855,7 +1855,7 @@ static long writeback_sb_inodes(struct super_block
*sb,

wbc_detach_inode(&wbc);
work->nr_pages -= write_chunk - wbc.nr_to_write;
- wrote += write_chunk - wbc.nr_to_write;
+ wrote += write_chunk - wbc.nr_to_write - wbc.pages_skipped;

if (need_resched()) {
/*
, or following is better(It looks awkward.):

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index e524c0a1749c..5f310e53bf1e 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1780,6 +1780,7 @@ static long writeback_sb_inodes(struct super_block
*sb,
while (!list_empty(&wb->b_io)) {
struct inode *inode = wb_inode(wb->b_io.prev);
struct bdi_writeback *tmp_wb;
+ long tmp_wrote;

if (inode->i_sb != sb) {
if (work->sb) {
@@ -1854,8 +1855,11 @@ static long writeback_sb_inodes(struct
super_block *sb,
__writeback_single_inode(inode, &wbc);

wbc_detach_inode(&wbc);
- work->nr_pages -= write_chunk - wbc.nr_to_write;
- wrote += write_chunk - wbc.nr_to_write;
+ tmp_wrote = write_chunk - wbc.nr_to_write >=
wbc.pages_skipped ?
+ write_chunk - wbc.nr_to_write -
wbc.pages_skipped :
+ write_chunk - wbc.nr_to_write;workaround
+ work->nr_pages -= tmp_wrote;
+ wrote += tmp_wrote;

if (need_resched()) {
/*
It depends on how specific filesystem behaves after invoking
redirty_page_for_writepage(). Most filesystems return
AOP_WRITEPAGE_ACTIVATE or 0 after invoking redirty_page_for_writepage()
in writepage, but there still exist accidential examples:
1. metapage_writepage() could returns -EIO after
redirty_page_for_writepage()
2. ext4_writepage() could returns -ENOMEM after redirty_page_for_writepage()

write_cache_pages
error = (*writepage)(page, wbc, data);
if (unlikely(error)) {
...
break;
}
--wbc->nr_to_write // Skip if 'error < 0'. And if writepage invokes
redirty_page_for_writepage(), wrote could be negative.

I think the root cause is fsync gets buffer head's lock without locking
corresponding page, fixing 'progess' and flushing plug are both workarounds.

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