Re: [PATCH 5/6] writeback: sync expired inodes first in backgroundwriteback

From: Wu Fengguang
Date: Tue Apr 26 2011 - 10:06:10 EST

> I scratched a patch (totally untested) which will guarantee any kind
> of starvation inside an inode. Will this be too overweight?
>
> Thanks,
> Fengguang
> ---
> Subject: writeback: livelock prevention inside actively dirtied files
> Date: Tue Apr 26 21:35:47 CST 2011
>
> - refresh dirtied_when on every full writeback_index cycle
> (pages may be skipped on SYNC_NONE, but as long as they are retried in
> next cycle..)
>
> - do tagged sync when writeback_index not cycled for too long time
> (the arbitrarily 60s may lead to more page tagging overheads in
> "large dirty threshold but slow storage" system..)
>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
> fs/fs-writeback.c | 1 +
> include/linux/fs.h | 1 +
> include/linux/pagemap.h | 16 ++++++++++++++++
> mm/page-writeback.c | 24 ++++++++++++++++++------
> 4 files changed, 36 insertions(+), 6 deletions(-)

FYI to make it easier, the above patch is actually based upon this one.
---
Subject: writeback: quit on wrap for .range_cyclic
Date: Mon Apr 25 14:58:19 CST 2011

Convert wbc.range_cyclic to new behavior: when past EOF, abort the
writeback of the current inode, which will instruct
writeback_single_inode() to redirty_tail() it.

This is the right behavior for
- sync writeback (is already so with range_whole)
we have scanned the inode address space, and don't care any more newly
dirtied pages. So shall update its i_dirtied_when and exclude it from
the todo list.
- periodic writeback
any more newly dirtied pages should be associated with a new expire
time. This also prevents pointless IO for busy overwriters.
- background writeback
irrelevant because it generally don't care the dirty timestamp.

That should get rid of one inefficient IO pattern of .range_cyclic when
writeback_index wraps, in which the submitted pages may be consisted of
two distant ranges: submit [10000-10100], (wrap), submit [0-100].

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
drivers/staging/pohmelfs/inode.c | 25 ++++++++-----------------
fs/afs/write.c | 7 +++----
fs/btrfs/extent_io.c | 21 ++++++---------------
fs/cifs/file.c | 15 +++------------
fs/ext4/inode.c | 18 ++++--------------
fs/gfs2/aops.c | 16 ++--------------
mm/page-writeback.c | 27 +++++----------------------
7 files changed, 31 insertions(+), 98 deletions(-)

--- linux-next.orig/mm/page-writeback.c 2011-04-25 14:56:33.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-04-25 14:57:09.000000000 +0800
@@ -868,35 +868,27 @@ int write_cache_pages(struct address_spa
int done = 0;
struct pagevec pvec;
int nr_pages;
- pgoff_t uninitialized_var(writeback_index);
pgoff_t index;
pgoff_t end; /* Inclusive */
pgoff_t done_index;
- int cycled;
int range_whole = 0;
int tag;

pagevec_init(&pvec, 0);
if (wbc->range_cyclic) {
- writeback_index = mapping->writeback_index; /* prev offset */
- index = writeback_index;
- if (index == 0)
- cycled = 1;
- else
- cycled = 0;
+ index = mapping->writeback_index; /* prev offset */
end = -1;
} else {
index = wbc->range_start >> PAGE_CACHE_SHIFT;
end = wbc->range_end >> PAGE_CACHE_SHIFT;
if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
range_whole = 1;
- cycled = 1; /* ignore range_cyclic tests */
}
if (wbc->sync_mode == WB_SYNC_ALL)
tag = PAGECACHE_TAG_TOWRITE;
else
tag = PAGECACHE_TAG_DIRTY;
-retry:
+
if (wbc->sync_mode == WB_SYNC_ALL)
tag_pages_for_writeback(mapping, index, end);
done_index = index;
@@ -905,8 +897,10 @@ retry:

nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
- if (nr_pages == 0)
+ if (nr_pages == 0) {
+ done_index = 0;
break;
+ }

for (i = 0; i < nr_pages; i++) {
struct page *page = pvec.pages[i];
@@ -998,17 +992,6 @@ continue_unlock:
pagevec_release(&pvec);
cond_resched();
}
- if (!cycled && !done) {
- /*
- * range_cyclic:
- * We hit the last page and there is more work to be done: wrap
- * back to the start of the file
- */
- cycled = 1;
- index = 0;
- end = writeback_index - 1;
- goto retry;
- }
if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
mapping->writeback_index = done_index;

--- linux-next.orig/drivers/staging/pohmelfs/inode.c 2011-03-31 13:53:02.000000000 +0800
+++ linux-next/drivers/staging/pohmelfs/inode.c 2011-04-25 14:57:03.000000000 +0800
@@ -148,7 +148,6 @@ static int pohmelfs_writepages(struct ad
int nr_pages;
pgoff_t index;
pgoff_t end; /* Inclusive */
- int scanned = 0;
int range_whole = 0;

if (wbc->range_cyclic) {
@@ -159,17 +158,18 @@ static int pohmelfs_writepages(struct ad
end = wbc->range_end >> PAGE_CACHE_SHIFT;
if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
range_whole = 1;
- scanned = 1;
}
-retry:
+
while (!done && (index <= end)) {
unsigned int i = min(end - index, (pgoff_t)psb->trans_max_pages);
int path_len;
struct netfs_trans *trans;

err = pohmelfs_inode_has_dirty_pages(mapping, index);
- if (!err)
+ if (!err) {
+ index = 0;
break;
+ }

err = pohmelfs_path_length(pi);
if (err < 0)
@@ -196,15 +196,16 @@ retry:
dprintk("%s: t: %p, nr_pages: %u, end: %lu, index: %lu, max: %u.\n",
__func__, trans, nr_pages, end, index, trans->page_num);

- if (!nr_pages)
+ if (!nr_pages) {
+ index = 0;
goto err_out_reset;
+ }

err = pohmelfs_write_inode_create(inode, trans);
if (err)
goto err_out_reset;

err = 0;
- scanned = 1;

for (i = 0; i < trans->page_num; i++) {
struct page *page = trans->pages[i];
@@ -214,7 +215,7 @@ retry:
if (unlikely(page->mapping != mapping))
goto out_continue;

- if (!wbc->range_cyclic && page->index > end) {
+ if (page->index > end) {
done = 1;
goto out_continue;
}
@@ -262,16 +263,6 @@ err_out_reset:
break;
}

- if (!scanned && !done) {
- /*
- * We hit the last page and there is more work to be done: wrap
- * back to the start of the file
- */
- scanned = 1;
- index = 0;
- goto retry;
- }
-
if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
mapping->writeback_index = index;

--- linux-next.orig/fs/btrfs/extent_io.c 2011-04-20 17:08:47.000000000 +0800
+++ linux-next/fs/btrfs/extent_io.c 2011-04-25 14:57:50.000000000 +0800
@@ -2464,10 +2464,9 @@ static int extent_write_cache_pages(stru
int done = 0;
int nr_to_write_done = 0;
struct pagevec pvec;
- int nr_pages;
+ int nr_pages = 1;
pgoff_t index;
pgoff_t end; /* Inclusive */
- int scanned = 0;

pagevec_init(&pvec, 0);
if (wbc->range_cyclic) {
@@ -2476,16 +2475,14 @@ static int extent_write_cache_pages(stru
} else {
index = wbc->range_start >> PAGE_CACHE_SHIFT;
end = wbc->range_end >> PAGE_CACHE_SHIFT;
- scanned = 1;
}
-retry:
+
while (!done && !nr_to_write_done && (index <= end) &&
(nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
PAGECACHE_TAG_DIRTY, min(end - index,
(pgoff_t)PAGEVEC_SIZE-1) + 1))) {
unsigned i;

- scanned = 1;
for (i = 0; i < nr_pages; i++) {
struct page *page = pvec.pages[i];

@@ -2506,7 +2503,7 @@ retry:
continue;
}

- if (!wbc->range_cyclic && page->index > end) {
+ if (page->index > end) {
done = 1;
unlock_page(page);
continue;
@@ -2543,15 +2540,9 @@ retry:
pagevec_release(&pvec);
cond_resched();
}
- if (!scanned && !done) {
- /*
- * We hit the last page and there is more work to be done: wrap
- * back to the start of the file
- */
- scanned = 1;
- index = 0;
- goto retry;
- }
+ if (!nr_pages)
+ mapping->writeback_index = 0;
+
return ret;
}

--- linux-next.orig/fs/cifs/file.c 2011-04-20 17:08:47.000000000 +0800
+++ linux-next/fs/cifs/file.c 2011-04-25 14:58:00.000000000 +0800
@@ -1200,7 +1200,6 @@ static int cifs_writepages(struct addres
struct page *page;
struct pagevec pvec;
int rc = 0;
- int scanned = 0;
int xid;

cifs_sb = CIFS_SB(mapping->host->i_sb);
@@ -1241,9 +1240,8 @@ static int cifs_writepages(struct addres
end = wbc->range_end >> PAGE_CACHE_SHIFT;
if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
range_whole = 1;
- scanned = 1;
}
-retry:
+
while (!done && (index <= end) &&
(nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
PAGECACHE_TAG_DIRTY,
@@ -1276,7 +1274,7 @@ retry:
break;
}

- if (!wbc->range_cyclic && page->index > end) {
+ if (page->index > end) {
done = 1;
unlock_page(page);
break;
@@ -1403,15 +1401,8 @@ retry_write:

pagevec_release(&pvec);
}
- if (!scanned && !done) {
- /*
- * We hit the last page and there is more work to be done: wrap
- * back to the start of the file
- */
- scanned = 1;
+ if (!nr_pages)
index = 0;
- goto retry;
- }
if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
mapping->writeback_index = index;

--- linux-next.orig/fs/ext4/inode.c 2011-04-12 11:13:27.000000000 +0800
+++ linux-next/fs/ext4/inode.c 2011-04-25 15:01:30.000000000 +0800
@@ -2893,7 +2893,7 @@ static int ext4_da_writepages(struct add
struct inode *inode = mapping->host;
int pages_written = 0;
unsigned int max_pages;
- int range_cyclic, cycled = 1, io_done = 0;
+ int range_cyclic, io_done = 0;
int needed_blocks, ret = 0;
long desired_nr_to_write, nr_to_writebump = 0;
loff_t range_start = wbc->range_start;
@@ -2930,8 +2930,6 @@ static int ext4_da_writepages(struct add
range_cyclic = wbc->range_cyclic;
if (wbc->range_cyclic) {
index = mapping->writeback_index;
- if (index)
- cycled = 0;
wbc->range_start = index << PAGE_CACHE_SHIFT;
wbc->range_end = LLONG_MAX;
wbc->range_cyclic = 0;
@@ -2974,7 +2972,6 @@ static int ext4_da_writepages(struct add
wbc->nr_to_write = desired_nr_to_write;
}

-retry:
if (wbc->sync_mode == WB_SYNC_ALL)
tag_pages_for_writeback(mapping, index, end);

@@ -3034,20 +3031,13 @@ retry:
pages_written += mpd.pages_written;
ret = 0;
io_done = 1;
- } else if (wbc->nr_to_write)
+ } else if (wbc->nr_to_write > 0) {
/*
* There is no more writeout needed
- * or we requested for a noblocking writeout
- * and we found the device congested
*/
+ index = 0;
break;
- }
- if (!io_done && !cycled) {
- cycled = 1;
- index = 0;
- wbc->range_start = index << PAGE_CACHE_SHIFT;
- wbc->range_end = mapping->writeback_index - 1;
- goto retry;
+ }
}

/* Update index */
--- linux-next.orig/fs/gfs2/aops.c 2011-03-31 13:53:21.000000000 +0800
+++ linux-next/fs/gfs2/aops.c 2011-04-25 14:57:03.000000000 +0800
@@ -283,7 +283,7 @@ static int gfs2_write_jdata_pagevec(stru
continue;
}

- if (!wbc->range_cyclic && page->index > end) {
+ if (page->index > end) {
ret = 1;
unlock_page(page);
continue;
@@ -335,7 +335,6 @@ static int gfs2_write_cache_jdata(struct
int nr_pages;
pgoff_t index;
pgoff_t end;
- int scanned = 0;
int range_whole = 0;

pagevec_init(&pvec, 0);
@@ -347,15 +346,12 @@ static int gfs2_write_cache_jdata(struct
end = wbc->range_end >> PAGE_CACHE_SHIFT;
if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
range_whole = 1;
- scanned = 1;
}

-retry:
while (!done && (index <= end) &&
(nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
PAGECACHE_TAG_DIRTY,
min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
- scanned = 1;
ret = gfs2_write_jdata_pagevec(mapping, wbc, &pvec, nr_pages, end);
if (ret)
done = 1;
@@ -366,16 +362,8 @@ retry:
cond_resched();
}

- if (!scanned && !done) {
- /*
- * We hit the last page and there is more work to be done: wrap
- * back to the start of the file
- */
- scanned = 1;
+ if (!nr_pages)
index = 0;
- goto retry;
- }
-
if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
mapping->writeback_index = index;
return ret;
--- linux-next.orig/fs/afs/write.c 2011-03-02 17:41:41.000000000 +0800
+++ linux-next/fs/afs/write.c 2011-04-25 14:57:03.000000000 +0800
@@ -476,8 +476,10 @@ static int afs_writepages_region(struct
do {
n = find_get_pages_tag(mapping, &index, PAGECACHE_TAG_DIRTY,
1, &page);
- if (!n)
+ if (!n) {
+ index = 0;
break;
+ }

_debug("wback %lx", page->index);

@@ -549,9 +551,6 @@ int afs_writepages(struct address_space
start = mapping->writeback_index;
end = -1;
ret = afs_writepages_region(mapping, wbc, start, end, &next);
- if (start > 0 && wbc->nr_to_write > 0 && ret == 0)
- ret = afs_writepages_region(mapping, wbc, 0, start,
- &next);
mapping->writeback_index = next;
} else if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX) {
end = (pgoff_t)(LLONG_MAX >> PAGE_CACHE_SHIFT);
--
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