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[PATCH 4/6] writeback: pay attention to wbc->nr_to_write in write_cache_

To: linux-kernel@xxxxxxxxxxxxxxx
Subject: [PATCH 4/6] writeback: pay attention to wbc->nr_to_write in write_cache_pages
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 25 May 2010 20:54:10 +1000
Cc: xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, tytso@xxxxxxx, jens.axboe@xxxxxxxxxx
In-reply-to: <1274784852-30502-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1274784852-30502-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

If a filesystem writes more than one page in ->writepage, write_cache_pages
fails to notice this and continues to attempt writeback when wbc->nr_to_write
has gone negative - this trace was captured from XFS:


    wbc_writeback_start: towrt=1024
    wbc_writepage: towrt=1024
    wbc_writepage: towrt=0
    wbc_writepage: towrt=-1
    wbc_writepage: towrt=-5
    wbc_writepage: towrt=-21
    wbc_writepage: towrt=-85

This has adverse effects on filesystem writeback behaviour. write_cache_pages()
needs to terminate after a certain number of pages are written, not after a
certain number of calls to ->writepage are made.  This is a regression
introduced by 17bc6c30cf6bfffd816bdc53682dd46fc34a2cf4, but cannot be reverted
directly due to subsequent bug fixes that have gone in on top of it.

This commit adds a ->writepage tracepoint inside write_cache_pages() (how the
above trace was generated) and does the revert manually leaving the subsequent
bug fixes in tact. ext4 is not affected by this as a previous commit in the
series stops ext4 from using the generic function.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 include/linux/writeback.h        |    9 ---------
 include/trace/events/writeback.h |    1 +
 mm/page-writeback.c              |   16 ++++++----------
 3 files changed, 7 insertions(+), 19 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index a74837e..488ac1c 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -55,15 +55,6 @@ struct writeback_control {
        unsigned for_reclaim:1;         /* Invoked from the page allocator */
        unsigned range_cyclic:1;        /* range_start is cyclic */
        unsigned more_io:1;             /* more io to be dispatched */
-       /*
-        * write_cache_pages() won't update wbc->nr_to_write and
-        * mapping->writeback_index if no_nrwrite_index_update
-        * is set.  write_cache_pages() may write more than we
-        * requested and we want to make sure nr_to_write and
-        * writeback_index are updated in a consistent manner
-        * so we use a single control to update them
-        */
-       unsigned no_nrwrite_index_update:1;
 
        /*
         * For WB_SYNC_ALL, the sb must always be pinned. For WB_SYNC_NONE,
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 72c1a12..5dda40e 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -244,6 +244,7 @@ DEFINE_WBC_EVENT(wbc_writeback_wait);
 DEFINE_WBC_EVENT(wbc_balance_dirty_start);
 DEFINE_WBC_EVENT(wbc_balance_dirty_written);
 DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
+DEFINE_WBC_EVENT(wbc_writepage);
 
 #endif /* _TRACE_WRITEBACK_H */
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 68eb727..0fe713d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -839,7 +839,6 @@ int write_cache_pages(struct address_space *mapping,
        pgoff_t done_index;
        int cycled;
        int range_whole = 0;
-       long nr_to_write = wbc->nr_to_write;
 
        pagevec_init(&pvec, 0);
        if (wbc->range_cyclic) {
@@ -921,6 +920,7 @@ continue_unlock:
                        if (!clear_page_dirty_for_io(page))
                                goto continue_unlock;
 
+                       trace_wbc_writepage(wbc);
                        ret = (*writepage)(page, wbc, data);
                        if (unlikely(ret)) {
                                if (ret == AOP_WRITEPAGE_ACTIVATE) {
@@ -939,11 +939,10 @@ continue_unlock:
                                        done = 1;
                                        break;
                                }
-                       }
+                       }
 
-                       if (nr_to_write > 0) {
-                               nr_to_write--;
-                               if (nr_to_write == 0 &&
+                       if (wbc->nr_to_write > 0) {
+                               if (--wbc->nr_to_write == 0 &&
                                    wbc->sync_mode == WB_SYNC_NONE) {
                                        /*
                                         * We stop writing back only if we are
@@ -974,11 +973,8 @@ continue_unlock:
                end = writeback_index - 1;
                goto retry;
        }
-       if (!wbc->no_nrwrite_index_update) {
-               if (wbc->range_cyclic || (range_whole && nr_to_write > 0))
-                       mapping->writeback_index = done_index;
-               wbc->nr_to_write = nr_to_write;
-       }
+       if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
+               mapping->writeback_index = done_index;
 
        return ret;
 }
-- 
1.5.6.5

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