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

To: linux-fsdevel@xxxxxxxxxxxxxxx
Subject: [PATCH 4/6] writeback: pay attention to wbc->nr_to_write in write_cache_pages
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 4 Jun 2010 09:55:26 +1000
Cc: linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx
In-reply-to: <1275609328-12514-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1275609328-12514-1-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 ("vfs: Add
no_nrwrite_index_update writeback control flag"), 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 intact. 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/ext4.h |    5 +----
 mm/page-writeback.c         |   15 +++++----------
 3 files changed, 6 insertions(+), 23 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index cc97d6c..52e82f3 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -56,15 +56,6 @@ 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/ext4.h b/include/trace/events/ext4.h
index f5b1ba9..f3865c7 100644
--- a/include/trace/events/ext4.h
+++ b/include/trace/events/ext4.h
@@ -306,7 +306,6 @@ TRACE_EVENT(ext4_da_writepages_result,
                __field(        int,    pages_written           )
                __field(        long,   pages_skipped           )
                __field(        char,   more_io                 )       
-               __field(        char,   no_nrwrite_index_update )
                __field(       pgoff_t, writeback_index         )
        ),
 
@@ -317,16 +316,14 @@ TRACE_EVENT(ext4_da_writepages_result,
                __entry->pages_written  = pages_written;
                __entry->pages_skipped  = wbc->pages_skipped;
                __entry->more_io        = wbc->more_io;
-               __entry->no_nrwrite_index_update = wbc->no_nrwrite_index_update;
                __entry->writeback_index = inode->i_mapping->writeback_index;
        ),
 
-       TP_printk("dev %s ino %lu ret %d pages_written %d pages_skipped %ld 
more_io %d no_nrwrite_index_update %d writeback_index %lu",
+       TP_printk("dev %s ino %lu ret %d pages_written %d pages_skipped %ld 
more_io %d writeback_index %lu",
                  jbd2_dev_to_name(__entry->dev),
                  (unsigned long) __entry->ino, __entry->ret,
                  __entry->pages_written, __entry->pages_skipped,
                  __entry->more_io,
-                 __entry->no_nrwrite_index_update,
                  (unsigned long) __entry->writeback_index)
 );
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index caaf954..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) {
@@ -940,11 +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
@@ -975,11 +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;
 }
-- 
1.7.1

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