Commit 0b564927 authored by Dave Chinner's avatar Dave Chinner Committed by Linus Torvalds
Browse files

writeback: pay attention to wbc->nr_to_write in write_cache_pages 



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 17bc6c30 ("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.

Signed-off-by: default avatarDave Chinner <dchinner@redhat.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8d7458da

include/linux/writeback.h

+0 −9
Original line number Diff line number Diff line
@@ -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;

};



/*

include/trace/events/ext4.h

+1 −4
Original line number Diff line number Diff line
@@ -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)

);

mm/page-writeback.c

+5 −10
Original line number Diff line number Diff line
@@ -835,7 +835,6 @@ 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) {
@@ -937,9 +936,8 @@ continue_unlock: 
				}

			}



			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
@@ -970,11 +968,8 @@ continue_unlock: 
		end = writeback_index - 1;

		goto retry;

	}

	if (!wbc->no_nrwrite_index_update) {

		if (wbc->range_cyclic || (range_whole && nr_to_write > 0))

	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))

		mapping->writeback_index = done_index;

		wbc->nr_to_write = nr_to_write;

	}



	return ret;

}

CRA Git | Maintained and supported by SUSTech CRA and CCSE