writeback: make writeback_control.nr_to_write straight

Pass struct wb_writeback_work all the way down to writeback_sb_inodes(),
and initialize the struct writeback_control there.

struct writeback_control is basically designed to control writeback of a
single file, but we keep abuse it for writing multiple files in
writeback_sb_inodes() and its callers.

It immediately clean things up, e.g. suddenly wbc.nr_to_write vs
work->nr_pages starts to make sense, and instead of saving and restoring
pages_skipped in writeback_sb_inodes it can always start with a clean
zero value.

It also makes a neat IO pattern change: large dirty files are now
written in the full 4MB writeback chunk size, rather than whatever
remained quota in wbc->nr_to_write.

Acked-by: Jan Kara <jack@suse.cz>
Proposed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

1 files changed, 111 insertions, 85 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 6caa98247a5b..2c947da39f6e 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -30,11 +30,21 @@
 #include "internal.h"
 
 /*
+ * The maximum number of pages to writeout in a single bdi flush/kupdate
+ * operation.  We do this so we don't hold I_SYNC against an inode for
+ * enormous amounts of time, which would block a userspace task which has
+ * been forced to throttle against that inode.  Also, the code reevaluates
+ * the dirty each time it has written this many pages.
+ */
+#define MAX_WRITEBACK_PAGES     1024L
+
+/*
  * Passed into wb_writeback(), essentially a subset of writeback_control
  */
 struct wb_writeback_work {
         long nr_pages;
         struct super_block *sb;
+        unsigned long *older_than_this;
         enum writeback_sync_modes sync_mode;
         unsigned int tagged_writepages:1;
         unsigned int for_kupdate:1;
@@ -472,7 +482,6 @@ writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
                          * No need to add it back to the LRU.
                          */
                         list_del_init(&inode->i_wb_list);
-                        wbc->inodes_written++;
                 }
         }
         inode_sync_complete(inode);
@@ -506,6 +515,31 @@ static bool pin_sb_for_writeback(struct super_block *sb)
         return false;
 }
 
+static long writeback_chunk_size(struct wb_writeback_work *work)
+{
+        long pages;
+
+        /*
+         * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
+         * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
+         * here avoids calling into writeback_inodes_wb() more than once.
+         *
+         * The intended call sequence for WB_SYNC_ALL writeback is:
+         *
+         *      wb_writeback()
+         *          writeback_sb_inodes()       <== called only once
+         *              write_cache_pages()     <== called once for each inode
+         *                   (quickly) tag currently dirty pages
+         *                   (maybe slowly) sync all tagged pages
+         */
+        if (work->sync_mode == WB_SYNC_ALL || work->tagged_writepages)
+                pages = LONG_MAX;
+        else
+                pages = min(MAX_WRITEBACK_PAGES, work->nr_pages);
+
+        return pages;
+}
+
 /*
  * Write a portion of b_io inodes which belong to @sb.
  *
@@ -513,18 +547,30 @@ static bool pin_sb_for_writeback(struct super_block *sb)
  * inodes. Otherwise write only ones which go sequentially
  * in reverse order.
  *
- * Return 1, if the caller writeback routine should be
- * interrupted. Otherwise return 0.
+ * Return the number of pages and/or inodes written.
  */
-static int writeback_sb_inodes(struct super_block *sb, struct bdi_writeback *wb,
-                struct writeback_control *wbc, bool only_this_sb)
+static long writeback_sb_inodes(struct super_block *sb,
+                                struct bdi_writeback *wb,
+                                struct wb_writeback_work *work)
 {
+        struct writeback_control wbc = {
+                .sync_mode              = work->sync_mode,
+                .tagged_writepages      = work->tagged_writepages,
+                .for_kupdate            = work->for_kupdate,
+                .for_background         = work->for_background,
+                .range_cyclic           = work->range_cyclic,
+                .range_start            = 0,
+                .range_end              = LLONG_MAX,
+        };
+        unsigned long start_time = jiffies;
+        long write_chunk;
+        long wrote = 0;  /* count both pages and inodes */
+
         while (!list_empty(&wb->b_io)) {
-                long pages_skipped;
                 struct inode *inode = wb_inode(wb->b_io.prev);
 
                 if (inode->i_sb != sb) {
-                        if (only_this_sb) {
+                        if (work->sb) {
                                 /*
                                  * We only want to write back data for this
                                  * superblock, move all inodes not belonging
@@ -539,7 +585,7 @@ static int writeback_sb_inodes(struct super_block *sb, struct bdi_writeback *wb,
                          * Bounce back to the caller to unpin this and
                          * pin the next superblock.
                          */
-                        return 0;
+                        break;
                 }
 
                 /*
@@ -553,12 +599,18 @@ static int writeback_sb_inodes(struct super_block *sb, struct bdi_writeback *wb,
                         requeue_io(inode, wb);
                         continue;
                 }
-
                 __iget(inode);
+                write_chunk = writeback_chunk_size(work);
+                wbc.nr_to_write = write_chunk;
+                wbc.pages_skipped = 0;
+
+                writeback_single_inode(inode, wb, &wbc);
 
-                pages_skipped = wbc->pages_skipped;
-                writeback_single_inode(inode, wb, wbc);
-                if (wbc->pages_skipped != pages_skipped) {
+                work->nr_pages -= write_chunk - wbc.nr_to_write;
+                wrote += write_chunk - wbc.nr_to_write;
+                if (!(inode->i_state & I_DIRTY))
+                        wrote++;
+                if (wbc.pages_skipped) {
                         /*
                          * writeback is not making progress due to locked
                          * buffers.  Skip this inode for now.
@@ -570,17 +622,25 @@ static int writeback_sb_inodes(struct super_block *sb, struct bdi_writeback *wb,
                 iput(inode);
                 cond_resched();
                 spin_lock(&wb->list_lock);
-                if (wbc->nr_to_write <= 0)
-                        return 1;
+                /*
+                 * bail out to wb_writeback() often enough to check
+                 * background threshold and other termination conditions.
+                 */
+                if (wrote) {
+                        if (time_is_before_jiffies(start_time + HZ / 10UL))
+                                break;
+                        if (work->nr_pages <= 0)
+                                break;
+                }
         }
-        /* b_io is empty */
-        return 1;
+        return wrote;
 }
 
-static void __writeback_inodes_wb(struct bdi_writeback *wb,
-                                  struct writeback_control *wbc)
+static long __writeback_inodes_wb(struct bdi_writeback *wb,
+                                  struct wb_writeback_work *work)
 {
-        int ret = 0;
+        unsigned long start_time = jiffies;
+        long wrote = 0;
 
         while (!list_empty(&wb->b_io)) {
                 struct inode *inode = wb_inode(wb->b_io.prev);
@@ -590,33 +650,37 @@ static void __writeback_inodes_wb(struct bdi_writeback *wb,
                         requeue_io(inode, wb);
                         continue;
                 }
-                ret = writeback_sb_inodes(sb, wb, wbc, false);
+                wrote += writeback_sb_inodes(sb, wb, work);
                 drop_super(sb);
 
-                if (ret)
-                        break;
+                /* refer to the same tests at the end of writeback_sb_inodes */
+                if (wrote) {
+                        if (time_is_before_jiffies(start_time + HZ / 10UL))
+                                break;
+                        if (work->nr_pages <= 0)
+                                break;
+                }
         }
         /* Leave any unwritten inodes on b_io */
+        return wrote;
 }
 
-void writeback_inodes_wb(struct bdi_writeback *wb,
-                struct writeback_control *wbc)
+long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages)
 {
+        struct wb_writeback_work work = {
+                .nr_pages       = nr_pages,
+                .sync_mode      = WB_SYNC_NONE,
+                .range_cyclic   = 1,
+        };
+
         spin_lock(&wb->list_lock);
         if (list_empty(&wb->b_io))
-                queue_io(wb, wbc->older_than_this);
-        __writeback_inodes_wb(wb, wbc);
+                queue_io(wb, NULL);
+        __writeback_inodes_wb(wb, &work);
         spin_unlock(&wb->list_lock);
-}
 
-/*
- * The maximum number of pages to writeout in a single bdi flush/kupdate
- * operation.  We do this so we don't hold I_SYNC against an inode for
- * enormous amounts of time, which would block a userspace task which has
- * been forced to throttle against that inode.  Also, the code reevaluates
- * the dirty each time it has written this many pages.
- */
-#define MAX_WRITEBACK_PAGES     1024
+        return nr_pages - work.nr_pages;
+}
 
 static inline bool over_bground_thresh(void)
 {
@@ -646,42 +710,13 @@ static inline bool over_bground_thresh(void)
 static long wb_writeback(struct bdi_writeback *wb,
                          struct wb_writeback_work *work)
 {
-        struct writeback_control wbc = {
-                .sync_mode              = work->sync_mode,
-                .tagged_writepages      = work->tagged_writepages,
-                .older_than_this        = NULL,
-                .for_kupdate            = work->for_kupdate,
-                .for_background         = work->for_background,
-                .range_cyclic           = work->range_cyclic,
-        };
+        long nr_pages = work->nr_pages;
         unsigned long oldest_jif;
-        long wrote = 0;
-        long write_chunk = MAX_WRITEBACK_PAGES;
         struct inode *inode;
-
-        if (!wbc.range_cyclic) {
-                wbc.range_start = 0;
-                wbc.range_end = LLONG_MAX;
-        }
-
-        /*
-         * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
-         * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
-         * here avoids calling into writeback_inodes_wb() more than once.
-         *
-         * The intended call sequence for WB_SYNC_ALL writeback is:
-         *
-         *      wb_writeback()
-         *          writeback_sb_inodes()       <== called only once
-         *              write_cache_pages()     <== called once for each inode
-         *                   (quickly) tag currently dirty pages
-         *                   (maybe slowly) sync all tagged pages
-         */
-        if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_writepages)
-                write_chunk = LONG_MAX;
+        long progress;
 
         oldest_jif = jiffies;
-        wbc.older_than_this = &oldest_jif;
+        work->older_than_this = &oldest_jif;
 
         spin_lock(&wb->list_lock);
         for (;;) {
@@ -711,24 +746,17 @@ static long wb_writeback(struct bdi_writeback *wb,
                 if (work->for_kupdate) {
                         oldest_jif = jiffies -
                                 msecs_to_jiffies(dirty_expire_interval * 10);
-                        wbc.older_than_this = &oldest_jif;
+                        work->older_than_this = &oldest_jif;
                 }
 
-                wbc.nr_to_write = write_chunk;
-                wbc.pages_skipped = 0;
-                wbc.inodes_written = 0;
-
-                trace_wbc_writeback_start(&wbc, wb->bdi);
+                trace_writeback_start(wb->bdi, work);
                 if (list_empty(&wb->b_io))
-                        queue_io(wb, wbc.older_than_this);
+                        queue_io(wb, work->older_than_this);
                 if (work->sb)
-                        writeback_sb_inodes(work->sb, wb, &wbc, true);
+                        progress = writeback_sb_inodes(work->sb, wb, work);
                 else
-                        __writeback_inodes_wb(wb, &wbc);
-                trace_wbc_writeback_written(&wbc, wb->bdi);
-
-                work->nr_pages -= write_chunk - wbc.nr_to_write;
-                wrote += write_chunk - wbc.nr_to_write;
+                        progress = __writeback_inodes_wb(wb, work);
+                trace_writeback_written(wb->bdi, work);
 
                 /*
                  * Did we write something? Try for more
@@ -738,9 +766,7 @@ static long wb_writeback(struct bdi_writeback *wb,
                  * mean the overall work is done. So we keep looping as long
                  * as made some progress on cleaning pages or inodes.
                  */
-                if (wbc.nr_to_write < write_chunk)
-                        continue;
-                if (wbc.inodes_written)
+                if (progress)
                         continue;
                 /*
                  * No more inodes for IO, bail
@@ -753,8 +779,8 @@ static long wb_writeback(struct bdi_writeback *wb,
                  * we'll just busyloop.
                  */
                 if (!list_empty(&wb->b_more_io))  {
+                        trace_writeback_wait(wb->bdi, work);
                         inode = wb_inode(wb->b_more_io.prev);
-                        trace_wbc_writeback_wait(&wbc, wb->bdi);
                         spin_lock(&inode->i_lock);
                         inode_wait_for_writeback(inode, wb);
                         spin_unlock(&inode->i_lock);
@@ -762,7 +788,7 @@ static long wb_writeback(struct bdi_writeback *wb,
         }
         spin_unlock(&wb->list_lock);
 
-        return wrote;
+        return nr_pages - work->nr_pages;
 }
 
 /*