[PATCH 8/9] libxfs: don't repeatedly shake unwritable buffers

[Dave Chinner <david@xxxxxxxxxxxxx>]

From: Dave Chinner <dchinner@xxxxxxxxxx>

now that we try to write dirty buffers before we release them, we
can get buildup of unwritable dirty buffers on the LRU lists, This
results in the cache shaker repeatedly trying to write out these
buffers every time the cache fills up. This results in more
corruption warnings, and takes up a lot of time doing reclaiming
nothing. This can effectively livelock the processing parts of phase
4.

Fix this by not trying to write buffers with corruption errors on
them. These errors will get cleared when the buffer is re-read and
fixed and them marked dirty again. At which point, we'll be able to
write them and so the cache can reclaim them successfully.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 libxfs/rdwr.c | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c
index 0337a21..a1f0029 100644
--- a/libxfs/rdwr.c
+++ b/libxfs/rdwr.c
@@ -1103,7 +1103,6 @@ int
 libxfs_writebufr(xfs_buf_t *bp)
 {
        int     fd = libxfs_device_to_fd(bp->b_target->dev);
-       int     error = 0;
 
        /*
         * we never write buffers that are marked stale. This indicates they
@@ -1134,7 +1133,7 @@ libxfs_writebufr(xfs_buf_t *bp)
        }
 
        if (!(bp->b_flags & LIBXFS_B_DISCONTIG)) {
-               error = __write_buf(fd, bp->b_addr, bp->b_bcount,
+               bp->b_error = __write_buf(fd, bp->b_addr, bp->b_bcount,
                                    LIBXFS_BBTOOFF64(bp->b_bn), bp->b_flags);
        } else {
                int     i;
@@ -1144,11 +1143,10 @@ libxfs_writebufr(xfs_buf_t *bp)
                        off64_t offset = LIBXFS_BBTOOFF64(bp->b_map[i].bm_bn);
                        int len = BBTOB(bp->b_map[i].bm_len);
 
-                       error = __write_buf(fd, buf, len, offset, bp->b_flags);
-                       if (error) {
-                               bp->b_error = error;
+                       bp->b_error = __write_buf(fd, buf, len, offset,
+                                                 bp->b_flags);
+                       if (bp->b_error)
                                break;
-                       }
                        buf += len;
                }
        }
@@ -1157,14 +1155,14 @@ libxfs_writebufr(xfs_buf_t *bp)
        printf("%lx: %s: wrote %u bytes, blkno=%llu(%llu), %p, error %d\n",
                        pthread_self(), __FUNCTION__, bp->b_bcount,
                        (long long)LIBXFS_BBTOOFF64(bp->b_bn),
-                       (long long)bp->b_bn, bp, error);
+                       (long long)bp->b_bn, bp, bp->b_error);
 #endif
-       if (!error) {
+       if (!bp->b_error) {
                bp->b_flags |= LIBXFS_B_UPTODATE;
                bp->b_flags &= ~(LIBXFS_B_DIRTY | LIBXFS_B_EXIT |
                                 LIBXFS_B_UNCHECKED);
        }
-       return error;
+       return bp->b_error;
 }
 
 int
@@ -1266,15 +1264,22 @@ libxfs_bulkrelse(
        return count;
 }
 
+/*
+ * When a buffer is marked dirty, the error is cleared. Hence if we are trying
+ * to flush a buffer prior to cache reclaim that has an error on it it means
+ * we've already tried to flush it and it failed. Prevent repeated corruption
+ * errors from being reported by skipping such buffers - when the corruption is
+ * fixed the buffer will be marked dirty again and we can write it again.
+ */
 static int
 libxfs_bflush(
        struct cache_node       *node)
 {
        struct xfs_buf          *bp = (struct xfs_buf *)node;
 
-       if (bp->b_flags & LIBXFS_B_DIRTY)
+       if (!bp->b_error && bp->b_flags & LIBXFS_B_DIRTY)
                return libxfs_writebufr(bp);
-       return 0;
+       return bp->b_error;
 }
 
 void
-- 
2.5.0