Re: [PATCH] btrfs: Use the normal writeback path for flushing delalloc

On Mon, Jan 4, 2021 at 5:06 PM Josef Bacik josef@toxicpanda.com wrote:

This is a revert for 38d715f494f2 ("btrfs: use btrfs_start_delalloc_roots in shrink_delalloc"). A user reported a problem where performance was significantly worse with this patch applied. The problem needs to be fixed with proper pre-flushing, and changes to how we deal with the work queues for the inodes. However that work is much more complicated than is acceptable for stable, and simply reverting this patch fixes the problem. The original patch was a cleanup of the code, so it's fine to revert it. My numbers for the original reported test, which was untarring a copy of the firefox sources, are as follows

5.9 0m54.258s 5.10 1m26.212s Fix 0m35.038s

cc: stable@vger.kernel.org # 5.10 Reported-by: René Rebe rene@exactcode.de Fixes: 38d715f494f2 ("btrfs: use btrfs_start_delalloc_roots in shrink_delalloc") Signed-off-by: Josef Bacik josef@toxicpanda.com

---

Dave, this is ontop of linus's branch, because we've changed the arguments for btrfs_start_delalloc_roots in misc-next, and this needs to go back to 5.10 ASAP. I can send a misc-next version if you want to have it there as well while we're waiting for it to go into linus's tree, just let me know.

Adding this to stable releases will also make the following fix not work on stable releases:

https://lore.kernel.org/linux-btrfs/39c2a60aa682f69f9823f51aa119d37ef4b9f834...

Since now the async reclaim task can trigger writeback through writeback_inodes_sb_nr() and not only through btrfs_start_delalloc_roots(). Other than changing that patch to make extent_write_cache_pages() do nothing when the inode has the bit BTRFS_INODE_NO_DELALLOC_FLUSH set, I'm not seeing other simple ways to do it.

Thanks.

fs/btrfs/space-info.c | 54 ++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 53 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c index 64099565ab8f..a2b322275b8d 100644 --- a/fs/btrfs/space-info.c +++ b/fs/btrfs/space-info.c @@ -465,6 +465,28 @@ void btrfs_dump_space_info(struct btrfs_fs_info *fs_info, up_read(&info->groups_sem); }

+static void btrfs_writeback_inodes_sb_nr(struct btrfs_fs_info *fs_info,

•                                    unsigned long nr_pages, u64 nr_items)
  

+{

•   struct super_block *sb = fs_info->sb;
  

•   if (down_read_trylock(&sb->s_umount)) {
  

•           writeback_inodes_sb_nr(sb, nr_pages, WB_REASON_FS_FREE_SPACE);
  

•           up_read(&sb->s_umount);
  

•   } else {
  

•           /*
  

•            * We needn't worry the filesystem going from r/w to r/o though
  

•            * we don't acquire ->s_umount mutex, because the filesystem
  

•            * should guarantee the delalloc inodes list be empty after
  

•            * the filesystem is readonly(all dirty pages are written to
  

•            * the disk).
  

•            */
  

•           btrfs_start_delalloc_roots(fs_info, nr_items);
  

•           if (!current->journal_info)
  

•                   btrfs_wait_ordered_roots(fs_info, nr_items, 0, (u64)-1);
  

•   }
  

+}

static inline u64 calc_reclaim_items_nr(struct btrfs_fs_info *fs_info, u64 to_reclaim) { @@ -490,8 +512,10 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info, struct btrfs_trans_handle *trans; u64 delalloc_bytes; u64 dio_bytes;

•   u64 async_pages;
    u64 items;
    long time_left;
  

•   unsigned long nr_pages;
    int loops;
  
    /* Calc the number of the pages we need flush for space reservation */
  

@@ -532,8 +556,36 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info,

    loops = 0;
    while ((delalloc_bytes || dio_bytes) && loops < 3) {

•           btrfs_start_delalloc_roots(fs_info, items);
  

•           nr_pages = min(delalloc_bytes, to_reclaim) >> PAGE_SHIFT;
  

•           /*
  

•            * Triggers inode writeback for up to nr_pages. This will invoke
  

•            * ->writepages callback and trigger delalloc filling
  

•            *  (btrfs_run_delalloc_range()).
  

•            */
  

•           btrfs_writeback_inodes_sb_nr(fs_info, nr_pages, items);
  

•           /*
  

•            * We need to wait for the compressed pages to start before
  

•            * we continue.
  

•            */
  

•           async_pages = atomic_read(&fs_info->async_delalloc_pages);
  

•           if (!async_pages)
  

•                   goto skip_async;
  

•           /*
  

•            * Calculate how many compressed pages we want to be written
  

•            * before we continue. I.e if there are more async pages than we
  

•            * require wait_event will wait until nr_pages are written.
  

•            */
  

•           if (async_pages <= nr_pages)
  

•                   async_pages = 0;
  

•           else
  

•                   async_pages -= nr_pages;
  
  

•           wait_event(fs_info->async_submit_wait,
  

•                      atomic_read(&fs_info->async_delalloc_pages) <=
  

•                      (int)async_pages);
  

+skip_async: loops++; if (wait_ordered && !trans) { btrfs_wait_ordered_roots(fs_info, items, 0, (u64)-1); -- 2.26.2