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    Subject[PATCH 4/6] writeback: pay attention to wbc->nr_to_write in write_cache_pages
    Date
    From: Dave Chinner <dchinner@redhat.com>

    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, 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 in tact. 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@redhat.com>
    ---
    include/linux/writeback.h | 9 ---------
    include/trace/events/writeback.h | 1 +
    mm/page-writeback.c | 16 ++++++----------
    3 files changed, 7 insertions(+), 19 deletions(-)

    diff --git a/include/linux/writeback.h b/include/linux/writeback.h
    index a74837e..488ac1c 100644
    --- a/include/linux/writeback.h
    +++ b/include/linux/writeback.h
    @@ -55,15 +55,6 @@ 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/writeback.h b/include/trace/events/writeback.h
    index 72c1a12..5dda40e 100644
    --- a/include/trace/events/writeback.h
    +++ b/include/trace/events/writeback.h
    @@ -244,6 +244,7 @@ DEFINE_WBC_EVENT(wbc_writeback_wait);
    DEFINE_WBC_EVENT(wbc_balance_dirty_start);
    DEFINE_WBC_EVENT(wbc_balance_dirty_written);
    DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
    +DEFINE_WBC_EVENT(wbc_writepage);

    #endif /* _TRACE_WRITEBACK_H */

    diff --git a/mm/page-writeback.c b/mm/page-writeback.c
    index 68eb727..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) {
    @@ -921,6 +920,7 @@ continue_unlock:
    if (!clear_page_dirty_for_io(page))
    goto continue_unlock;

    + trace_wbc_writepage(wbc);
    ret = (*writepage)(page, wbc, data);
    if (unlikely(ret)) {
    if (ret == AOP_WRITEPAGE_ACTIVATE) {
    @@ -939,11 +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
    @@ -974,11 +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;
    }
    --
    1.5.6.5


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