From 3efaf0faba6793cd91298c76315e15de59c13ae0 Mon Sep 17 00:00:00 2001 From: Wu Fengguang Date: Thu, 16 Dec 2010 22:22:00 -0600 Subject: [PATCH] writeback: skip balance_dirty_pages() for in-memory fs This avoids unnecessary checks and dirty throttling on tmpfs/ramfs. Notes about the tmpfs/ramfs behavior changes: As for 2.6.36 and older kernels, the tmpfs writes will sleep inside balance_dirty_pages() as long as we are over the (dirty+background)/2 global throttle threshold. This is because both the dirty pages and threshold will be 0 for tmpfs/ramfs. Hence this test will always evaluate to TRUE: dirty_exceeded = (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh) || (nr_reclaimable + nr_writeback >= dirty_thresh); For 2.6.37, someone complained that the current logic does not allow the users to set vm.dirty_ratio=0. So commit 4cbec4c8b9 changed the test to dirty_exceeded = (bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh) || (nr_reclaimable + nr_writeback > dirty_thresh); So 2.6.37 will behave differently for tmpfs/ramfs: it will never get throttled unless the global dirty threshold is exceeded (which is very unlikely to happen; once happen, will block many tasks). I'd say that the 2.6.36 behavior is very bad for tmpfs/ramfs. It means for a busy writing server, tmpfs write()s may get livelocked! The "inadvertent" throttling can hardly bring help to any workload because of its "either no throttling, or get throttled to death" property. So based on 2.6.37, this patch won't bring more noticeable changes. CC: Hugh Dickins Acked-by: Rik van Riel Acked-by: Peter Zijlstra Reviewed-by: Minchan Kim Signed-off-by: Wu Fengguang --- mm/page-writeback.c | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index b8be62381396..b2529f8f8be0 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -244,13 +244,8 @@ void task_dirty_inc(struct task_struct *tsk) static void bdi_writeout_fraction(struct backing_dev_info *bdi, long *numerator, long *denominator) { - if (bdi_cap_writeback_dirty(bdi)) { - prop_fraction_percpu(&vm_completions, &bdi->completions, + prop_fraction_percpu(&vm_completions, &bdi->completions, numerator, denominator); - } else { - *numerator = 0; - *denominator = 1; - } } static inline void task_dirties_fraction(struct task_struct *tsk, @@ -495,6 +490,9 @@ static void balance_dirty_pages(struct address_space *mapping, bool dirty_exceeded = false; struct backing_dev_info *bdi = mapping->backing_dev_info; + if (!bdi_cap_account_dirty(bdi)) + return; + for (;;) { struct writeback_control wbc = { .sync_mode = WB_SYNC_NONE, -- 2.20.1