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Date: Tue, 14 Sep 2021 10:13:04 +1000 From: NeilBrown <neilb@...e.de> To: Andrew Morton <akpm@...ux-foundation.org>, Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, "Darrick J. Wong" <djwong@...nel.org>, Matthew Wilcox <willy@...radead.org>, Mel Gorman <mgorman@...e.com> Cc: linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective. Only 4 subsystems call set_bdi_congested() or clear_bdi_congested(): block/pktcdvd, fs/ceph fs/fuse fs/nfs It may make sense to use congestion_wait() or wait_iff_congested() within these subsystems, but they have no value outside of these. Add documentation comments to these functions to discourage further use. Signed-off-by: NeilBrown <neilb@...e.de> --- include/linux/backing-dev.h | 7 +++++++ mm/backing-dev.c | 9 +++++++++ 2 files changed, 16 insertions(+) diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h index ac7f231b8825..cc9513840351 100644 --- a/include/linux/backing-dev.h +++ b/include/linux/backing-dev.h @@ -153,6 +153,13 @@ static inline int wb_congested(struct bdi_writeback *wb, int cong_bits) return wb->congested & cong_bits; } +/* NOTE congestion_wait() and wait_iff_congested() are + * largely useless except as documentation. + * congestion_wait() will (almost) always wait for the given timeout. + * wait_iff_congested() will (almost) never wait, but will call + * cond_resched(). + * Were possible an alternative waiting strategy should be found. + */ long congestion_wait(int sync, long timeout); long wait_iff_congested(int sync, long timeout); diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 4a9d4e27d0d9..53472ab38796 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -1023,6 +1023,11 @@ EXPORT_SYMBOL(set_bdi_congested); * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit * write congestion. If no backing_devs are congested then just wait for the * next write to be completed. + * + * NOTE: in the current implementation, hardly any backing_devs are ever + * marked as congested, and write-completion is rarely reported (see calls + * to clear_bdi_congested). So this should not be assumed to ever wake before + * the timeout. */ long congestion_wait(int sync, long timeout) { @@ -1054,6 +1059,10 @@ EXPORT_SYMBOL(congestion_wait); * The return value is 0 if the sleep is for the full timeout. Otherwise, * it is the number of jiffies that were still remaining when the function * returned. return_value == timeout implies the function did not sleep. + * + * NOTE: in the current implementation, hardly any backing_devs are ever + * marked as congested, and write-completion is rarely reported (see calls + * to clear_bdi_congested). So this should not be assumed to sleep at all. */ long wait_iff_congested(int sync, long timeout) {
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