kmap() is being deprecated in favor of kmap_local_page(). There are two main problems with kmap(): (1) It comes with an overhead as the mapping space is restricted and protected by a global lock for synchronization and (2) it also requires global TLB invalidation when the kmap’s pool wraps and it might block when the mapping space is fully utilized until a slot becomes available. With kmap_local_page() the mappings are per thread, CPU local, can take page faults, and can be called from any context (including interrupts). It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore, the tasks can be preempted and, when they are scheduled to run again, the kernel virtual addresses are restored and still valid. Since its use in fs/ufs is safe everywhere, it should be preferred. Therefore, replace kmap() with kmap_local_page() in fs/ufs. kunmap_local() requires the mapping address, so return that address from ufs_get_page() to be used in ufs_put_page(). This series could have not been ever made because nothing prevented the previous patch from working properly but Al Viro made a long series of very appreciated comments about how many unnecessary and redundant lines of code I could have removed. He could see things I was entirely unable to notice. Furthermore, he also provided solutions and details about how I could decompose a single patch into a small series of three independent units.[1][2][3] I want to thank him so much for the patience, kindness and the time he decided to spend to provide those analysis and write three messages full of interesting insights. I hope to have not misunderstood too many things, however I'm pretty sure that I made many mistakes due to my scarce knowledge of filesystem and, above all, lack of experience :-) I decided to get rid of the previous numbers and start from scratch (i.e., version 1) because this series has too little to share with the design of the previous patch.[4] [1] https://lore.kernel.org/lkml/Y4E++JERgUMoqfjG@ZenIV/ [2] https://lore.kernel.org/lkml/Y4FG0O7VWTTng5yh@ZenIV/ [3] https://lore.kernel.org/lkml/Y4ONIFJatIGsVNpf@ZenIV/ [4] https://lore.kernel.org/lkml/20221016163855.8173-1-fmdefrancesco@xxxxxxxxx/ Cc: Ira Weiny <ira.weiny@xxxxxxxxx> Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Signed-off-by: Fabio M. De Francesco <fmdrefrancesco@xxxxxxxxx> Fabio M. De Francesco (3): fs/ufs: Use the offset_in_page() helper fs/ufs: Change the signature of ufs_get_page() fs/ufs: Replace kmap() with kmap_local_page() fs/ufs/dir.c | 140 +++++++++++++++++++++++++++------------------------ 1 file changed, 73 insertions(+), 67 deletions(-) -- 2.38.1
