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.[1][2][3] Changes from v1: 1/3: No changes. 2/3: Restore the return of "err" that was mistakenly deleted together with the removal of the "out" label in ufs_add_link(). Thanks to Al Viro.[4] Return the address of the kmap()'ed page instead of a pointer to a pointer to the mapped page; a page_address() had been overlooked in ufs_get_page(). Thanks to Al Viro.[5] 3/3: Return the kernel virtual address got from the call to kmap_local_page() after conversion from kmap(). Again thanks to Al Viro.[6] Changes from v2: 1/3: No changes. 2/3: Rework ufs_get_page() because the previous version had two errors: (1) It could return an invalid pages with the out argument "page" and (2) it could return "page_address(page)" also in cases where read_mapping_page() returned an error and the page is never kmap()'ed. Thanks to Al Viro.[7] 3/3: Rework ufs_get_page() after conversion to kmap_local_page(), in accordance to the last changes in 2/3. [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/Y5Zc0qZ3+zsI74OZ@ZenIV/ [5] https://lore.kernel.org/lkml/Y5ZZy23FFAnQDR3C@ZenIV/ [6] https://lore.kernel.org/lkml/Y5ZcMPzPG9h6C9eh@ZenIV/ [7] https://lore.kernel.org/lkml/Y5glgpD7fFifC4Fi@ZenIV/#t The cover letter of the v1 series is at https://lore.kernel.org/lkml/20221211213111.30085-1-fmdefrancesco@xxxxxxxxx/ The cover letter of the v2 series is at https://lore.kernel.org/lkml/20221212231906.19424-1-fmdefrancesco@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 | 134 +++++++++++++++++++++++++++------------------------ 1 file changed, 71 insertions(+), 63 deletions(-) -- 2.39.0
