Re: [PATCH v3 0/3] fs/ufs: replace kmap() with kmap_local_page

From: Fabio M. De Francesco
Date: Wed Dec 21 2022 - 12:46:31 EST


On sabato 17 dicembre 2022 19:47:46 CET Fabio M. De Francesco wrote:
> 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

Please drop this v3 because of a mistake in patch 3/3.
The updated v4 series is at
https://lore.kernel.org/lkml/20221221172802.18743-1-fmdefrancesco@xxxxxxxxx/T/

Thanks,

Fabio