linux-kernel - Re: [PATCH v3 0/3] fs/ufs: replace kmap() with kmap_local

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Date:   Wed, 21 Dec 2022 18:46:08 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Evgeniy Dushistov <dushistov@...l.ru>, bpf@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Cc:     Al Viro <viro@...iv.linux.org.uk>, Ira Weiny <ira.weiny@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/3] fs/ufs: replace kmap() with kmap_local_page

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@gmail.com/
> The cover letter of the v2 series is at
> https://lore.kernel.org/lkml/20221212231906.19424-1-fmdefrancesco@gmail.com/
> 
> 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@gmail.com/T/

Thanks,

Fabio