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

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