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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Liam R. Howlett" <Liam.Howlett@Oracle.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org,
        Shanker Donthineni <sdonthineni@nvidia.com>
Subject: Re: your mail
In-Reply-To: <20230515192708.sypitem5hzycdois@revolver>
References: <87mt2cxb7y.ffs@tglx> <20230515192708.sypitem5hzycdois@revolver>
Date:   Wed, 17 May 2023 00:47:53 +0200
Message-ID: <875y8r7v1i.ffs@tglx>
MIME-Version: 1.0
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On Mon, May 15 2023 at 15:27, Liam R. Howlett wrote:
> * Thomas Gleixner <tglx@linutronix.de> [230510 15:01]:
>> The documentation of mt_next() claims that it starts the search at the
>> provided index. That's incorrect as it starts the search after the provided
>> index.
>> 
>> The documentation of mt_find() is slightly confusing. "Handles locking" is
>> not really helpful as it does not explain how the "locking" works.
>
> More locking notes can be found in Documentation/core-api/maple_tree.rst
> which lists mt_find() under the "Takes RCU read lock" list.  I'm okay
> with duplicating the comment of taking the RCU read lock in here.

Without a reference to the actual locking documentation such comments
are not super helpful.

>> Fix similar issues for mt_find_after() and mt_prev().
>> 
>> Remove the completely confusing and pointless "Note: Will not return the
>> zero entry." comment from mt_for_each() and document @__index correctly.
>
> The zero entry concept is an advanced API concept which allows you to
> store something that cannot be seen by the mt_* family of users, so it
> will not be returned and, instead, it will return NULL.  Think of it as
> a reservation for an entry that isn't fully initialized.  Perhaps it
> should read "Will not return the XA_ZERO_ENTRY" ?
>>
>> - *
>> - * Note: Will not return the zero entry.
>
> This function "will not return the zero entry", meaning it will return
> NULL if xa_is_zero(entry).

If I understand correctly, this translates to:

  This iterator skips entries, which have been reserved for future use
  but have not yet been fully initialized.

Right?

>> @@ -6487,9 +6493,14 @@ EXPORT_SYMBOL(mtree_destroy);
>>   * mt_find() - Search from the start up until an entry is found.
>>   * @mt: The maple tree
>>   * @index: Pointer which contains the start location of the search
>> - * @max: The maximum value to check
>> + * @max: The maximum value of the search range
>> + *
>> + * Takes RCU read lock internally to protect the search, which does not
>> + * protect the returned pointer after dropping RCU read lock.
>>   *
>> - * Handles locking.  @index will be incremented to one beyond the range.
>> + * In case that an entry is found @index contains the index of the found
>> + * entry plus one, so it can be used as iterator index to find the next
>> + * entry.
>
> What about:
> "In case that an entry is found @index contains the last index of the
> found entry plus one"

Still confusing to the casual reader like me :)

    "In case that an entry is found @index is updated to point to the next
     possible entry independent whether the found entry is occupying a
     single index or a range if indices."

Hmm?

Thanks,

        tglx