linux-kernel - Re: [PATCH RFC 1/4] lib/radix: add universal radix_tree_fill

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Message-ID: <CALYGNiO6QEhA-Oa1TT+j9CkZxz9UerrnVoZg-fV0u46FdFwQQQ@mail.gmail.com>
Date:   Mon, 29 Aug 2016 21:15:53 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     Matthew Wilcox <mawilcox@...rosoft.com>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 1/4] lib/radix: add universal radix_tree_fill_range

On Mon, Aug 29, 2016 at 9:08 PM, Matthew Wilcox <mawilcox@...rosoft.com> wrote:
> The DAX lock bit is analogous to the PageLock.  You can't serialise on the mapping lock; the contention will be too high.

I mean DAX lock bit is protected by mapping lock thus we can set/clear
it for all entries if needed.

>
> And the point of having the radix tree support the same entry for many indices is that we don't have to go and probe the radix tree multiple times looking for the first or last entry.  We just look up the index, then use the entry we got back.

Right, but lookup function can also return pointer for node --
fininding first or last entries is trivial here.
We could simply scan back/forward or if huge DAX order is known then
just align slot pointer to first or last entry.

>
> -----Original Message-----
> From: Konstantin Khlebnikov [mailto:koct9i@...il.com]
> Sent: Monday, August 29, 2016 12:14 PM
> To: Matthew Wilcox <mawilcox@...rosoft.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH RFC 1/4] lib/radix: add universal radix_tree_fill_range
>
> On Mon, Aug 29, 2016 at 6:21 PM, Matthew Wilcox <mawilcox@...rosoft.com> wrote:
>> Thanks, Ross.
>>
>> Konstantin, I think there are problems with the concept behind this series.  You have multiple entries in the tree with the same value.  That works out fine when the entry is a pointer (eg to a struct page), but not so well when it's an exceptional entry (eg a swap cache entry or a DAX radix tree entry).  If you look at the recent DAX work, you'll see there's a lock bit, and having multiple lock bits is a recipe for disaster.
>>
>
> I see no problem here. They could use lock bit at first or last entry.
> Anyway all changes should be protecred by lock at mapping.
>
>