From: Matthew Wilcox <willy@infradead.org>
To: Wei Yang <richard.weiyang@gmail.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/9] XArray: internal node is a xa_node when it is bigger than XA_ZERO_ENTRY
Date: Mon, 30 Mar 2020 17:06:49 -0700 [thread overview]
Message-ID: <20200331000649.GG22483@bombadil.infradead.org> (raw)
In-Reply-To: <20200330222013.34nkqen2agujhd6j@master>
On Mon, Mar 30, 2020 at 10:20:13PM +0000, Wei Yang wrote:
> On Mon, Mar 30, 2020 at 07:27:08AM -0700, Matthew Wilcox wrote:
> >On Mon, Mar 30, 2020 at 02:13:50PM +0000, Wei Yang wrote:
> >> On Mon, Mar 30, 2020 at 06:49:03AM -0700, Matthew Wilcox wrote:
> >> >On Mon, Mar 30, 2020 at 01:45:19PM +0000, Wei Yang wrote:
> >> >> On Mon, Mar 30, 2020 at 05:50:06AM -0700, Matthew Wilcox wrote:
> >> >> >On Mon, Mar 30, 2020 at 12:36:40PM +0000, Wei Yang wrote:
> >> >> >> As the comment mentioned, we reserved several ranges of internal node
> >> >> >> for tree maintenance, 0-62, 256, 257. This means a node bigger than
> >> >> >> XA_ZERO_ENTRY is a normal node.
> >> >> >>
> >> >> >> The checked on XA_ZERO_ENTRY seems to be more meaningful.
> >> >> >
> >> >> >257-1023 are also reserved, they just aren't used yet. XA_ZERO_ENTRY
> >> >> >is not guaranteed to be the largest reserved entry.
> >> >>
> >> >> Then why we choose 4096?
> >> >
> >> >Because 4096 is the smallest page size supported by Linux, so we're
> >> >guaranteed that anything less than 4096 is not a valid pointer.
> >>
>
> So you want to say, the 4096 makes sure XArray will not store an address in
> first page? If this is the case, I have two suggestions:
>
> * use PAGE_SIZE would be more verbose?
But also incorrect, because it'll be different on different architectures.
It's 4096. That's all.
> * a node is an internal entry, do we need to compare with xa_mk_internal()
> instead?
No. 4096 is better because it's a number which is easily expressible in
many CPU instruction sets. 4094 is much less likely to be an easy number
to encode.
> >(it is slightly out of date; the XArray actually supports storing unaligned
> >pointers now, but that's not relevant to this discussion)
>
> Ok, maybe this document need to update.
Did you want to send a patch?
next prev parent reply other threads:[~2020-03-31 0:06 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-30 12:36 [PATCH 0/9] XArray: several cleanups Wei Yang
2020-03-30 12:36 ` [PATCH 1/9] XArray: fix comment on Zero/Retry entry Wei Yang
2020-03-30 12:46 ` Matthew Wilcox
2020-03-30 13:42 ` Wei Yang
2020-03-30 12:36 ` [PATCH 2/9] XArray: simplify the calculation of shift Wei Yang
2020-03-30 13:20 ` Matthew Wilcox
2020-03-30 14:07 ` Wei Yang
2020-03-30 12:36 ` [PATCH 3/9] XArray: handle a NULL head by itself Wei Yang
2020-03-30 12:36 ` [PATCH 4/9] XArray: don't expect to have more nr_values than count Wei Yang
2020-03-30 12:36 ` [PATCH 5/9] XArray: entry in last level is not expected to be a node Wei Yang
2020-03-30 12:48 ` Matthew Wilcox
2020-03-30 14:15 ` Wei Yang
2020-03-30 14:28 ` Matthew Wilcox
2020-03-30 22:10 ` Wei Yang
2020-03-31 13:42 ` Wei Yang
2020-03-31 16:42 ` Matthew Wilcox
2020-03-31 22:04 ` Wei Yang
2020-03-31 23:59 ` Matthew Wilcox
2020-04-01 22:10 ` Wei Yang
2020-04-01 22:20 ` Matthew Wilcox
2020-04-02 12:36 ` Wei Yang
2020-04-03 22:39 ` Wei Yang
2020-04-04 15:37 ` Matthew Wilcox
2020-04-05 11:07 ` Wei Yang
2020-04-05 21:56 ` Matthew Wilcox
2020-04-06 1:14 ` Wei Yang
2020-04-06 1:24 ` Wei Yang
2020-04-11 13:56 ` Wei Yang
2020-04-28 21:24 ` Wei Yang
2020-03-30 12:36 ` [PATCH 6/9] XArray: internal node is a xa_node when it is bigger than XA_ZERO_ENTRY Wei Yang
2020-03-30 12:50 ` Matthew Wilcox
2020-03-30 13:45 ` Wei Yang
2020-03-30 13:49 ` Matthew Wilcox
2020-03-30 14:13 ` Wei Yang
2020-03-30 14:27 ` Matthew Wilcox
2020-03-30 22:20 ` Wei Yang
2020-03-31 0:06 ` Matthew Wilcox [this message]
2020-03-31 13:40 ` Wei Yang
2020-03-30 12:36 ` [PATCH 7/9] XArray: the NULL xa_node condition is handled in xas_top Wei Yang
2020-03-30 12:36 ` [PATCH 8/9] XArray: take xas_error() handling for clearer logic Wei Yang
2020-03-30 12:36 ` [PATCH 9/9] XArray: adjust xa_offset till it gets the correct node Wei Yang
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