public inbox for [email protected]
help / color / mirror / Atom feedFrom: Drouvot, Bertrand <[email protected]>
To: Robert Haas <[email protected]>
To: Andres Freund <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Cc: Ibrar Ahmed <[email protected]>
Cc: Amit Khandekar <[email protected]>
Cc: [email protected]
Cc: tushar <[email protected]>
Cc: Rahila Syed <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Minimal logical decoding on standbys
Date: Thu, 15 Dec 2022 11:20:18 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <CA+TgmoZ8-DXtDMMDGggFkSYyO8nmNdBKWjV6Ts-zYGR9p5sK5g@mail.gmail.com>
References: <[email protected]>
<[email protected]>
<[email protected]>
<[email protected]>
<CA+TgmoY0df9X+5ENg8P0BGj0odhM45sdQ7kB4JMo4NKaoFy-Vg@mail.gmail.com>
<[email protected]>
<CA+Tgmoa1cZSo1tUTzxS+RyniMuNtkpxUeEhnD2+-mvJodVtdSQ@mail.gmail.com>
<[email protected]>
<[email protected]>
<CA+TgmoaVcu_mbxbH=EccvKG6u8+MdQf9zx98uAL9zsStFwrYsQ@mail.gmail.com>
<[email protected]>
<CA+TgmoZ8-DXtDMMDGggFkSYyO8nmNdBKWjV6Ts-zYGR9p5sK5g@mail.gmail.com>
Hi,
On 12/14/22 6:48 PM, Robert Haas wrote:
> On Wed, Dec 14, 2022 at 12:35 PM Andres Freund <[email protected]> wrote:
>>> typedef struct xl_heap_prune
>>>
>>> I think this is unsafe on alignment-picky machines. I think it will
>>> cause the offset numbers to be aligned at an odd address.
>>> heap_xlog_prune() doesn't copy the data into aligned memory, so I
>>> think this will result in a misaligned pointer being passed down to
>>> heap_page_prune_execute.
>>
>> I think the offset numbers are stored separately from the record, even
>> though it doesn't quite look like that in the above due to the way the
>> 'OFFSET NUMBERS' is embedded in the struct. As they're stored with the
>> block reference 0, the added boolean shouldn't make a difference
>> alignment wise?
>>
>> Or am I misunderstanding your point?
>
> Oh, you're right. So this is another case similar to
> xl_btree_reuse_page. In heap_xlog_prune(), we access the offset number
> data like this:
>
> redirected = (OffsetNumber *)
> XLogRecGetBlockData(record, 0, &datalen);
> end = (OffsetNumber *) ((char *) redirected + datalen);
> nowdead = redirected + (nredirected * 2);
> nowunused = nowdead + ndead;
> nunused = (end - nowunused);
> heap_page_prune_execute(buffer,
>
> redirected, nredirected,
> nowdead, ndead,
>
> nowunused, nunused);
>
> This is only safe if the return value of XLogRecGetBlockData is
> guaranteed to be properly aligned,
Why, could you please elaborate?
It looks to me that here we are "just" accessing the
members of the xl_heap_prune struct to get the numbers.
Then, the actual data will be read later in heap_page_prune_execute() from the buffer/page based on the numbers we got from xl_heap_prune.
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
view thread (196+ messages) latest in thread
reply
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: [email protected]
Cc: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]
Subject: Re: Minimal logical decoding on standbys
In-Reply-To: <[email protected]>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox