Horton

Re: pg_verify_checksums and -fno-strict-aliasing

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Michael Banck <michael.banck@credativ.de>, Magnus Hagander <magnus@hagander.net>, Fabien COELHO <coelho@cri.ensmp.fr>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-08-30T23:02:15Z
Lists: pgsql-hackers

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Andres Freund <andres@anarazel.de> writes:
> On 2018-08-30 18:11:40 -0400, Tom Lane wrote:
>> I suspect people will complain about the added cost of doing that.

> I think the compiler will just optimize it away.

Maybe.  In any case, the attached version avoids any need for memcpy
and is, I think, cleaner code anyhow.  It fixes the problem for me
with Fedora's gcc, though I'm not sure that it'd be enough to get rid
of the warning Michael sees (I wonder what compiler he's using).

>> BTW, not to mention the elephant in the room, but: is it *really* OK
>> that pg_checksum_page scribbles on the page buffer, even temporarily?
>> It's certainly quite horrid that there aren't large warnings about
>> that in the function's API comment.

> It certainly should be warned about.  Practically I don't think it's a
> problem, because we pretty much always operate on a copy of the page
> when writing out, as otherwise concurrently set hint bits would be
> troublesome.

The write end of things is not a problem IMO, since presumably the caller
would be about to overwrite the checksum field anyway.  The issue is for
reading and/or verifying, where it's much less obvious that clobbering
the page image is safe.

			regards, tom lane

Commits

  1. Avoid using potentially-under-aligned page buffers.

  2. Code review for pg_verify_checksums.c.

  3. Make checksum_impl.h safe to compile with -fstrict-aliasing.