Avoid using potentially-under-aligned page buffers

Enterprise / PostgreSQL - Tom Lane [sss.pgh.pa.us] - 1 September 2018 19:27 EDT

There's a project policy against using plain "char buf[BLCKSZ]" local or static variables as page buffers; preferred style is to palloc or malloc each buffer to ensure it is MAXALIGN'd. However, that policy's been ignored in an increasing number of places. We've apparently got away with it so far, probably because (a) relatively few people use platforms on which misalignment causes core dumps and/or (b) the
variables chance to be sufficiently aligned anyway. But this is not something to rely on. Moreover, even if we don't get a core dump, we might be paying a lot of cycles for misaligned accesses.

To fix, invent new union types PGAlignedBlock and PGAlignedXLogBlock that the compiler must allocate with sufficient alignment, and use those in place of plain char arrays.

I used these types even for variables where there's no risk of a misaligned access, since ensuring proper alignment should make kernel data transfers faster. I also changed some places where we had been palloc'ing short-lived buffers, for coding style uniformity and to save palloc/pfree overhead.

Since this seems to be a live portability hazard (despite the lack of field reports), back-patch to all supported versions.

Patch by me; thanks to Michael Paquier for review.

Discussion: https://postgr.es/m/1535618100.1286.3.camel@credativ.de

44cac93464 Avoid using potentially-under-aligned page buffers.
contrib/bloom/blinsert.c | 12 ++++-----
contrib/pg_prewarm/pg_prewarm.c | 4 +--
contrib/pg_standby/pg_standby.c | 10 +++-----
src/backend/access/gin/ginentrypage.c | 6 ++---
src/backend/access/gin/ginfast.c | 11 +++-----
src/backend/access/hash/hashpage.c | 8 +++---
src/backend/access/heap/heapam.c | 16 +++---------
src/backend/access/heap/visibilitymap.c | 11 +++-----
src/backend/access/transam/generic_xlog.c | 21 +++++++--------
src/backend/access/transam/xlog.c | 26 +++++++++----------
src/backend/access/transam/xloginsert.c | 14 +++++-----
src/backend/access/transam/xlogreader.c | 6 ++---
src/backend/commands/tablecmds.c | 20 +++++----------
src/backend/replication/walsender.c | 6 ++---
src/backend/storage/file/buffile.c | 10 ++++----
src/backend/storage/freespace/freespace.c | 11 +++-----
src/backend/utils/sort/logtape.c | 6 ++---
src/bin/pg_basebackup/walmethods.c | 20 +++++----------
src/bin/pg_resetwal/pg_resetwal.c | 14 +++++-----
src/bin/pg_rewind/copy_fetch.c | 6 ++---
src/bin/pg_upgrade/file.c | 31 ++++++++---------------
src/bin/pg_verify_checksums/pg_verify_checksums.c | 8 +++---
src/bin/pg_waldump/pg_waldump.c | 6 ++---
src/include/c.h | 26 +++++++++++++++++++
24 files changed, 141 insertions(+), 168 deletions(-)

Upstream: git.postgresql.org


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