[PATCH v8 5/6] elf: Align large load segments to PMD huge page size for THP

[WANG Rui]

On Sun, Apr 12, 2026 at 11:51 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Sun, Apr 5, 2026 at 11:56 AM WANG Rui <wangrui@loongson.cn> wrote:
> >
> > Mapping segments that are at least the size of a PMD huge page to
> > huge-page-aligned addresses helps make them eligible for Transparent
> > Huge Pages (THP).
> >
> > This patch introduces a Linux-specific helper, `_dl_map_segment_align`,
> > to determine an appropriate maximum alignment for ELF load segments based
> > on the system THP policy. The optimization is enabled only when the glibc
> > tunable `glibc.elf.thp=1` is set and THP is configured to be used
> > unconditionally.
> >
> > The optimization depends on Linux kernel support for file-backed THP,
> > specifically:
> >
> > * `CONFIG_READ_ONLY_THP_FOR_FS` (available since Linux kernel 5.4), and
> > * `CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS`.
> >
> > When enabled, the helper queries the default THP page size and uses it
> > to align sufficiently large load segments that are already properly
> > aligned in both virtual address and file offset (e.g., zero).
> >
> > For eligible segments, the alignment is bumped to the THP page size,
> > which improves THP eligibility, reduces TLB pressure, and improves
> > performance for large objects. To avoid excessive address space padding
> > on systems with very large THP sizes, the alignment is capped at 32MB.
> > The optimization is applied only to non-writable segments, matching
> > typical THP usage.
> >
> > Signed-off-by: WANG Rui <wangrui@loongson.cn>
> > ---
> >  sysdeps/unix/sysv/linux/Makefile              |  1 +
> >  .../unix/sysv/linux/dl-map-segment-align.c    | 53 +++++++++++++++++++
> >  .../unix/sysv/linux/dl-map-segment-align.h    | 27 ++++++++++
> >  3 files changed, 81 insertions(+)
> >  create mode 100644 sysdeps/unix/sysv/linux/dl-map-segment-align.c
> >  create mode 100644 sysdeps/unix/sysv/linux/dl-map-segment-align.h
> >
> > diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
> > index c6bd97abf1a..9879fec9a93 100644
> > --- a/sysdeps/unix/sysv/linux/Makefile
> > +++ b/sysdeps/unix/sysv/linux/Makefile
> > @@ -673,6 +673,7 @@ endif
> >
> >  ifeq ($(subdir),elf)
> >  dl-routines += \
> > +  dl-map-segment-align \
> >    dl-rseq-symbols \
> >    # dl-routines
> >
> > diff --git a/sysdeps/unix/sysv/linux/dl-map-segment-align.c b/sysdeps/unix/sysv/linux/dl-map-segment-align.c
> > new file mode 100644
> > index 00000000000..75fb0496730
> > --- /dev/null
> > +++ b/sysdeps/unix/sysv/linux/dl-map-segment-align.c
> > @@ -0,0 +1,53 @@
> > +/* _dl_map_segment_align.  Linux version.
> > +   Copyright (C) 2026 Free Software Foundation, Inc.
> > +   Copyright The GNU Toolchain Authors.
> > +   This file is part of the GNU C Library.
> > +
> > +   The GNU C Library is free software; you can redistribute it and/or
> > +   modify it under the terms of the GNU Lesser General Public
> > +   License as published by the Free Software Foundation; either
> > +   version 2.1 of the License, or (at your option) any later version.
> > +
> > +   The GNU C Library is distributed in the hope that it will be useful,
> > +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> > +   Lesser General Public License for more details.
> > +
> > +   You should have received a copy of the GNU Lesser General Public
> > +   License along with the GNU C Library; if not, see
> > +   <https://www.gnu.org/licenses/>.  */
> > +
> > +#include <dl-map-segment-align.h>
> > +#include <dl-tunables.h>
> > +#include <hugepages.h>
> > +
> > +ElfW (Addr)
> > +_dl_map_segment_align (const struct loadcmd *c, ElfW (Addr) p_align_max)
> > +{
> > +  static enum thp_mode_t thp_mode = thp_mode_not_supported;
> > +  static unsigned long int thp_pagesize;
> > +
> > +  if (TUNABLE_GET (glibc, elf, thp, int32_t, NULL) == 0)
> > +    return p_align_max;
> > +
>
> We should add GLRO(dl_thp_control) and set it in _dl_sysdep_start
> to avoid calling TUNABLE_GET for every PT_LOAD segment.

Since tunables are already almost for free once initialized,
essentially just a global variable access.
I'm wondering if adding a separate GLRO(dl_thp_control) is really
necessary in this patch.

Thanks,
Rui