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

[Wilco Dijkstra]

Hi HJ,

> A couple issues:
>
> 1.  We shouldn't assume what are in /sys/kernel/mm/transparent_hugepage/enabled
> nor /sys/kernel/mm/transparent_hugepage/hpage_pmd_size.   We can
> add a system call or AT_XXX values to get them.

Indeed, reading those files turned out to be a bad idea. Adding AT_XXX sounds like
a good idea - it's cheap for the loader to process a few extra entries.

However until we have a way to easily get the THP settings at startup, we need an
alternative without reading those files. We also need to override the PMD size if
it is too large - the current patch limits it to 32MB and for AArch64 I force the THP size
to 2MB for all base page sizes.

> 2.  We should support THP enabled with madvise.  If the current kernel doesn't
> support it, it should be updated.

In the future, sure, but it is not a requirement for Rui's patch series.

> 3. Glibc can map PIE and DSO to address aligned to THP page size after
>
> commit 718fdd87b1b98ef88e883a37d9c18867256fa5a4
> Author: Rongwei Wang <rongwei.wang@linux.alibaba.com>
> Date:   Fri Dec 10 20:39:10 2021 +0800
>
>    elf: Properly align PT_LOAD segments [BZ #28676]
>
> If THP is enabled with madvise, should kernel map PIE to address aligned to
> THP page size just like THP enabled with always?  If not,  should we add a
> marker in PIE to let kernel know that a PIE should be mapped to address
> aligned to THP page size?

The kernel should do the alignment when it believes there is an expected benefit -
the exact cases for which that is true are likely to change over time depending on how
it is implemented in the kernel. If we use AT_XXX as the mechanism, it could actually
give GLIBC what it believes is the best alignment for THP (potentially the alignment for
file and anonymous mappings might be different).

Cheers,
Wilco