Re: [PATCH] mm: remove VM_EXEC requirement for THP eligibility

[Yang Shi]

On Wed, Dec 20, 2023 at 8:53 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
>
> On Wed, Dec 20, 2023 at 3:42 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
> >
> > On Tue, Dec 19, 2023 at 9:41 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
> > >
> > > Commit e6be37b2e7bd ("mm/huge_memory.c: add missing read-only THP
> > > checking in transparent_hugepage_enabled()") introduced the VM_EXEC
> > > requirement, which is not strictly needed.
> > >
> > > lld's default --rosegment option and GNU ld's -z separate-code option
> > > (default on Linux/x86 since binutils 2.31) create a read-only PT_LOAD
> > > segment without the PF_X flag, which should be eligible for THP.
> > >
> > > Certain architectures support medium and large code models, where
> > > .lrodata may be placed in a separate read-only PT_LOAD segment, which
> > > should be eligible for THP as well.
> >
> > Yeah, it doesn't have to be VM_EXEC. The original implementation was
> > restricted to VM_EXEC to minimize the blast radius and the targe use
> > case is for large text segments. Out of curiosity, did you see any
> > noticeable improvement with this change?
>
> Hi Yang,
>
> Thanks for the comment. Frankly, I am not familiar with huge pages...
> I noticed this VM_EXEC condition when I was writing this
> hugepage-related section in
> https://maskray.me/blog/2023-12-17-exploring-the-section-layout-in-linker-output#transparent-huge-pages-for-mapped-files
> (Thanks to Alexander Monakov's comment about
> CONFIG_READ_ONLY_THP_FOR_FS in
> https://mazzo.li/posts/check-huge-page.html).

Thanks for sharing the article, learnt something about linker and loader.

>
> As dTLB for read-only data is also an important optimization of
> file-backed THP, it seems straightforward that we should drop the
> VM_EXEC condition :)

Yeah, as long as the use case is valid, it is definitely fine to lift
the restriction.

>
> On my Arch linux machine, the r--p page gets split if I invoke
> madvise(__ehdr_start, HPAGE_SIZE, MADV_HUGEPAGE) I haven't figured out
> why it behaves so in the presence of the VM_EXEC check.

What do you mean about "split"? THP got split into small pages? It
depends on the address of __ehdr_start. If it is in the middle of a
VMA, the VMA is going to be split due to the different huge page
attributes.

>
> % g++ test.cc -o ~/tmp/test -O2 -fuse-ld=lld
> -Wl,-z,max-page-size=2097152 && sudo ~/tmp/test
> __ehdr_start: 0x55f3b1c00000
> 55f3b1c00000-55f3b1e00000 r--p 00000000 103:03 555277119
>   /home/ray/tmp/test
> 55f3b1e00000-55f3b1e01000 r--p 00200000 103:03 555277119
>   /home/ray/tmp/test
> 55f3b2000000-55f3b2002000 r-xp 00200000 103:03 555277119
>   /home/ray/tmp/test
> 55f3b2201000-55f3b2202000 r--p 00201000 103:03 555277119
>   /home/ray/tmp/test
> 55f3b2401000-55f3b2402000 rw-p 00201000 103:03 555277119
>   /home/ray/tmp/test
> 55f3b3a9a000-55f3b3abb000 rw-p 00000000 00:00 0                          [heap]
>
>
> It'd be greatly appreciated if someone familiar with
> CONFIG_READ_ONLY_THP_FOR_FS could provide some notes on how to use
> this feature:)

I think your blog covered all the points. If you don't mind, you could
add some notes in Documentation/admin-guide/mm/transhuge.rst.

>
> > >
> > > Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
> > > ---
> > >  include/linux/huge_mm.h | 1 -
> > >  1 file changed, 1 deletion(-)
> > >
> > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > index fa0350b0812a..4c9e67e9000f 100644
> > > --- a/include/linux/huge_mm.h
> > > +++ b/include/linux/huge_mm.h
> > > @@ -126,7 +126,6 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
> > >         inode = vma->vm_file->f_inode;
> > >
> > >         return (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)) &&
> > > -              (vma->vm_flags & VM_EXEC) &&
> > >                !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
> > >  }
> > >
> > > --
> > > 2.43.0.472.g3155946c3a-goog
> > >
>
>
>
> --
> 宋方睿