linux-kernel - Re: [PATCH v2] mm: align larger anonymous mappings on THP boundaries

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Date: Wed, 17 Jan 2024 16:09:46 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Yang Shi <shy828301@...il.com>
Cc: Jiri Slaby <jirislaby@...nel.org>, Rik van Riel <riel@...riel.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, kernel-team@...com, 
	Matthew Wilcox <willy@...radead.org>, Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH v2] mm: align larger anonymous mappings on THP boundaries

On Wed, Jan 17, 2024 at 4:07 PM Yang Shi <shy828301@...il.com> wrote:
>
> On Tue, Jan 16, 2024 at 4:09 AM Jiri Slaby <jirislaby@...nel.org> wrote:
> >
> > On 16. 01. 24, 12:53, Jiri Slaby wrote:
> > > Hi,
> > >
> > > On 09. 08. 22, 20:24, Rik van Riel wrote:
> > >> Align larger anonymous memory mappings on THP boundaries by
> > >> going through thp_get_unmapped_area if THPs are enabled for
> > >> the current process.
> > >>
> > >> With this patch, larger anonymous mappings are now THP aligned.
> > >> When a malloc library allocates a 2MB or larger arena, that
> > >> arena can now be mapped with THPs right from the start, which
> > >> can result in better TLB hit rates and execution time.
> > >
> > > This appears to break 32bit processes on x86_64 (at least). In
> > > particular, 32bit kernel or firefox builds in our build system.
> > >
> > > Reverting this on top of 6.7 makes it work again.
> > >
> > > Downstream report:
> > >   https://bugzilla.suse.com/show_bug.cgi?id=1218841
> > >
> > > So running:
> > > pahole -J --btf_gen_floats -j --lang_exclude=rust
> > > --skip_encoding_btf_inconsistent_proto --btf_gen_optimized .tmp_vmlinux.btf
> > >
> > > crashes or errors out with some random errors:
> > > [182671] STRUCT idr's field 'idr_next' offset=128 bit_size=0 type=181346
> > > Error emitting field
> > >
> > > strace shows mmap() fails with ENOMEM right before the errors:
> > > 1223  mmap2(NULL, 5783552, PROT_READ|PROT_WRITE,
> > > MAP_PRIVATE|MAP_ANONYMOUS, -1, 0 <unfinished ...>
> > > ...
> > > 1223  <... mmap2 resumed>)              = -1 ENOMEM (Cannot allocate
> > > memory)
> > >
> > > Note the .tmp_vmlinux.btf above can be arbitrary, but likely large
> > > enough. For reference, one is available at:
> > > https://decibel.fi.muni.cz/~xslaby/n/btf
> > >
> > > Any ideas?
> >
> > This works around the problem, of course (but is a band-aid, not a fix):
> >
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1829,7 +1829,7 @@ get_unmapped_area(struct file *file, unsigned long
> > addr, unsigned long len,
> >                   */
> >                  pgoff = 0;
> >                  get_area = shmem_get_unmapped_area;
> > -       } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> > +       } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> > !in_32bit_syscall()) {
> >                  /* Ensures that larger anonymous mappings are THP
> > aligned. */
> >                  get_area = thp_get_unmapped_area;
> >          }
> >
> >
> > thp_get_unmapped_area() does not take care of the legacy stuff...
>
> Could you please help test the below patch? It is compiled, but I
> don't have 32 bit userspace or machine to test it.

Hmm. I think you misunderstood me. This is happening on x86_64 emulated system.

>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 94ef5c02b459..a4d0839e5f31 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -811,6 +811,9 @@ static unsigned long
> __thp_get_unmapped_area(struct file *filp,
>         loff_t off_align = round_up(off, size);
>         unsigned long len_pad, ret;
>
> +       if (IS_ENABLED(CONFIG_32BIT) || in_compat_syscall())
> +               return 0;
> +
>         if (off_end <= off_align || (off_end - off_align) < size)
>                 return 0;
>
>
> >
> > regards,
> > --
> > js
> > suse labs
> >
>