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

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Message-ID: <CAHbLzkp-s1rr3496rpgx_td75waE=P=sv=5OCK9HWvWjmz-0GA@mail.gmail.com>
Date: Tue, 16 Jan 2024 12:55:08 -0800
From: Yang Shi <shy828301@...il.com>
To: Jiri Slaby <jirislaby@...nel.org>
Cc: 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 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_vmlinuxbtf
> >
> > 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)

It should be due to the virtual address space size on 32-bit machine.

> >
> > 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...

Thanks for the report. I think we can just make
thp_get_unmapped_area() no-op on 32 bit.

>
> regards,
> --
> js
> suse labs
>