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

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Date: Tue, 16 Jan 2024 12:53:06 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: Rik van Riel <riel@...riel.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...com,
 Matthew Wilcox <willy@...radead.org>, Yang Shi <shy828301@...il.com>,
 Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH v2] mm: align larger anonymous mappings on THP boundaries

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?

> Signed-off-by: Rik van Riel <riel@...riel.com>
> ---
> v2: avoid the chicken & egg issue with MMF_VM_HUGEPAGE (Yang Shi)
> 
>   mm/mmap.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c035020d0c89..1d859893436d 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2229,6 +2229,9 @@ 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)) {
> +		/* Ensures that larger anonymous mappings are THP aligned. */
> +		get_area = thp_get_unmapped_area;
>   	}
>   
>   	addr = get_area(file, addr, len, pgoff, flags);

thanks,
-- 
js
suse labs