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

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Date: Tue, 16 Jan 2024 13:09:29 +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

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...

regards,
-- 
js
suse labs