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

[Jiri Slaby]

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