Re: Bug: Performance regression in 1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race

["Uschakow, Stanislav" <suschako@amazon.de>]

Hi David,

> From: David Hildenbrand <david@redhat.com>
> Sent: Monday, September 1, 2025 1:26 PM
> To: Jann Horn; Uschakow, Stanislav
> Cc: linux-mm@kvack.org; trix@redhat.com; ndesaulniers@google.com; nathan@kernel.org; akpm@linux-foundation.org; muchun.song@linux.dev; mike.kravetz@oracle.com; lorenzo.stoakes@oracle.com; liam.howlett@oracle.com; osalvador@suse.de; vbabka@suse.cz; stable@vger.kernel.org
> Subject: RE: [EXTERNAL] Bug: Performance regression in 1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race
>     
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> 
> 
> On 01.09.25 12:58, Jann Horn wrote:
> > Hi!
> >
> > On Fri, Aug 29, 2025 at 4:30 PM Uschakow, Stanislav <suschako@amazon.de> wrote:
> >> We have observed a huge latency increase using `fork()` after ingesting the CVE-2025-38085 fix which leads to the commit `1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race`. On large machines with 1.5TB of memory with 196 cores, we identified  mmapping of 1.2TB of shared memory and forking itself dozens or hundreds of times we see a increase of execution times of a factor of 4. The reproducer is at the end of the email.
> >
> > Yeah, every 1G virtual address range you unshare on unmap will do an
> > extra synchronous IPI broadcast to all CPU cores, so it's not very
> > surprising that doing this would be a bit slow on a machine with 196
> > cores.
> 
> What is the use case for this extreme usage of fork() in that context?
> Is it just something people noticed and it's suboptimal, or is this a
> real problem for some use cases?
> 

Yes, we have customer reporting huge performance regressions on their workloads. I don't know the software architecture or actual use case for their application though. A execution time increase of at least a factor of 4 is noticeable even with few forks() on those machines.

> --
> Cheers
> 
> David / dhildenb


Thanks

Stanislav

    



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