linux-kernel - Re: [PATCH 2/3] hugetlbfs: close race between MADV

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Date:   Tue, 26 Sep 2023 14:15:30 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     riel@...riel.com
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        linux-mm@...ck.org, muchun.song@...ux.dev, mike.kravetz@...cle.com,
        leit@...a.com, willy@...radead.org
Subject: Re: [PATCH 2/3] hugetlbfs: close race between MADV_DONTNEED and
 page fault

On Mon, 25 Sep 2023 23:10:51 -0400 riel@...riel.com wrote:

> From: Rik van Riel <riel@...riel.com>
> 
> Malloc libraries, like jemalloc and tcalloc, take decisions on when
> to call madvise independently from the code in the main application.
> 
> This sometimes results in the application page faulting on an address,
> right after the malloc library has shot down the backing memory with
> MADV_DONTNEED.
> 
> Usually this is harmless, because we always have some 4kB pages
> sitting around to satisfy a page fault. However, with hugetlbfs
> systems often allocate only the exact number of huge pages that
> the application wants.
> 
> Due to TLB batching, hugetlbfs MADV_DONTNEED will free pages outside of
> any lock taken on the page fault path, which can open up the following
> race condition:
> 
>        CPU 1                            CPU 2
> 
>        MADV_DONTNEED
>        unmap page
>        shoot down TLB entry
>                                        page fault
>                                        fail to allocate a huge page
>                                        killed with SIGBUS
>        free page
> 
> Fix that race by pulling the locking from __unmap_hugepage_final_range
> into helper functions called from zap_page_range_single. This ensures
> page faults stay locked out of the MADV_DONTNEED VMA until the
> huge pages have actually been freed.
> 

Was a -stable backport considered?