linux-kernel - [BUG] Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp() with mmap_read

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Date:   Mon, 14 Aug 2023 22:36:52 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
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        Linux-MM <linux-mm@...ck.org>
Subject: [BUG] Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp()
 with mmap_read_lock()

On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <hughd@...gle.com> wrote:
> Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
> It does need mmap_read_lock(), but it does not need mmap_write_lock(),
> nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
> paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.

We can still have a racing userfaultfd operation at the "/* step 4:
remove page table */" point that installs a new PTE before the page
table is removed.

To reproduce, patch a delay into the kernel like this:


diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 9a6e0d507759..27cc8dfbf3a7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -20,6 +20,7 @@
 #include <linux/swapops.h>
 #include <linux/shmem_fs.h>
 #include <linux/ksm.h>
+#include <linux/delay.h>

 #include <asm/tlb.h>
 #include <asm/pgalloc.h>
@@ -1617,6 +1618,11 @@ int collapse_pte_mapped_thp(struct mm_struct
*mm, unsigned long addr,
        }

        /* step 4: remove page table */
+       if (strcmp(current->comm, "DELAYME") == 0) {
+               pr_warn("%s: BEGIN DELAY INJECTION\n", __func__);
+               mdelay(5000);
+               pr_warn("%s: END DELAY INJECTION\n", __func__);
+       }

        /* Huge page lock is still held, so page table must remain empty */
        pml = pmd_lock(mm, pmd);


And then run the attached reproducer against mm/mm-everything. You
should get this in dmesg:

[  206.578096] BUG: Bad rss-counter state mm:000000000942ebea
type:MM_ANONPAGES val:1

View attachment "khugepaged-vs-uffd.c" of type "text/x-csrc" (2942 bytes)