Re: [PATCH] mm/mremap: reset unfaulted VMA page offset for MREMAP_DONTUNMAP

[Kunwu Chan <kunwu.chan@gmail.com>]

On Wed, 26 Aug 2026 19:26:42 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Tue, 25 Aug 2026 08:55:26 +0100 "Lorenzo Stoakes (ARM)" <ljs@kernel.org> wrote:
> 
> > Uniquely an mremap() invocation using the MREMAP_DONTUNMAP flag can reset
> > a faulted VMA into an unfaulted one.
> > 
> > It does so after the page tables have been moved to the copied VMA with
> > MREMAP_DONTUNMAP leaving the old VMA in place which is naturally unfaulted
> > as the page tables it had are no longer present.
> > 
> > However, in doing so, it violates the invariant that the anonymous page
> > offset of an unfaulted VMA is vma->vm_start >> PAGE_SHIFT.
> > 
> > This is because a VMA may have been faulted in, mremap()'d (causing a delta
> > between its page offset and vma->vm_start >> PAGE_SHIFT), and then
> > mremap()'d again with MREMAP_DONTUNMAP resulting in the unfaulting.
> > 
> > This condition is a violation of a fundamental assumption in mm, but now
> > also triggers an assert in assert_sane_pgoff() which explicitly checks for
> > this condition.
> > 
> > Correct it by resetting the VMA's page offset at the point of completing
> > the MREMAP_DONTUNMAP operation.
> 
> Thanks.  I'll park this in mm-new until mm.git is all merged up
> (simplifying my life..)
> 
> 
> Unrelatedly, Sashiko thinks we're messing up locked_vm accounting with
> MREMAP_DONTUNMAP on a locked VMA.
> 	https://sashiko.dev/#/patchset/20260825-fix-mremap-dontunmap-pgoff-v1-1-39a40b2c98b3@kernel.org
> 
> 
> I had Sashiko write code to demonstrate this but am too lazy to test it
> on a current kernel.  If someone could oblige?

Hi Andrew,

I'll test the reproducer on a current kernel and check the mm->locked_vm 
accounting before and after MREMAP_DONTUNMAP.


Thanks,
Kunwu


> 
> 
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <unistd.h>
> #include <sys/mman.h>
> #include <fcntl.h>
> 
> /* Read VMLck (in kB) from /proc/self/status */
> static long get_vmlck_kb(void) {
>     FILE *f = fopen("/proc/self/status", "r");
>     if (!f) {
>         perror("fopen /proc/self/status");
>         return -1;
>     }
> 
>     char line[256];
>     long vmlck = -1;
>     while (fgets(line, sizeof(line), f)) {
>         if (strncmp(line, "VMLck:", 6) == 0) {
>             sscanf(line + 6, "%ld", &vmlck);
>             break;
>         }
>     }
>     fclose(f);
>     return vmlck;
> }
> 
> int main(void) {
>     size_t size = 4096 * 10; // 40 kB
>     long initial_vmlck, post_mlock, post_mremap, post_munmap;
> 
>     initial_vmlck = get_vmlck_kb();
>     printf("[1] Initial VMLck:      %ld kB\n", initial_vmlck);
> 
>     /* 1. Allocate initial VMA */
>     void *addr = mmap(NULL, size, PROT_READ | PROT_WRITE,
>                       MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>     if (addr == MAP_FAILED) {
>         perror("mmap initial");
>         return 1;
>     }
> 
>     /* 2. Lock the VMA (increments mm->locked_vm) */
>     if (mlock(addr, size) != 0) {
>         perror("mlock");
>         return 1;
>     }
>     post_mlock = get_vmlck_kb();
>     printf("[2] Post-mlock VMLck:   %ld kB (+%ld kB)\n", 
>            post_mlock, post_mlock - initial_vmlck);
> 
>     /* 3. mremap with MREMAP_DONTUNMAP 
>      * move_vma() increments mm->locked_vm for the destination VMA,
>      * while dontunmap_complete() clears VMA_LOCKED_MASK on source VMA
>      * without decrementing mm->locked_vm.
>      */
>     void *new_addr = mremap(addr, size, size, 
>                             MREMAP_MAYMOVE | MREMAP_DONTUNMAP, NULL);
>     if (new_addr == MAP_FAILED) {
>         perror("mremap MREMAP_DONTUNMAP");
>         return 1;
>     }
>     post_mremap = get_vmlck_kb();
>     printf("[3] Post-mremap VMLck:  %ld kB (+%ld kB from initial)\n", 
>            post_mremap, post_mremap - initial_vmlck);
> 
>     /* 4. Unmap source VMA 
>      * Since VMA_LOCKED_BIT was cleared on source VMA, 
>      * munmap fails to decrement mm->locked_vm for this region.
>      */
>     munmap(addr, size);
>     post_munmap = get_vmlck_kb();
>     printf("[4] Post-munmap source: %ld kB\n", post_munmap);
> 
>     /* 5. Clean up destination VMA */
>     munmap(new_addr, size);
>     long final_vmlck = get_vmlck_kb();
>     printf("[5] Final VMLck:        %ld kB\n", final_vmlck);
> 
>     /* Evaluation */
>     printf("\n--- Result ---\n");
>     if (final_vmlck > initial_vmlck) {
>         printf("BUG DEMONSTRATED: Leaked %ld kB in mm->locked_vm counter.\n",
>                final_vmlck - initial_vmlck);
>     } else {
>         printf("NO LEAK: mm->locked_vm returned to initial state.\n");
>     }
> 
>     return 0;
> }
> 
>