Re: [PATCH v3] selftests/mm: merge: keep the far mremap() targets in the carveout

[Gregory Price (Meta)]

On 2026-10-22 07:18 -0500, Danish Khateeb wrote:
> The merge tests map their areas at fixed addresses within a 30-page
> PROT_NONE carveout. Several mremap() tests move an area 1000 or 2000
> pages away to separate it from the others, but those targets are outside
> the carveout, and MREMAP_FIXED unmaps whatever is mapped there.
> 
> Under virtme-ng, the shared libraries are packed right above the
> carveout and the dynamic linker starts about 4 MiB above it. The moves
> then replace part of its text with an anonymous mapping, and the test
> crashes in its next lazily bound call:
> 
>   merge[6980]: segfault at 7f7ebb3c1e60 ip 00007f7ebb3c1e60 sp 00007ffd35fbaec8 error 14
> 
> Two to four of the mremap() tests then fail with "Test terminated
> unexpectedly by signal 11", depending on the exact layout. On a host
> where libc is mapped at a 2 MiB aligned address, the targets happen to
> land above the dynamic linker and the tests pass.
> 
> Since the areas are all within the first 30 pages, moving one by 30
> pages is enough to keep it apart from the others. Add FAR_PAGES for
> that distance, move areas by FAR_PAGES or 2 * FAR_PAGES pages, and make
> the carveout 3 * FAR_PAGES pages so that it covers every target.
> 
> Fixes: 4e25f85b9f85 ("tools/testing/selftests: add mremap() unfaulted/faulted test cases")
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> Signed-off-by: Danish Khateeb <danishkhateeb03@xxxxxxxxx>

Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>

-- 
~Gregory