Re: [PATCH v3] mm/secretmem: properly account locked pages

On 8/26/26 18:30, Lorenzo Stoakes (ARM) wrote:

secretmem accounts folios by treating memory as if it were mlock()'d and
thus limited by the RLIMIT_MEMLOCK limit.

However the folios are unevictable and remain so until the inode is
evicted, eliminating usual mlock() semantics - mapping folios then
unmapping them does not clear their unevictable state, since it depends on
AS_UNEVICTABLE, not PG_mlocked.

A user can therefore easily work around the RLIMIT_MEMLOCK limit - simply
map then unmap and VmLck no longer counts the secretmem range. Worse,
folios are not accounted in the process's RSS, meaning the OOM killer won't
know to kill the process.

Repeatedly mapping/unmapping (or forking) can then result in the
consumption of all available system memory with unevictable folios and
cause system instability.

A secretmem fd can be passed between processes and over fork so a
per-process limit simply does not make sense, so follow the precedent set
by io_uring, perf, skbuff, iommufd and xdp by tracking the number of locked
pages in user_struct->locked_vm.

Since the scope tracked is actually inode lifetime, the RLIMIT_MEMLOCK
applies per-user not per-process, so it doesn't make sense to bypass for
users with CAP_IPC_LOCK, therefore remove this bypass.

There is simply no reason to carry on marking the mapping as mlock()'d
since it's misleading and the lifecycle is now correctly handled, so remove
this too.

Note that secretmem does not support any form of truncation (including hole
punching) and the folios are unreclaimable, so the folios need only be
accounted on fault and unaccounted on inode destruction.

__secretmem_account_pages() is more or less a duplicate of the code that
io_uring etc. use, but since this is a bug fix that needs backporting,
defer any de-duplication efforts to a follow-up.

test_mlock_limit() asserts mlock_future_ok() on mmap(), however this has
been removed, so remove the test altogether for the fix. A new test will be
sent separately for upstream.

Reported-by: Daehyeon Ko <redacted>
Closes: https://lore.kernel.org/linux-mm/20260813225328.2010303-1-4ncienth@gmail.com/ (local)
Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas")
Cc: stable@vger.kernel.org
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
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LGTM thanks

-- 
Cheers,

David