fs/proc/task_mmu: read proc/pid/{smaps|numa_maps} under per-vma lock

Patch series "use vma locks for proc/pid/{smaps|numa_maps} reads", v2.

Use per-vma locks when reading /proc/pid/smaps and /proc/pid/numa_maps
similar to /proc/pid/maps to reduce contention on central mmap_lock.  One
major difference between maps and smaps/numa_maps reading is that the
latter executes page table walk which can't be done under RCU due to a
possibility of sleeping.  Therefore we drop RCU read lock before this walk
while keeping the VMA locked.  After the walk we retake RCU read lock,
reset VMA iterator and proceed with the next VMA.

The last two patches extend /proc/pid/maps test to cover /proc/pid/smaps
reading during concurrent address space modification.


This patch (of 3):

proc/pid/{smaps|numa_maps} can be read using the combination of RCU and
VMA read locks, similar to proc/pid/maps.  RCU is required to safely
traverse the VMA tree and VMA lock stabilizes the VMA being processed and
the pagetable walk.

Link: https://lore.kernel.org/20260426062718.1238437-1-surenb@google.com
Link: https://lore.kernel.org/20260426062718.1238437-2-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <liam@infradead.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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