mm: replace vm_lock and detached flag with a reference count

rw_semaphore is a sizable structure of 40 bytes and consumes considerable
space for each vm_area_struct. However vma_lock has two import

mm: replace vm_lock and detached flag with a reference count

rw_semaphore is a sizable structure of 40 bytes and consumes considerable
space for each vm_area_struct. However vma_lock has two important
specifics which can be used to replace rw_semaphore with a simpler
structure:

1. Readers never wait. They try to take the vma_lock and fall back to
mmap_lock if that fails.

2. Only one writer at a time will ever try to write-lock a vma_lock
because writers first take mmap_lock in write mode. Because of these
requirements, full rw_semaphore functionality is not needed and we can
replace rw_semaphore and the vma->detached flag with a refcount
(vm_refcnt).

When vma is in detached state, vm_refcnt is 0 and only a call to
vma_mark_attached() can take it out of this state. Note that unlike
before, now we enforce both vma_mark_attached() and vma_mark_detached() to
be done only after vma has been write-locked. vma_mark_attached() changes
vm_refcnt to 1 to indicate that it has been attached to the vma tree.
When a reader takes read lock, it increments vm_refcnt, unless the top
usable bit of vm_refcnt (0x40000000) is set, indicating presence of a
writer. When writer takes write lock, it sets the top usable bit to
indicate its presence. If there are readers, writer will wait using newly
introduced mm->vma_writer_wait. Since all writers take mmap_lock in write
mode first, there can be only one writer at a time. The last reader to
release the lock will signal the writer to wake up. refcount might
overflow if there are many competing readers, in which case read-locking
will fail. Readers are expected to handle such failures.

In summary:
1. all readers increment the vm_refcnt;
2. writer sets top usable (writer) bit of vm_refcnt;
3. readers cannot increment the vm_refcnt if the writer bit is set;
4. in the presence of readers, writer must wait for the vm_refcnt to drop
to 1 (plus the VMA_LOCK_OFFSET writer bit), indicating an attached vma
with no readers;
5. vm_refcnt overflow is handled by the readers.

While this vm_lock replacement does not yet result in a smaller
vm_area_struct (it stays at 256 bytes due to cacheline alignment), it
allows for further size optimization by structure member regrouping to
bring the size of vm_area_struct below 192 bytes.

[[email protected]: fix a crash due to vma_end_read() that should have been removed]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Suren Baghdasaryan <[email protected]>
Suggested-by: Peter Zijlstra <[email protected]>
Suggested-by: Matthew Wilcox <[email protected]>
Tested-by: Shivank Garg <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Reviewed-by: Vlastimil Babka <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Howells <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Klara Modin <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: "Paul E . McKenney" <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Sourav Panda <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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tools: fix atomic_set() definition to set the value correctly

Currently vma test is failing because of the new vma_assert_attached()
assertion. The check is failing because previous refcount_set()

tools: fix atomic_set() definition to set the value correctly

Currently vma test is failing because of the new vma_assert_attached()
assertion. The check is failing because previous refcount_set() inside
vma_mark_attached() is a NoOp. Fix the definition of atomic_set() to
correctly set the value of the atomic.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 9325b8b5a1cb ("tools: add skeleton code for userland testing of VMA logic")
Signed-off-by: Suren Baghdasaryan <[email protected]>
Reviewed-by: Lorenzo Stoakes <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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tools: add skeleton code for userland testing of VMA logic

Establish a new userland VMA unit testing implementation under
tools/testing which utilises existing logic providing maple tree support
in

tools: add skeleton code for userland testing of VMA logic

Establish a new userland VMA unit testing implementation under
tools/testing which utilises existing logic providing maple tree support
in userland utilising the now-shared code previously exclusive to radix
tree testing.

This provides fundamental VMA operations whose API is defined in mm/vma.h,
while stubbing out superfluous functionality.

This exists as a proof-of-concept, with the test implementation functional
and sufficient to allow userland compilation of vma.c, but containing only
cursory tests to demonstrate basic functionality.

Link: https://lkml.kernel.org/r/533ffa2eec771cbe6b387dd049a7f128a53eb616.1722251717.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Tested-by: SeongJae Park <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Brendan Higgins <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: David Gow <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Rae Moar <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Pengfei Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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