linux-stable.git/tools/testing/vma/linux, branch v6.17

mm: replace vm_lock and detached flag with a reference count

2025-03-17T05:06:20+00:00

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.

[surenb@google.com: fix a crash due to vma_end_read() that should have been removed]
  Link: https://lkml.kernel.org/r/20250220200208.323769-1-surenb@google.com
Link: https://lkml.kernel.org/r/20250213224655.1680278-13-surenb@google.com
Signed-off-by: Suren Baghdasaryan 
Suggested-by: Peter Zijlstra 
Suggested-by: Matthew Wilcox 
Tested-by: Shivank Garg 
  Link: https://lkml.kernel.org/r/5e19ec93-8307-47c2-bb13-3ddf7150624e@amd.com
Reviewed-by: Vlastimil Babka 
Cc: Christian Brauner 
Cc: David Hildenbrand 
Cc: David Howells 
Cc: Davidlohr Bueso 
Cc: Hugh Dickins 
Cc: Jann Horn 
Cc: Johannes Weiner 
Cc: Jonathan Corbet 
Cc: Klara Modin 
Cc: Liam R. Howlett 
Cc: Lokesh Gidra 
Cc: Lorenzo Stoakes 
Cc: Mateusz Guzik 
Cc: Mel Gorman 
Cc: Michal Hocko 
Cc: Minchan Kim 
Cc: Oleg Nesterov 
Cc: Pasha Tatashin 
Cc: "Paul E . McKenney" 
Cc: Peter Xu 
Cc: Shakeel Butt 
Cc: Sourav Panda 
Cc: Wei Yang 
Cc: Will Deacon 
Cc: Heiko Carstens 
Cc: Stephen Rothwell 
Signed-off-by: Andrew Morton

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

2025-01-13T03:03:35+00:00

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/20241227222220.1726384-1-surenb@google.com
Fixes: 9325b8b5a1cb ("tools: add skeleton code for userland testing of VMA logic")
Signed-off-by: Suren Baghdasaryan 
Reviewed-by: Lorenzo Stoakes 
Cc: Jann Horn 
Cc: Liam R. Howlett 
Cc: Vlastimil Babka 
Cc: 
Signed-off-by: Andrew Morton

tools: add skeleton code for userland testing of VMA logic

2024-09-02T03:25:55+00:00

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 
Tested-by: SeongJae Park 
Acked-by: Vlastimil Babka 
Reviewed-by: Liam R. Howlett 
Cc: Alexander Viro 
Cc: Brendan Higgins 
Cc: Christian Brauner 
Cc: David Gow 
Cc: Eric W. Biederman 
Cc: Jan Kara 
Cc: Kees Cook 
Cc: Matthew Wilcox (Oracle) 
Cc: Rae Moar 
Cc: Shuah Khan 
Cc: Suren Baghdasaryan 
Cc: Pengfei Xu 
Signed-off-by: Andrew Morton