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 u/pTLK5g 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Howlett wrote: > > Cc linux-mm > > > > * Carlos Llamas [230424 16:56]: > > > This reverts commit a43cfc87caaf46710c8027a8c23b8a55f1078f19. > > > > > > This patch fixed an issue reported by syzkaller in [1]. However, this > > > turned out to be only a band-aid in binder. The root cause, as bisected > > > by syzkaller, was fixed by commit 5789151e48ac ("mm/mmap: undo ->mmap() > > > when mas_preallocate() fails"). We no longer need the patch for binder. > > > > > > Reverting such patch allows us to have a lockless access to alloc->vma > > > in specific cases where the mmap_lock is not required. > > > > Can you elaborate on the situation where recording a VMA pointer and > > later accessing it outside the mmap_lock is okay? > > The specifics are in the third patch of this patchset but the gist of it > is that during ->mmap() handler, binder will complete the initialization > of the binder_alloc structure. With the last step of this process being > the caching of the vma pointer. Since the ordering is protected with a > barrier we can then check alloc->vma to determine if the initialization > has been completed. > > Since this check is part of the critical path for every single binder > transaction, the performance plummeted when we started contending for > the mmap_lock. In this particular case, binder doesn't actually use the > vma. So why does binder_update_page_range() take the mmap_read_lock then use the cached vma in the reverted patch? If you want to use it as a flag to see if the driver is initialized, why not use the cached address != 0? Or better yet, >It only needs to know if the internal structure has been fully > initialized and it is safe to use it. This seems like a good reason to use your own rwsem. This is, essentially, rolling your own lock with smp_store_release()/smp_load_acquire() and a pointer which should not be cached. > > FWIW, this had been the design for ~15 years. The original patch is > this: https://git.kernel.org/torvalds/c/457b9a6f09f0