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Howlett" To: Alice Ryhl Cc: Miguel Ojeda , Matthew Wilcox , Lorenzo Stoakes , Vlastimil Babka , John Hubbard , Andrew Morton , Greg Kroah-Hartman , Arnd Bergmann , Jann Horn , Suren Baghdasaryan , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?B?QmrDtnJu?= Roy Baron , Benno Lossin , Andreas Hindborg , Trevor Gross , linux-kernel@vger.kernel.org, linux-mm@kvack.org, rust-for-linux@vger.kernel.org Subject: Re: [PATCH v13 2/8] mm: rust: add vm_area_struct methods that require read access Message-ID: Mail-Followup-To: "Liam R. 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 yFfKWJo5 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Howlett wrote: > > > > > > > + let vma =3D unsafe { bindings::vma_lookup(self.mm.as= _raw(), vma_addr) }; > > > > > > > + > > > > > > > + if vma.is_null() { > > > > > > > + None > > > > > > > + } else { > > > > > > > + // SAFETY: We just checked that a vma was found,= so the pointer is valid. Furthermore, > > > > > > > + // the returned area will borrow from this read = lock guard, so it can only be used > > > > > > > + // while the mmap read lock is still held. > > > > > > > > > > > > So We have complicated the locking of the vmas with rcu and per= -vma > > > > > > locking recently. We are now able to look up and use a vma und= er the > > > > > > rcu read lock. Does this translate to rust model? > > > > > > > > > > > > I believe this is true in recent version of binder as well? > > > > > > > > > > Yes. The safety requirements of VmAreaRef is that you must hold t= he > > > > > mmap read lock *or* the vma read lock while you have a VmAreaRef > > > > > reference. This particular method achieves that requirement by ho= lding > > > > > the mmap read lock. But there is also a Rust lock_vma_under_rcu()= , see > > > > > patch 4 for that. > > > > > > > > Right, okay. Thanks. You can get the reference by only holding th= e rcu > > > > read lock, but you should hold the vma lock to ensure that the vma > > > > itself (and not just the pointer) is safe to use. > > > > > > Hmm... To modify the vma, you must hold the mmap *and* vma write lock= , > > > so holding the mmap read lock prevents mutations? > > > > Sorry, I think I confused things with my answer. Your code is fine. > > The phrasing of the "only be used while the mmap read lock is still > > held" made me wonder about further clarification on the locking here > > (because the locking is confusing). > > > > Yes, mmap read lock means there are no writers that can modify the vma. > > Essentially, you are using the lock to ensure the entire vma space isn'= t > > changed during your operation - which is heavier than just locking one > > vma. >=20 > I could extend the safety comment like this: >=20 > SAFETY: We just checked that a vma was found, so the pointer is valid. > Furthermore, the returned area will borrow from this read lock guard, > so it can only be used while the mmap read lock is still held. This > ensures that there are no writers because writers must hold both the > mmap and vma write lock. How about just changing the last part to: Furthermore, the returned vma is still under the protection of the read lock guard and can be used while the mmap read lock is still held. Thanks, Liam