From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BBFAA379974; Mon, 3 Aug 2026 11:11:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785755516; cv=none; b=dn/FMi2Le5klQkBgBh4ihjalXtQxSEjne4HGPQ0yIBadKILwS+deTD4uvqx0vgMC4noq2x3FmaQva72fGPNRVK3IeNO18zLOnCPWiKS+UAHtK030ODo+LhScyXWB4NM7cjSuqlwF96B0YPPvPI4rZW9FlagQ1nPi+OSVScyX4sQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785755516; c=relaxed/simple; bh=WJjA9p12tFsV6zkH5AhowSQn8yCnJRFh98dmbQ7S74A=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Rw75IOjdpuYSbTMgRwDnvcsNvmQtH00QOFr7EJ07Mtha+nkfAPeeH2FjW+wqrkuyb+Yg9Re7gY1mbK/SEfpzFVL2PGGSpoACz6FU9x3DwXzewpCWs2x5+IgPXYGPrvG8iM018lRUqv9gB59TBRkEgXMM13uqVSIOAD1vAxbUUTo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=VYy7zN9D; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="VYy7zN9D" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F10E51F000E9; Mon, 3 Aug 2026 11:11:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785755515; bh=WJjA9p12tFsV6zkH5AhowSQn8yCnJRFh98dmbQ7S74A=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=VYy7zN9DQKeubwiuG+IrgbF+ryVDfowZogKwDDz4NWIOW5URub+pUL/pVNmMh24YZ tpQUlAF7BYw18DWmWlNETeMUjNpu9opaVkR9p1HpuWg0MN4zG9782rUewY1BG+7cyO Xo0jlKsFEPT4Ar0HfyxhRJtEy1IurhpiRKDK+IOiu2qFXH9JxL9D2eMsfRjz9lK/+f RPVTiqwaX4ty51kutN6Q8ZKy9cpufqJH56nA5zFp0w3wfg0QYMnNu4WDpiSA5qcAvs H+jL4E4WW4PpVHOgclHuw5ywFycKD25HzsTuONfE0U55xDuEy+13soDMv86Kv8S27G ZImetKrLspSxQ== Date: Mon, 3 Aug 2026 12:11:37 +0100 From: "Lorenzo Stoakes (ARM)" To: Alice Ryhl Cc: Suren Baghdasaryan , akpm@linux-foundation.org, dave.hansen@linux.intel.com, Liam.Howlett@oracle.com, david@redhat.com, willy@infradead.org, shakeel.butt@linux.dev, vbabka@kernel.org, jannh@google.com, arve@android.com, cmllamas@google.com, christian@brauner.io, tkjos@android.com, dsahern@kernel.org, davem@davemloft.net, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, netdev@vger.kernel.org Subject: Re: [PATCH v3 2/5] binder: Make shrinker rely solely on per-VMA lock Message-ID: References: <20260802215459.2769283-1-surenb@google.com> <20260802215459.2769283-3-surenb@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Aug 03, 2026 at 11:50:41AM +0100, Lorenzo Stoakes (ARM) wrote: > On Mon, Aug 03, 2026 at 09:48:19AM +0000, Alice Ryhl wrote: > > On Sun, Aug 02, 2026 at 02:54:56PM -0700, Suren Baghdasaryan wrote: > > > From: Dave Hansen > > > > > > tl;dr: lock_vma_under_rcu() is already a trylock. No need to do both > > > it and mmap_read_trylock(). > > > > > > Long Version: > > > > > > == Background == > > > > > > Historically, binder used an mmap_read_trylock() in its shrinker code. > > > This ensures that reclaim is not blocked on an mmap_lock. Commit > > > 95bc2d4a9020 ("binder: use per-vma lock in page reclaiming") added > > > support for the per-VMA lock, but left mmap_read_trylock() as a > > > fallback. > > > > > > This was presumably because the per-VMA locking can fail for several > > > reasons and most (all?) lock_vma_under_rcu() callers have a fallback > > > to mmap_read_trylock(). > > > > > > == Problem == > > > > > > The fallback is not worth the complexity here. lock_vma_under_rcu() is > > > essentially already a non-blocking trylock. The main reason it fails > > > is also the reason mmap_read_trylock() fails: something is holding > > > mmap_write_lock(). > > > > > > The only remedy for a collision with mmap_write_lock() is to wait, > > > which this code can not do. So the "fallback" after > > > lock_vma_under_rcu() failure is not really a fallback: it is really > > > likely to just be retrying in vain. That retry in an of itself isn't > > > horrible. But it adds complexity. > > > > > > == Solution == > > > > > > Now that per-VMA locks are universally available, lock_vma_under_rcu() > > > will not persistently fail. Rely on it alone and simplify the code. > > > > > > Full disclosure: I originally tried to do this with > > > lock_vma_under_rcu_wait(), but it did not fit well with the mmap_lock > > > trylock semantics. Claude caught this in a review and suggested the > > > approach in this path. It seemed sane to me. So, Suggesed-by: Claude, > > > I guess. > > > > > > Signed-off-by: Dave Hansen > > > Signed-off-by: Suren Baghdasaryan > > > > Reviewed-by: Alice Ryhl > > Is there anything on the rust side that needs to be changed also? Ah ok addressed in 4/5, disregard :) > > -- > Cheers, Lorenzo -- Cheers, Lorenzo