From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f170.google.com (mail-pl1-f170.google.com [209.85.214.170]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3FEEE38758D for ; Mon, 10 Aug 2026 18:57:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.170 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786388268; cv=none; b=WCFv+FU4t3/9noWCo1DdbM2zkOQ8eS+NSVypGaTg3yykannwqGaxXbkgnw22rEFrEEssDC9eno6DcZOvI2Hi1CGFC+xQujpj+93BIDrNJb4tnUTOQHv0QmzUYP/ibAzjQnsJVnNo+ZhWdZZIdFwbpM3tm+sw95e/lTePj/qCDuk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786388268; c=relaxed/simple; bh=12d/6ewGNujC9HUSogExdbgQR+eqrRrh1/mwtxpRJ14=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=bRCt1UVT1/+lXsLdzje3ScDFnDe+TTtTx3czfnC+APsE9PaJ2ThzoExunVBA+57+6k7atNZz+kynnsAqYx05lfE8Y9YbGJYa2qVf32/IvDIpljFNiCKMbOdkpVW3TsE5MIGATABmylYpPdu2g7CxivGc5lxqSGHaz4+xB+CKK1M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=Ts/uQXUH; arc=none smtp.client-ip=209.85.214.170 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="Ts/uQXUH" Received: by mail-pl1-f170.google.com with SMTP id d9443c01a7336-2ccdf36f63dso20965ad.0 for ; Mon, 10 Aug 2026 11:57:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1786388264; x=1786993064; darn=vger.kernel.org; h=in-reply-to:content-transfer-encoding:content-disposition :content-type:mime-version:references:message-id:subject:cc:to:from :date:from:to:cc:subject:date:message-id:reply-to:content-type; bh=srMva8ELPlGiijDY3h4l4HTa6KEy7A4N7csv16dIv1c=; b=Ts/uQXUHT/g0ev/J2eK0vIMp9KIoxuEpRFlZsJiAO9ox5R2jPyNpl+1FtGDauYeTxa lJhHVAk+Gwyy7cfQ0kBYKG07cYSklqK/sSei4XUksLnQeof9KfrcdTIsMH4Onltbsbfl iJJWztSZuh++eotnoNsb++b8cDqWgi8ilXdycPDDNvaPqIi3DUkuFonAK87M46p7aj1e 5hQovB2bh+A4rBtXP+4IzwAvABdsBidrBlE9g+eC23oRgmoWZJ8EPUyvX8mnvUokCYMa ktblQNveFB8+CrOl1PuRxIyh2dOAdtLxkUvHbN7yUqFy4SFmYoZIgq5/7KKz7mJzi/WY qF5w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786388264; x=1786993064; h=in-reply-to:content-transfer-encoding:content-disposition :content-type:mime-version:references:message-id:subject:cc:to:from :date:x-gm-gg:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to:content-type; bh=srMva8ELPlGiijDY3h4l4HTa6KEy7A4N7csv16dIv1c=; b=G6ytVhN5dH902hcZc6VH2mWFhDX8UhdVWHE08kG7cO+is+cPkNFTjmhhx7JPcojM05 0lip2F89MmoVdxXtqh+vYQrBsWANfkiFfxNoCPajJhb9kGuyN5gFNhXKRfJnON9xXFQ7 LL6ePawyJCdhuZjPWC1SNHQ89Q9mOqMQrVI/G16kCQeruK1n6V6dv0PCMPW4NRnF4pE+ yVfhdec+Xk5cmWZBtyaqDPuH6JqVEZkS8ckHP5nSs9etOai7qGPFKXwhz4+Dn1tCbbny GixyGLmAjkl1xS7ytK5vEIulSmDq3dIXjd7+XtwEeQM9rRyh8ApE06qkorDy+sDOHncr L6UQ== X-Forwarded-Encrypted: i=1; AHgh+Rop/DnaOJDX8paWEDNO/Jab+q4quPAlpqcPzQy+qajE7zoEwq0/a4sIE2xrpIoEKn/tPWP3iD0=@vger.kernel.org X-Gm-Message-State: AOJu0YyPcLIDUdVDQ43WqEOxWw/w3Db22bBYK0AMiBnDqN0du57jrWhR os5RUKcfmSKzRBztY9S3mbOoL0tmUaApBktus3TVXmgI7Q4zS/RYCnhV8i6RfmqjqQ== X-Gm-Gg: AR+sD13GgccEzBe3GRJ2Neh+ibLMOm5RXsUbEEA9NRim3N3qBUythdN/spAVtFwY4LY MTpF0qF8z9/ASoqXXyypKatYnJvlTnmER55gOFxxpPwydRvDXf3mz/OmKQFMcEVeG7o9AmYwf5q +4igwthI9RrD9CVaaBXflSlLWlLskx9UntmM4snXHInPXQQslTxGYznWqwqw7P/hb8ZDAogNwjE CILsNOyKe70XXcSeqFWRVlC/ds6uYr/a+YmqJJnK6JLXDkPaXU9Km7OcNc1uw/OJldcF+Cq6e/5 DLcTyr3Ew7eKH0UjeB135KFaOtBciU1dqQKcTUbLrkM+JwHLh1lD/iiUt0hdyZFupSIgeLrmCex 11gf8O3utqO+jffnQkchuGad8drUeijHcuCEsH0aIx8AlrrtfdJaZkPvDmWHh9+8l5IjfdXrPDP MiK7rJrbZekfhWnwiOdodTVqYUc45ivyZ/UWsw/91lP6GBb8uVKjmSxQviH72otA7Ia3/O8j76M rCvjdFiFXyxrh7vzGssz+1rLEHOpYb4UyoKSSRo/SrO2NzgNUVa7laDrs4kvCxfYV5d5k03K5fK vD01TogB8MeqHN8nhQ== X-Received: by 2002:a17:902:d081:b0:2d1:1d3d:97c3 with SMTP id d9443c01a7336-2d31088e36bmr1036735ad.13.1786388263842; Mon, 10 Aug 2026 11:57:43 -0700 (PDT) Received: from google.com (193.67.125.34.bc.googleusercontent.com. [34.125.67.193]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-392d4050827sm647419a91.2.2026.08.10.11.57.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 10 Aug 2026 11:57:43 -0700 (PDT) Date: Mon, 10 Aug 2026 18:57:38 +0000 From: Carlos Llamas To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, dave.hansen@linux.intel.com, Liam.Howlett@oracle.com, ljs@kernel.org, david@kernel.org, willy@infradead.org, shakeel.butt@linux.dev, vbabka@kernel.org, jannh@google.com, aliceryhl@google.com, arve@android.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 v4 2/5] binder: Make shrinker rely solely on per-VMA lock Message-ID: References: <20260806200548.3124802-1-surenb@google.com> <20260806200548.3124802-3-surenb@google.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Aug 10, 2026 at 06:32:15PM +0000, Carlos Llamas wrote: > On Thu, Aug 06, 2026 at 01:05:45PM -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. > > The removal of the fallback does not affect NOMMU case because binder > > driver depends on CONFIG_MMU. > > > > 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 > > Cc: Andrew Morton > > Cc: "Liam R. Howlett" > > Cc: Vlastimil Babka > > Cc: Shakeel Butt > > Cc: linux-mm@kvack.org > > Cc: Greg Kroah-Hartman > > Cc: Arve Hjønnevåg > > Cc: Todd Kjos > > Cc: Christian Brauner > > Cc: Carlos Llamas > > Cc: Alice Ryhl > > Cc: "David S. Miller" > > Cc: David Ahern > > Cc: netdev@vger.kernel.org > > --- FWIW, I think we should just simply do the following: --- diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c index e4488ad86a65..b0b618c1d1da 100644 --- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -1142,7 +1142,6 @@ enum lru_status binder_alloc_free_page(struct list_head *item, struct vm_area_struct *vma; struct page *page_to_free; unsigned long page_addr; - int mm_locked = 0; size_t index; if (!mmget_not_zero(mm)) @@ -1153,13 +1152,8 @@ enum lru_status binder_alloc_free_page(struct list_head *item, /* attempt per-vma lock first */ vma = lock_vma_under_rcu(mm, page_addr); - if (!vma) { - /* fall back to mmap_lock */ - if (!mmap_read_trylock(mm)) - goto err_mmap_read_lock_failed; - mm_locked = 1; - vma = vma_lookup(mm, page_addr); - } + if (!vma) + goto err_vma_lock_failed; if (!mutex_trylock(&alloc->mutex)) goto err_get_alloc_mutex_failed; @@ -1191,10 +1185,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item, } mutex_unlock(&alloc->mutex); - if (mm_locked) - mmap_read_unlock(mm); - else - vma_end_read(vma); + vma_end_read(vma); mmput_async(mm); binder_free_page(page_to_free); @@ -1203,11 +1194,8 @@ enum lru_status binder_alloc_free_page(struct list_head *item, err_invalid_vma: mutex_unlock(&alloc->mutex); err_get_alloc_mutex_failed: - if (mm_locked) - mmap_read_unlock(mm); - else - vma_end_read(vma); -err_mmap_read_lock_failed: + vma_end_read(vma); +err_vma_lock_failed: mmput_async(mm); err_mmget: return LRU_SKIP;