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 7FFE6846A; Mon, 3 Aug 2026 11:10:40 +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=1785755441; cv=none; b=soY8q5KQ7qW62xu9rgdwQhDDqAIvf/DPN/QAE/ZiAOM7SFVo0WW1fTh9pVI81H1SiniojACsFAAVDIvxfL7OjJPloM8eHQOBw5DhBumDvUPv1/ZKiuMgH8dGa591iGucv/COTUnrC+zXUZnXB7bDNUPOku+pNlwc94haJjbhs54= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785755441; c=relaxed/simple; bh=t4eylFBYpRTL1bMl+UUVRy730w8kyrOjmUo/hO5oSFw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=RNvRVWPpA4aGdFfIEmnyJV/kWf7dvlkvcGy/UsA4q02hoRrgeY+egHeEkQxNP7YGuWWupJL/M1K0jjE6p3bupzchTBt8A4WqTUDLUN3jFhS2Eqga87rOA+V6LFy8DhF/qh6UCs9ZLkJmNK8ncGnqVO2IDqUQknvniPSA2/VGSXY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JiUiJCOy; 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="JiUiJCOy" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C4D121F000E9; Mon, 3 Aug 2026 11:10:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785755440; bh=X1AHQgHItUAjyCfSWii/TMz98UWdEFUg8DqjV+05t70=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=JiUiJCOye7/jl1rGGRpqCOJgVh16/YcyclOxKfogRjggIMDGxj9QsmGN6ZXUVIUPm JPfO9GDePDAomLfbjEXVKJYDg1y26wrH1mfyhczkWxGXX+KL0b2KoZpRJa9VNRiRjJ FuJyDqfl4tLeONgNL6s4ZEWnK3XEziDZrqsvuBlzZQ2HWisN91O0dHjbOHDrsAlbdH AKvDLK9NKVCzwK+PB1ArR80DX8LuGlNUPcERKHVlgJeQ1D2JbjmQd02AolrO3A5BaJ T+IOZuDn1ZLtBUtPMQpIFRz6TeZFH6OV/SkPfXgqpzdUEXavltTrZrml7ugpIlduzo tMzhGgF2PDKHQ== Date: Mon, 3 Aug 2026 12:10:21 +0100 From: "Lorenzo Stoakes (ARM)" To: Suren Baghdasaryan Cc: 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, aliceryhl@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=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260802215459.2769283-3-surenb@google.com> 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 > 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 > --- > drivers/android/binder_alloc.c | 29 +++++++++++++++-------------- > 1 file changed, 15 insertions(+), 14 deletions(-) > > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c > index e4488ad86a65..84104ba04e30 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)) > @@ -1151,14 +1150,20 @@ enum lru_status binder_alloc_free_page(struct list_head *item, > index = mdata->page_index; > page_addr = alloc->vm_start + index * PAGE_SIZE; > > - /* attempt per-vma lock first */ > + /* > + * Attempt per-vma lock. This is essentially a > + * "trylock". It can fail even if the VMA exists > + * for 'page_addr'. > + */ This makes me wonder whether lock_vma_under_rcu() should really become vma_trylock() at some point in time? :) Or at least have 'trylock' in the name. > 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 the vma exists, we can't continue because we cannot > + * remove the page from the vma. However, if the vma was > + * unmapped, it's okay to continue. > + */ > + if (binder_alloc_is_mapped(alloc)) > + goto err_vma_lock_failed; Hmm, it seems a bit odd to me that you also have: if (vma && !binder_alloc_is_mapped(alloc)) goto err_invalid_vma; Below? So you have: Before: |binder_alloc_is_mapped()? |yes no --------|----------------- vma is mapped? yes |OK abort no |OK OK Now: |binder_alloc_is_mapped()? |yes no --------|----------------- vma is mapped? maybe |abort OK yes |OK abort no |OK OK The 'maybe' is because the VMA trylock failed. So the issue is you might have a case where the VMA _is_ mapped but !binder_alloc_is_mapped(), which previously aborted because of the vma && !binder_alloc_is_mapped() check. It seems like: /* * Since a binder_alloc can only be mapped once, we ensure * the vma corresponds to this mapping by checking whether * the binder_alloc is still mapped. */ if (vma && !binder_alloc_is_mapped(alloc)) goto err_invalid_vma; Is testing for a specific scenario 'we found a VMA but it turns out it's invalid' and aborting if so. So either this check should be removed or you should uncondtionally abort if !vma I think? > } > > if (!mutex_trylock(&alloc->mutex)) > @@ -1191,9 +1196,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item, > } > > mutex_unlock(&alloc->mutex); > - if (mm_locked) > - mmap_read_unlock(mm); > - else > + if (vma) > vma_end_read(vma); > mmput_async(mm); > binder_free_page(page_to_free); > @@ -1203,11 +1206,9 @@ 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 > + if (vma) > vma_end_read(vma); > -err_mmap_read_lock_failed: > +err_vma_lock_failed: > mmput_async(mm); > err_mmget: > return LRU_SKIP; > -- > 2.55.0.508.g3f0d502094-goog > -- Cheers, Lorenzo