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[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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 72E2AA0003 X-Rspam-User: X-Stat-Signature: k1cgpfmh1fypeddr8n5mgs7oipuobk15 X-HE-Tag: 1786388265-485262 X-HE-Meta: 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 xpI95D8v 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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;