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Howlett" , linux-mm@kvack.org, Lorenzo Stoakes , Shakeel Butt , Suren Baghdasaryan , Vlastimil Babka From: Dave Hansen Date: Wed, 29 Apr 2026 11:19:57 -0700 References: <20260429181954.F50224AE@davehans-spike.ostc.intel.com> In-Reply-To: <20260429181954.F50224AE@davehans-spike.ostc.intel.com> Message-Id: <20260429181957.7511C256@davehans-spike.ostc.intel.com> X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 5537F40008 X-Rspam-User: X-Stat-Signature: paorq41cigci9wfbdpdn1hzzomomyiuo X-HE-Tag: 1777486801-568482 X-HE-Meta: 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 iu8QhNgp HnSxKpFuatiPBrHFmE4y+T2P2KYcrBydelWMbrxaOP46exCsgyKmdjLXy9dhfoADoSygAkk1pMtYshbZ9sV4TNFkHLct+CaQEJSWKez8ljqiL3bT0+VR5LzGrX1d1fK7d8xa8DiG9jedY8/ca9t1xygaVLFCzDkOP2QFRIZhbrtUoysp+AGRbUAwtyUJNed+bbBkRyFaDBZcamnJeDKGvFppGjDm2N3SqRZVlAVFCgAdBJLr9LUuFjEcWev5ouEE/dmllsu0C18ARhqdk7ikq6kwyAQFKtEASgrhQHCh7Yf1gNnuOETsOJaM8OdBnREyEQhWEfySN1pCCfuk4OIw7knMGvlU+bYEdUH89+2LnEtS5v0E= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 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 Cc: Suren Baghdasaryan Cc: Andrew Morton Cc: "Liam R. Howlett" Cc: Lorenzo Stoakes Cc: Vlastimil Babka Cc: Shakeel Butt Cc: linux-mm@kvack.org --- b/drivers/android/binder_alloc.c | 22 +++++----------------- 1 file changed, 5 insertions(+), 17 deletions(-) diff -puN drivers/android/binder_alloc.c~binder-try-vma-lock drivers/android/binder_alloc.c --- a/drivers/android/binder_alloc.c~binder-try-vma-lock 2026-04-29 11:18:50.066607065 -0700 +++ b/drivers/android/binder_alloc.c 2026-04-29 11:18:50.069607180 -0700 @@ -1142,7 +1142,6 @@ enum lru_status binder_alloc_free_page(s 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)) @@ -1151,15 +1150,10 @@ enum lru_status binder_alloc_free_page(s index = mdata->page_index; page_addr = alloc->vm_start + index * PAGE_SIZE; - /* attempt per-vma lock first */ + /* attempt per-vma lock */ 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_mmap_read_lock_failed; if (!mutex_trylock(&alloc->mutex)) goto err_get_alloc_mutex_failed; @@ -1191,10 +1185,7 @@ enum lru_status binder_alloc_free_page(s } 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); @@ -1203,10 +1194,7 @@ enum lru_status binder_alloc_free_page(s err_invalid_vma: mutex_unlock(&alloc->mutex); err_get_alloc_mutex_failed: - if (mm_locked) - mmap_read_unlock(mm); - else - vma_end_read(vma); + vma_end_read(vma); err_mmap_read_lock_failed: mmput_async(mm); err_mmget: _