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h=Date:From:To:Cc:Subject:References:In-Reply-To; b=laxXRj3+r8yZ9K+RSpG6vi6zKPDvYe3GBYTALOFtRWTH76t/I70VijHNz5j7r1YnN K42uEbRi7TYxjqJpNq+sI8ickY/webiCvSvJ4kA92iCsWF1MF+nH2sJgSjPaxlVNaA nkFiYLwbaCdJmmm2ra700kIvVrWHk+HsMAQen60pOIryMAaBgjOWl0fkQzZzefnGmL tywpJe+9RccEmg8ZJLN9bo//HnItnHLK/3+HLNxSC1noJ3RzfapmRypwjRqBQrWqLP 78wZoIX9HcVY45Kr80Wpuk4lxUiGHBOySzTDyOnpoRxcpSJjtjHfNbCTMLSf98w+Up pqe/HkGpyJsTw== Date: Mon, 3 Aug 2026 12:33:59 +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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: gz4a6fj6z594rchr3dthjw566mtyhkow X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 4852D40010 X-Rspam-User: X-HE-Tag: 1785756859-378877 X-HE-Meta: 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 ZVMaIrrY ya9txYqtN87znHWQRdrggthnYPKT1tfrKlNrnacI904i57kbE3bvD9zfFIWM8I+sSTlXNRCctazgmmSjT0unu1uo3yTejBh6ihDY4XjHgkIL37Nyi5iYPYnMLdoAsCDnXwlI3qPzyPDPFvn5ghoBk7pZPGmP+ItP+ipRqbz2WxPg+Rbu7KPjpbtOCFauDKnd4FkC7eeCTeO7WaeGj/A0LvHbbBnMliTM9IvE/7OF3q89YmZ7C5qX7c4N9hug58QIB9JUQu3VY408/+YGlGU/ScLz1meBeGnKudHN5S+TGNlvqm8QOeFFAViShJN9akwy3theNE9sxGfCdQ/DThVAZWX2m/zMj2XAXaiwysfXpvDtJTzrtFnWd/j39WA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 03, 2026 at 12:11:42PM +0100, Lorenzo Stoakes (ARM) wrote: > 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 :) OK disregard the disregard :)) that addresses something different. So question remains :P -- Cheers, Lorenzo