linux-kernel - Re: [PATCH v4] mm: use per_vma lock for MADV

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Message-ID: <CAGsJ_4xVH6DT_8t=oDvHCJ-iDwrpms6FhMn9UdKWMwDRv+hunA@mail.gmail.com>
Date: Wed, 18 Jun 2025 17:52:28 +0800
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, David Hildenbrand <david@...hat.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Lokesh Gidra <lokeshgidra@...gle.com>, 
	Tangquan Zheng <zhengtangquan@...o.com>, Qi Zheng <zhengqi.arch@...edance.com>, 
	Lance Yang <ioworker0@...il.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Zi Li <zi.li@...ux.dev>
Subject: Re: [PATCH v4] mm: use per_vma lock for MADV_DONTNEED

On Wed, Jun 18, 2025 at 10:25 AM Lance Yang <lance.yang@...ux.dev> wrote:
>
> Hi all,
>
> Crazy, the per-VMA lock for madvise is an absolute game-changer ;)
>
> On 2025/6/17 21:38, Lorenzo Stoakes wrote:
> [...]
> >
> > On Sun, Jun 08, 2025 at 10:01:50AM +1200, Barry Song wrote:
> >> From: Barry Song <v-songbaohua@...o.com>
> >>
> >> Certain madvise operations, especially MADV_DONTNEED, occur far more
> >> frequently than other madvise options, particularly in native and Java
> >> heaps for dynamic memory management.
> >>
> >> Currently, the mmap_lock is always held during these operations, even when
> >> unnecessary. This causes lock contention and can lead to severe priority
> >> inversion, where low-priority threads—such as Android's HeapTaskDaemon—
> >> hold the lock and block higher-priority threads.
> >>
> >> This patch enables the use of per-VMA locks when the advised range lies
> >> entirely within a single VMA, avoiding the need for full VMA traversal. In
> >> practice, userspace heaps rarely issue MADV_DONTNEED across multiple VMAs.
> >>
> >> Tangquan’s testing shows that over 99.5% of memory reclaimed by Android
> >> benefits from this per-VMA lock optimization. After extended runtime,
> >> 217,735 madvise calls from HeapTaskDaemon used the per-VMA path, while
> >> only 1,231 fell back to mmap_lock.
> >>
> >> To simplify handling, the implementation falls back to the standard
> >> mmap_lock if userfaultfd is enabled on the VMA, avoiding the complexity of
> >> userfaultfd_remove().
> >>
> >> Many thanks to Lorenzo's work[1] on:
> >> "Refactor the madvise() code to retain state about the locking mode
> >> utilised for traversing VMAs.
> >>
> >> Then use this mechanism to permit VMA locking to be done later in the
> >> madvise() logic and also to allow altering of the locking mode to permit
> >> falling back to an mmap read lock if required."
> >>
> >> One important point, as pointed out by Jann[2], is that
> >> untagged_addr_remote() requires holding mmap_lock. This is because
> >> address tagging on x86 and RISC-V is quite complex.
> >>
> >> Until untagged_addr_remote() becomes atomic—which seems unlikely in
> >> the near future—we cannot support per-VMA locks for remote processes.
> >> So for now, only local processes are supported.
>
> Just to put some numbers on it, I ran a micro-benchmark with 100
> parallel threads, where each thread calls madvise() on its own 1GiB
> chunk of 64KiB mTHP-backed memory. The performance gain is huge:
>
> 1) MADV_DONTNEED saw its average time drop from 0.0508s to 0.0270s (~47%
> faster)
> 2) MADV_FREE     saw its average time drop from 0.3078s to 0.1095s (~64%
> faster)

Thanks for the report, Lance. I assume your micro-benchmark includes some
explicit or implicit operations that may require mmap_write_lock().
As  mmap_read_lock() only waits for writers and does not block other
mmap_read_lock() calls.

By the way, I would expect that per-VMA locking for madvise_dontneed or
madvise_free would benefit nearly all Linux and Android systems, as long
as they use a dynamic C/Java memory allocator.

>
> Thanks,
> Lance

Thanks
Barry