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Message-ID: <Zy6QC71iEy3w5FYA@google.com>
Date: Fri, 8 Nov 2024 22:26:19 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Arve Hjønnevåg <arve@...roid.com>,
Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
Joel Fernandes <joel@...lfernandes.org>,
Christian Brauner <brauner@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>,
linux-kernel@...r.kernel.org, kernel-team@...roid.com,
David Hildenbrand <david@...hat.com>,
Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v3 2/8] binder: concurrent page installation
On Fri, Nov 08, 2024 at 03:41:41PM -0500, Liam R. Howlett wrote:
> * Carlos Llamas <cmllamas@...gle.com> [241108 14:11]:
> > Allow multiple callers to install pages simultaneously by downgrading
> > the mmap_sem to non-exclusive mode.
>
> Since we actually allow downgrading of a lock, it would be more clear to
> say that you are changing from a read lock to a write lock. I was
> searching for the lock downgrade in this patch :)
ah, I can see how the wording here can be misleading. I'll rephrase that
to avoid confusion.
> > - ret = vm_insert_page(alloc->vma, addr, page);
> > - if (ret) {
> > + mmap_read_lock(alloc->mm);
>
> Ah, I debate bring this up, but you can do this without the mmap read
> lock. You can use rcu and the per-vma locking like in the page fault
> path. If you look at how that is done using the lock_vma_under_rcu()
> (mm/memory.c) then you can see that pages are installed today without
> the mmap locking (on some architectures - which includes x86_64 and
> arm64).
Right, per-vma locking is implemented in patch 7 of this series:
https://lore.kernel.org/all/20241108191057.3288442-8-cmllamas@google.com/
> userfaultfd had to do something like this as well, and created some
> abstraction to facilitate either mmap or rcu, based on the arch support.
> Going to rcu really helped performance there [1]. There was also a
> chance of the vma being missed, so it is checked again under the mmap
> read lock, but realistically that never happens and exists to ensure
> correctness.
hmm, if there are more users of this pattern in the future then perhaps
it might be worth adding a common helper?
> You also mention the shrinker and using the alloc->mutex, well zapping
> pages can also be done under the vma specific lock - if that helps?
>
> Freeing page tables is different though, that needs more locking, but I
> don't think this is an issue for you.
>
> [1]. https://lore.kernel.org/all/20240215182756.3448972-1-lokeshgidra@google.com/
ha, I was not aware of this. I'll have a look thanks.
>
> > + vma = vma_lookup(alloc->mm, addr);
>
> Thank you for not saving a pointer anymore.
lol, thanks to you for initiating this effort!
Cheers,
Carlos Llamas
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