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Message-ID: <ZyzdUyHr7S_cpsso@google.com>
Date: Thu, 7 Nov 2024 15:31:31 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: Suren Baghdasaryan <surenb@...gle.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>,
linux-kernel@...r.kernel.org, kernel-team@...roid.com,
David Hildenbrand <david@...hat.com>,
Barry Song <v-songbaohua@...o.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [PATCH v2 2/8] binder: concurrent page installation
On Thu, Nov 07, 2024 at 07:10:28AM -0800, Suren Baghdasaryan wrote:
> On Wed, Nov 6, 2024 at 8:02 PM Carlos Llamas <cmllamas@...gle.com> wrote:
> >
> > Allow multiple callers to install pages simultaneously by downgrading
> > the mmap_sem to non-exclusive mode. Races to the same PTE are handled
> > using get_user_pages_remote() to retrieve the already installed page.
> > This method significantly reduces contention in the mmap semaphore.
> >
> > To ensure safety, vma_lookup() is used (instead of alloc->vma) to avoid
> > operating on an isolated VMA. In addition, zap_page_range_single() is
> > called under the alloc->mutex to avoid racing with the shrinker.
> >
> > Many thanks to Barry Song who posted a similar approach [1].
> >
> > Link: https://lore.kernel.org/all/20240902225009.34576-1-21cnbao@gmail.com/ [1]
> > Cc: David Hildenbrand <david@...hat.com>
> > Cc: Barry Song <v-songbaohua@...o.com>
> > Cc: Suren Baghdasaryan <surenb@...gle.com>
> > Cc: Liam R. Howlett <Liam.Howlett@...cle.com>
> > Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
> > ---
> > drivers/android/binder_alloc.c | 64 +++++++++++++++++++++-------------
> > 1 file changed, 40 insertions(+), 24 deletions(-)
> >
> > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> > index 7241bf4a3ff2..acdc05552603 100644
> > --- a/drivers/android/binder_alloc.c
> > +++ b/drivers/android/binder_alloc.c
> > @@ -221,26 +221,14 @@ static int binder_install_single_page(struct binder_alloc *alloc,
> > struct binder_lru_page *lru_page,
> > unsigned long addr)
> > {
> > + struct vm_area_struct *vma;
> > struct page *page;
> > - int ret = 0;
> > + long npages;
> > + int ret;
> >
> > if (!mmget_not_zero(alloc->mm))
> > return -ESRCH;
> >
> > - /*
> > - * Protected with mmap_sem in write mode as multiple tasks
> > - * might race to install the same page.
> > - */
> > - mmap_write_lock(alloc->mm);
> > - if (binder_get_installed_page(lru_page))
> > - goto out;
> > -
> > - if (!alloc->vma) {
> > - pr_err("%d: %s failed, no vma\n", alloc->pid, __func__);
> > - ret = -ESRCH;
> > - goto out;
> > - }
> > -
> > page = alloc_page(GFP_KERNEL | __GFP_HIGHMEM | __GFP_ZERO);
> > if (!page) {
> > pr_err("%d: failed to allocate page\n", alloc->pid);
> > @@ -248,19 +236,47 @@ static int binder_install_single_page(struct binder_alloc *alloc,
> > goto out;
> > }
> >
> > - ret = vm_insert_page(alloc->vma, addr, page);
> > - if (ret) {
> > - pr_err("%d: %s failed to insert page at offset %lx with %d\n",
> > - alloc->pid, __func__, addr - alloc->buffer, ret);
> > + mmap_read_lock(alloc->mm);
> > + vma = vma_lookup(alloc->mm, addr);
> > + if (!vma || vma != alloc->vma) {
> > + mmap_read_unlock(alloc->mm);
>
> nit: instead of unlocking here you could have another label before
> mmap_read_unlock() at the end and jump to it.
Sounds good, I'll do this.
>
> > __free_page(page);
> > - ret = -ENOMEM;
> > + pr_err("%d: %s failed, no vma\n", alloc->pid, __func__);
> > + ret = -ESRCH;
> > goto out;
> > }
> >
> > - /* Mark page installation complete and safe to use */
> > - binder_set_installed_page(lru_page, page);
> > + ret = vm_insert_page(vma, addr, page);
> > + switch (ret) {
> > + case -EBUSY:
> > + /*
> > + * EBUSY is ok. Someone installed the pte first but the
> > + * lru_page->page_ptr has not been updated yet. Discard
> > + * our page and look up the one already installed.
> > + */
> > + ret = 0;
> > + __free_page(page);
> > + npages = get_user_pages_remote(alloc->mm, addr, 1, 0, &page, NULL);
> > + if (npages <= 0) {
> > + pr_err("%d: failed to find page at offset %lx\n",
> > + alloc->pid, addr - alloc->buffer);
> > + ret = -ESRCH;
> > + break;
> > + }
> > + fallthrough;
> > + case 0:
> > + /* Mark page installation complete and safe to use */
> > + binder_set_installed_page(lru_page, page);
> > + break;
> > + default:
> > + __free_page(page);
> > + pr_err("%d: %s failed to insert page at offset %lx with %d\n",
> > + alloc->pid, __func__, addr - alloc->buffer, ret);
> > + ret = -ENOMEM;
>
> vm_insert_page() can return EINVAL (see
> validate_page_before_insert()). Instead of converting other codes into
> ENOMEM why not return "ret" as is?
This is purely historical, binder has always propagated -ENOMEM to
userspace on errors from vm_insert_page() and I'm not sure why.
FWIW, I've dropped the behavior in the last patch and now I just forward
whatever vm_insert_page() returns. I had a look at libbinder code and it
doesn't really make a difference.
Perhaps, I should be explicit about this move and do it in a separate
commit.
Thanks,
--
Carlos Llamas
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