On Tue, Sep 03, 2024 at 07:45:12PM +0800, Barry Song wrote: > On Tue, Sep 3, 2024 at 7:01 PM Hillf Danton <hdanton@xxxxxxxx> wrote: > > > > On Tue, Sep 03, 2024 at 10:50:09AM +1200, Barry Song wrote: > > > From: Barry Song <v-songbaohua@xxxxxxxx> > > > > > > The mmap_write_lock() can block all access to the VMAs, for example page > > > faults. Performing memory allocation while holding this lock may trigger > > > direct reclamation, leading to others being queued in the rwsem for an > > > extended period. > > > We've observed that the allocation can sometimes take more than 300ms, > > > significantly blocking other threads. The user interface sometimes > > > becomes less responsive as a result. To prevent this, let's move the > > > allocation outside of the write lock. Thanks for you patch Barry. So, we are aware of this contention and I've been working on a fix for it. See more about this below. > > > > I suspect concurrent allocators make things better wrt response, cutting > > alloc latency down to 10ms for instance in your scenario. Feel free to > > show figures given Tangquan's 48-hour profiling. > > Likely. > > Concurrent allocators are quite common in PFs which occur > in the same PTE. whoever gets PTL sets PTE, others free the allocated > pages. > > > > > > A potential side effect could be an extra alloc_page() for the second > > > thread executing binder_install_single_page() while the first thread > > > has done it earlier. However, according to Tangquan's 48-hour profiling > > > using monkey, the likelihood of this occurring is minimal, with a ratio > > > of only 1 in 2400. Compared to the significantly costly rwsem, this is > > > negligible. This is not negligible. In fact, it is the exact reason for the page allocation to be done with the mmap sem. If the first thread sleeps on vm_insert_page(), then binder gets into a bad state of multiple threads trying to reclaim pages that won't really be used. Memory pressure goes from bad to worst pretty quick. FWIW, I believe this was first talked about here: https://lore.kernel.org/all/ZWmNpxPXZSxdmDE1@xxxxxxxxxx/ > > > On the other hand, holding a write lock without making any VMA > > > modifications appears questionable and likely incorrect. While this > > > patch focuses on reducing the lock duration, future updates may aim > > > to eliminate the write lock entirely. > > > > If spin, better not before taking a look at vm_insert_page(). > > I have patch 2/3 transitioning to mmap_read_lock, and per_vma_lock is > currently in the > testing queue. At the moment, alloc->spin is in place, but I'm not > entirely convinced > it's the best replacement for the write lock. Let's wait for > Tangquan's test results. > > Patch 2 is detailed below, but it has only passed the build-test phase > so far, so > its result is uncertain. I'm sharing it early in case you find it > interesting. And I > am not convinced Commit d1d8875c8c13 ("binder: fix UAF of alloc->vma in > race with munmap()") is a correct fix to really avoid all UAF of alloc->vma. > > [PATCH] binder_alloc: Don't use mmap_write_lock for installing page > > Commit d1d8875c8c13 ("binder: fix UAF of alloc->vma in race with > munmap()") uses the mmap_rwsem write lock to protect against a race > condition with munmap, where the vma is detached by the write lock, > but pages are zapped by the read lock. This approach is extremely > expensive for the system, though perhaps less so for binder itself, > as the write lock can block all other operations. > > As an alternative, we could hold only the read lock and re-check > that the vma hasn't been detached. To protect simultaneous page > installation, we could use alloc->lock instead. > > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx> > --- > drivers/android/binder_alloc.c | 32 +++++++++++++++++--------------- > 1 file changed, 17 insertions(+), 15 deletions(-) > > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c > index f20074e23a7c..a2281dfacbbc 100644 > --- a/drivers/android/binder_alloc.c > +++ b/drivers/android/binder_alloc.c > @@ -228,24 +228,17 @@ static int binder_install_single_page(struct > binder_alloc *alloc, > return -ESRCH; > > /* > - * Don't allocate page in mmap_write_lock, this can block > - * mmap_rwsem for a long time; Meanwhile, allocation failure > - * doesn't necessarily need to return -ENOMEM, if lru_page > - * has been installed, we can still return 0(success). > + * Allocation failure doesn't necessarily need to return -ENOMEM, > + * if lru_page has been installed, we can still return 0(success). > + * So, defer the !page check until after binder_get_installed_page() > + * is completed. > */ > page = alloc_page(GFP_KERNEL | __GFP_HIGHMEM | __GFP_ZERO); > > - /* > - * 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)) { > - ret = 1; > - goto out; > - } > + mmap_read_lock(alloc->mm); > > - if (!alloc->vma) { > + /* vma might have been dropped or deattached */ > + if (!alloc->vma || !find_vma(alloc->mm, addr)) { > pr_err("%d: %s failed, no vma\n", alloc->pid, __func__); > ret = -ESRCH; > goto out; > @@ -257,18 +250,27 @@ static int binder_install_single_page(struct > binder_alloc *alloc, > goto out; > } > > + spin_lock(&alloc->lock); You can't hold a spinlock and then call vm_insert_page(). > + if (binder_get_installed_page(lru_page)) { > + spin_unlock(&alloc->lock); > + ret = 1; > + 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); > + spin_unlock(&alloc->lock); > ret = -ENOMEM; > goto out; > } > > /* Mark page installation complete and safe to use */ > binder_set_installed_page(lru_page, page); > + spin_unlock(&alloc->lock); > out: > - mmap_write_unlock(alloc->mm); > + mmap_read_unlock(alloc->mm); > mmput_async(alloc->mm); > if (ret && page) > __free_page(page); > -- > 2.39.3 (Apple Git-146) Sorry, but as I mentioned, I've been working on fixing this contention by supporting concurrent "faults" in binder_install_single_page(). This is the appropriate fix. I should be sending a patch soon after working out the conflicts with the shrinker's callback. Thanks, -- Carlos Llamas
