On Tue, Apr 29, 2025 at 8:04 PM Suren Baghdasaryan <sur...@google.com> wrote:
> On Tue, Apr 29, 2025 at 10:21 AM Jann Horn <ja...@google.com> wrote:
> >
> > Hi!
> >
> > (I just noticed that I incorrectly assumed that VMAs use kfree_rcu
> > (not SLAB_TYPESAFE_BY_RCU) when I wrote my review of this, somehow I
> > forgot all about that...)
>
> Does this fact affect your previous comments? Just want to make sure
> I'm not missing something...
When I suggested using "WRITE_ONCE(vma->vm_file, NULL)" when tearing
down a VMA, and using get_file_rcu() for the lockless lookup, I did
not realize that you could actually also race with all the other
places that set ->vm_file, like __mmap_new_file_vma() and so on; and I
did not think about whether any of those code paths might leave a VMA
with a dangling ->vm_file pointer.
I guess maybe that means you really do need to do the lookup from the
copied data, as you did in your patch; and that might require calling
get_file_active() on the copied ->vm_file pointer (instead of
get_file_rcu()), even though I think that is not really how
get_file_active() is supposed to be used (it's supposed to be used
when you know the original file hasn't been freed yet). Really what
you'd want for that is basically a raw __get_file_rcu(), but that is
static and I think Christian wouldn't want to expose more of these
internals outside VFS...
(In that case, all the stuff below about get_file_rcu() would be moot.)
Or you could pepper WRITE_ONCE() over all the places that write
->vm_file, and ensure that ->vm_file is always NULLed before its
reference is dropped... but that seems a bit more ugly to me.
> > On Tue, Apr 29, 2025 at 7:09 PM Suren Baghdasaryan <sur...@google.com>
> > wrote:
> > > On Tue, Apr 29, 2025 at 8:40 AM Jann Horn <ja...@google.com> wrote:
> > > > On Fri, Apr 18, 2025 at 7:50 PM Suren Baghdasaryan <sur...@google.com>
> > > > wrote:
> > > > > With maple_tree supporting vma tree traversal under RCU and vma and
> > > > > its important members being RCU-safe, /proc/pid/maps can be read under
> > > > > RCU and without the need to read-lock mmap_lock. However vma content
> > > > > can change from under us, therefore we make a copy of the vma and we
> > > > > pin pointer fields used when generating the output (currently only
> > > > > vm_file and anon_name). Afterwards we check for concurrent address
> > > > > space modifications, wait for them to end and retry. While we take
> > > > > the mmap_lock for reading during such contention, we do that
> > > > > momentarily
> > > > > only to record new mm_wr_seq counter. This change is designed to
> > > > > reduce
> > > > > mmap_lock contention and prevent a process reading /proc/pid/maps
> > > > > files
> > > > > (often a low priority task, such as monitoring/data collection
> > > > > services)
> > > > > from blocking address space updates.
> > > > [...]
> > > > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > > > index b9e4fbbdf6e6..f9d50a61167c 100644
> > > > > --- a/fs/proc/task_mmu.c
> > > > > +++ b/fs/proc/task_mmu.c
> > > > [...]
> > > > > +/*
> > > > > + * Take VMA snapshot and pin vm_file and anon_name as they are used
> > > > > by
> > > > > + * show_map_vma.
> > > > > + */
> > > > > +static int get_vma_snapshot(struct proc_maps_private *priv, struct
> > > > > vm_area_struct *vma)
> > > > > +{
> > > > > + struct vm_area_struct *copy = &priv->vma_copy;
> > > > > + int ret = -EAGAIN;
> > > > > +
> > > > > + memcpy(copy, vma, sizeof(*vma));
> > > > > + if (copy->vm_file && !get_file_rcu(©->vm_file))
> > > > > + goto out;
> > > >
> > > > I think this uses get_file_rcu() in a different way than intended.
> > > >
> > > > As I understand it, get_file_rcu() is supposed to be called on a
> > > > pointer which always points to a file with a non-zero refcount (except
> > > > when it is NULL). That's why it takes a file** instead of a file* - if
> > > > it observes a zero refcount, it assumes that the pointer must have
> > > > been updated in the meantime, and retries. Calling get_file_rcu() on a
> > > > pointer that points to a file with zero refcount, which I think can
> > > > happen with this patch, will cause an endless loop.
> > > > (Just as background: For other usecases, get_file_rcu() is supposed to
> > > > still behave nicely and not spuriously return NULL when the file* is
> > > > concurrently updated to point to another file*; that's what that loop
> > > > is for.)
> > >
> > > Ah, I see. I wasn't aware of this subtlety. I think this is fixable by
> > > checking the return value of get_file_rcu() and retrying speculation
> > > if it changed.
> >
> > I think you could probably still end up looping endlessly in get_file_rcu().
(Just to be clear: What I meant here is that get_file_rcu() loops
*internally*; get_file_rcu() is not guaranteed to ever return if the
pointed-to file has a zero refcount.)
> By "retrying speculation" I meant it in the sense of
> get_vma_snapshot() retry when it takes the mmap_read_lock and then
> does mmap_lock_speculate_try_begin to restart speculation. I'm also
> thinking about Liam's concern of guaranteeing forward progress for the
> reader. Thinking maybe I should not drop mmap_read_lock immediately on
> contention but generate some output (one vma or one page worth of
> vmas) before dropping mmap_read_lock and proceeding with speculation.
Hm, yeah, I guess you need that for forward progress...
> > > > (If my understanding is correct, maybe we should document that more
> > > > explicitly...)
> > >
> > > Good point. I'll add comments for get_file_rcu() as a separate patch.
> > >
> > > >
> > > > Also, I think you are introducing an implicit assumption that
> > > > remove_vma() does not NULL out the ->vm_file pointer (because that
> > > > could cause tearing and could theoretically lead to a torn pointer
> > > > being accessed here).
> > > >
> > > > One alternative might be to change the paths that drop references to
> > > > vma->vm_file (search for vma_close to find them) such that they first
> > > > NULL out ->vm_file with a WRITE_ONCE() and do the fput() after that,
> > > > maybe with a new helper like this:
> > > >
> > > > static void vma_fput(struct vm_area_struct *vma)
> > > > {
> > > > struct file *file = vma->vm_file;
> > > >
> > > > if (file) {
> > > > WRITE_ONCE(vma->vm_file, NULL);
> > > > fput(file);
> > > > }
> > > > }
> > > >
> > > > Then on the lockless lookup path you could use get_file_rcu() on the
> > > > ->vm_file pointer _of the original VMA_, and store the returned file*
> > > > into copy->vm_file.
> > >
> > > Ack. Except for storing the return value of get_file_rcu(). I think
> > > once we detect that get_file_rcu() returns a different file we should
> > > bail out and retry. The change in file is an indication that the vma
> > > got changed from under us, so whatever we have is stale.
> >
> > What does "different file" mean here - what file* would you compare
> > the returned one against?
>
> Inside get_vma_snapshot() I would pass the original vma->vm_file to
> get_file_rcu() and check if it returns the same value. If the value
> got changed we jump to /* Address space got modified, vma might be
> stale. Re-lock and retry. */ section. That should work, right?
Where do you get an "original vma->vm_file" from?
To be clear, get_file_rcu(p) returns one of the values that *p had
while get_file_rcu(p) is running.
