lkml.org 
[lkml]   [2023]   [Jan]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free
    On Mon 23-01-23 09:07:34, Suren Baghdasaryan wrote:
    > On Mon, Jan 23, 2023 at 8:55 AM Michal Hocko <mhocko@suse.com> wrote:
    > >
    > > On Mon 23-01-23 08:22:53, Suren Baghdasaryan wrote:
    > > > On Mon, Jan 23, 2023 at 1:56 AM Michal Hocko <mhocko@suse.com> wrote:
    > > > >
    > > > > On Fri 20-01-23 09:50:01, Suren Baghdasaryan wrote:
    > > > > > On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox <willy@infradead.org> wrote:
    > > > > [...]
    > > > > > > The page fault handler (or whatever other reader -- ptrace, proc, etc)
    > > > > > > should have a refcount on the mm_struct, so we can't be in this path
    > > > > > > trying to free VMAs. Right?
    > > > > >
    > > > > > Hmm. That sounds right. I checked process_mrelease() as well, which
    > > > > > operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps
    > > > > > VMAs without freeing them, so we are still good. Michal, do you agree
    > > > > > this is ok?
    > > > >
    > > > > Don't we need RCU procetions for the vma life time assurance? Jann has
    > > > > already shown how rwsem is not safe wrt to unlock and free without RCU.
    > > >
    > > > Jann's case requires a thread freeing the VMA to be blocked on vma
    > > > write lock waiting for the vma real lock to be released by a page
    > > > fault handler. However exit_mmap() means mm->mm_users==0, which in
    > > > turn suggests that there are no racing page fault handlers and no new
    > > > page fault handlers will appear. Is that a correct assumption? If so,
    > > > then races with page fault handlers can't happen while in exit_mmap().
    > > > Any other path (other than page fault handlers), accesses vma->lock
    > > > under protection of mmap_lock (for read or write, does not matter).
    > > > One exception is when we operate on an isolated VMA, then we don't
    > > > need mmap_lock protection, but exit_mmap() does not deal with isolated
    > > > VMAs, so out of scope here. exit_mmap() frees vm_area_structs under
    > > > protection of mmap_lock in write mode, so races with anything other
    > > > than page fault handler should be safe as they are today.
    > >
    > > I do not see you talking about #PF (RCU + vma read lock protected) with
    > > munmap. It is my understanding that the latter will synchronize over per
    > > vma lock (along with mmap_lock exclusive locking). But then we are back
    > > to the lifetime guarantees, or do I miss anything.
    >
    > munmap() or any VMA-freeing operation other than exit_mmap() will free
    > using call_rcu(), as implemented today. The suggestion is to free VMAs
    > directly, without RCU grace period only when done from exit_mmap().

    OK, I have clearly missed that. This makes more sense but it also adds
    some more complexity and assumptions - a harder to maintain code in the
    end. Whoever wants to touch this scheme in future would have to
    re-evaluate all of them. So, I would just avoid that special casing if
    that is feasible.

    Dealing with the flood of call_rcu during exit_mmap is a trivial thing
    to deal with as proposed elsewhere (just batch all of them in a single
    run). This will surely add some more code but at least the locking would
    consistent.
    --
    Michal Hocko
    SUSE Labs

    \
     
     \ /
      Last update: 2023-03-26 23:52    [W:4.495 / U:0.220 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site