Re: [PATCH 1/2] mm: fix the racy mm->locked_vm change in

[Oleg Nesterov]

On 09/30, Hugh Dickins wrote:
>
> On Tue, 29 Sep 2015, Oleg Nesterov wrote:
>
> > "mm->locked_vm += grow" and vm_stat_account() in acct_stack_growth()
> > are not safe; multiple threads using the same ->mm can do this at the
> > same time trying to expans different vma's under down_read(mmap_sem).
>                       expand
> > This means that one of the "locked_vm += grow" changes can be lost
> > and we can miss munlock_vma_pages_all() later.
>
> From the Cc list, I guess you are thinking this might be the fix to
> the "Bad state page (mlocked)" issues Andrey and Sasha have reported.

Yes, I found this when I tried to explain this problem, but I doubt
this change can fix it... Firstly I think it is very unlikely that
trinity hits this race. And even if mm->locked_vm is wrongly equal
to zero in exit_mmap(), it seems that page_remove_rmap() should do
clear_page_mlock(). But I do not understand this code enough. So if
this patch can actually help I would really like to know why ;)

And of course this can not explain other traces which look like
mm->mmap corruption.

> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

Thanks!

> with some hesitation.  I don't like very much that the preliminary
> mm->locked_vm + grow check is still done without complete locking,
> so racing threads could get more locked_vm than they're permitted;
> but I'm not sure that we care enough to put page_table_lock back
> over all of that (and security_vm_enough_memory wants to have final
> say on whether to go ahead); even if it was that way years ago.

Yes. Plus all these RLIMIT_MEMLOCK/etc and security_* checks assume
that we are going to expand current->mm, but this is not necessarily
true. Debugger or sys_process_vm_* can expand a foreign vma.

Oleg.

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