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SubjectRe: [PATCH v3 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap
On Wed, Dec 08, 2021 at 04:51:58PM +0100, Michal Hocko wrote:
> On Wed 08-12-21 15:01:24, Matthew Wilcox wrote:
> > On Tue, Dec 07, 2021 at 03:08:19PM -0800, Suren Baghdasaryan wrote:
> > > > > /**
> > > > > * @close: Called when the VMA is being removed from the MM.
> > > > > * Context: Caller holds mmap_lock.
> > >
> > > BTW, is the caller always required to hold mmap_lock for write or it
> > > *might* hold it?
> >
> > __do_munmap() might hold it for read, thanks to:
> >
> > if (downgrade)
> > mmap_write_downgrade(mm);
> >
> > Should probably say:
> >
> > * Context: User context. May sleep. Caller holds mmap_lock.
> >
> > I don't think we should burden the implementor of the vm_ops with the
> > knowledge that the VM chooses to not hold the mmap_lock under certain
> > circumstances when it doesn't matter whether it's holding the mmap_lock
> > or not.
>
> If we document it like that some code might depend on that lock to be
> held. I think we only want to document that the holder itself is not
> allowed to take mmap sem or a depending lock.

The only place where we're not currently holding the mmap_lock is at
task exit, where the mmap_lock is effectively held because nobody else
can modify the task's mm. Besides, Suren is changing that in this patch
series anyway, so it will be always true.

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