Re: [PATCH RFC v2 00/12] mm/hugetlb: Make huge_pte_offset() thread-safe for pmd unshare

[Mike Kravetz]

On 11/23/22 10:09, Peter Xu wrote:
> On Wed, Nov 23, 2022 at 10:40:40AM +0100, David Hildenbrand wrote:
> > Let me try understand the basic problem first:
> > 
> > hugetlb walks page tables semi-lockless: while we hold the mmap lock, we
> > don't grab the page table locks. That's very hugetlb specific handling and I
> > assume hugetlb uses different mechanisms to sync against MADV_DONTNEED,
> > concurrent page fault s... but that's no news. hugetlb is weird in many ways
> > :)
> > 
> > So, IIUC, you want a mechanism to synchronize against PMD unsharing. Can't
> > we use some very basic locking for that?
> 
> Yes we can in most cases.  Please refer to above paragraph [1] where I
> referred Mike's recent work on vma lock.  That's the basic locking we need
> so far to protect pmd unsharing.  I'll attach the link too in the next
> post, which is here:
> 
> https://lore.kernel.org/r/20220914221810.95771-1-mike.kravetz@xxxxxxxxxx
> 
> > 
> > Using RCU / disabling local irqs seems a bit excessive because we *are*
> > holding the mmap lock and only care about concurrent unsharing
> 
> The series wanted to address where the vma lock is not easy to take.  It
> originates from when I was reading Mike's other patch, I forgot why I did
> that but I just noticed there's some code path that we may not want to take
> a sleepable lock, e.g. in follow page code.

Yes, it was the patch suggested by David,

https://lore.kernel.org/linux-mm/20221030225825.40872-1-mike.kravetz@xxxxxxxxxx/

The issue was that FOLL_NOWAIT could be passed into follow_page_mask.  If so,
then we do not want potentially sleep on the mutex.

Since you both are on this thread, I thought of/noticed a related issue.  In
follow_hugetlb_page, it looks like we can call hugetlb_fault if FOLL_NOWAIT
is set.  hugetlb_fault certainly has the potential for sleeping.  Is this also
a similar issue?

-- 
Mike Kravetz

> The other one is huge_pmd_share() where we may have the mmap lock for
> current mm but we're fundamentally walking another mm.  It'll be tricky to
> take a sleepable lock in such condition too.
> 
> I mentioned these cases in the other paragraph above [2].  Let me try to
> expand that in my next post too.
> 
> It's debatable whether all the rest places can only work with either RCU or
> irq disabled, but the idea is at least it should speed up those paths when
> we still can.  Here, irqoff might be a bit heavy, but RCU lock should be
> always superior to vma lock when possible, the payoff is we may still see
> stale pgtable data (since unsharing can still happen in parallel), while
> that can be completely avoided when we take the vma lock.
> 
> Thanks,
> 
> -- 
> Peter Xu
>