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

[David Hildenbrand]

On 23.11.22 19:56, Peter Xu wrote:
> On Wed, Nov 23, 2022 at 10:21:30AM -0800, Mike Kravetz wrote:
> > 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?
>
> Yeah maybe the clean way to do this is when FAULT_FLAG_RETRY_NOWAIT is set
> we should always try to not sleep at all.

hva_to_pfn_slow() that sets FOLL_NOWAIT calls get_user_pages_unlocked(), 
which will just do a straight mmap_read_lock().

The interpretation of FOLL_NOWAIT should not be "don't take any 
sleepable locks" but instead more like "don't wait for a page to get 
swapped in".

#define FOLL_NOWAIT  0x20  /* if a disk transfer is needed, start the IO


I did not read the full replies yet (sorry, busy hacking :) ) but *any* 
code path that already takes the mmap_read_lock() can just take whatever 
other lock we want -- IMHO. No need to over-complicate our code trying 
to avoid locks in that case.

--
Thanks,

David / dhildenb