From: Mike Kravetz <mike.kravetz@oracle.com>
To: Peter Xu <peterx@redhat.com>
Cc: David Hildenbrand <david@redhat.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Rik van Riel <riel@surriel.com>,
Muchun Song <songmuchun@bytedance.com>,
Andrew Morton <akpm@linux-foundation.org>,
James Houghton <jthoughton@google.com>,
Nadav Amit <nadav.amit@gmail.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Miaohe Lin <linmiaohe@huawei.com>
Subject: Re: [PATCH RFC v2 00/12] mm/hugetlb: Make huge_pte_offset() thread-safe for pmd unshare
Date: Wed, 23 Nov 2022 10:21:30 -0800 [thread overview]
Message-ID: <Y35kqkYQGW8ohKEa@monkey> (raw)
In-Reply-To: <Y343kIQ9l2d8wViz@x1n>
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@oracle.com
>
> >
> > 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@oracle.com/
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
>
next prev parent reply other threads:[~2022-11-23 18:21 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-18 1:10 Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 01/12] mm/hugetlb: Let vma_offset_start() to return start Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 02/12] mm/hugetlb: Move swap entry handling into vma lock for fault Peter Xu
2022-11-18 1:35 ` Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 03/12] mm/hugetlb: Don't wait for migration entry during follow page Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 04/12] mm/hugetlb: Add pgtable walker lock Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 05/12] mm/hugetlb: Make userfaultfd_huge_must_wait() safe to pmd unshare Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 06/12] mm/hugetlb: Protect huge_pmd_share() with walker lock Peter Xu
2022-11-18 1:17 ` Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 07/12] mm/hugetlb: Use hugetlb walker lock in hugetlb_follow_page_mask() Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 08/12] mm/hugetlb: Use hugetlb walker lock in follow_hugetlb_page() Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 09/12] mm/hugetlb: Use hugetlb walker lock in hugetlb_vma_maps_page() Peter Xu
2022-11-18 1:10 ` [PATCH RFC v2 10/12] mm/hugetlb: Use hugetlb walker lock in walk_hugetlb_range() Peter Xu
2022-11-18 1:11 ` [PATCH RFC v2 11/12] mm/hugetlb: Use hugetlb walker lock in page_vma_mapped_walk() Peter Xu
2022-11-18 1:11 ` [PATCH RFC v2 12/12] mm/hugetlb: Introduce hugetlb_walk() Peter Xu
2022-11-23 9:40 ` [PATCH RFC v2 00/12] mm/hugetlb: Make huge_pte_offset() thread-safe for pmd unshare David Hildenbrand
2022-11-23 15:09 ` Peter Xu
2022-11-23 18:21 ` Mike Kravetz [this message]
2022-11-23 18:56 ` Peter Xu
2022-11-23 19:31 ` David Hildenbrand
2022-11-25 9:43 ` David Hildenbrand
2022-11-25 13:55 ` Peter Xu
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