From: Mike Kravetz <mike.kravetz@oracle.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Xu <peterx@redhat.com>, Linux-MM <linux-mm@kvack.org>,
kernel list <linux-kernel@vger.kernel.org>,
Naoya Horiguchi <naoya.horiguchi@linux.dev>,
David Hildenbrand <david@redhat.com>,
Axel Rasmussen <axelrasmussen@google.com>,
Mina Almasry <almasrymina@google.com>,
Rik van Riel <riel@surriel.com>, Vlastimil Babka <vbabka@suse.cz>,
Matthew Wilcox <willy@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v8 2/2] mm: remove zap_page_range and change callers to use zap_vma_range
Date: Thu, 10 Nov 2022 14:17:39 -0800 [thread overview]
Message-ID: <Y214g5vrDqOi6Tmw@monkey> (raw)
In-Reply-To: <C16276B9-F40A-4767-AB3A-8566FA61A8DF@gmail.com>
On 11/10/22 14:02, Nadav Amit wrote:
> On Nov 10, 2022, at 1:27 PM, Peter Xu <peterx@redhat.com> wrote:
>
> > Hi, Nadav,
> >
> > On Thu, Nov 10, 2022 at 01:09:43PM -0800, Nadav Amit wrote:
> >> But, are the callers really able to guarantee that the ranges are all in a
> >> single VMA? I am not familiar with the users, but how for instance
> >> tcp_zerocopy_receive() can guarantee that no one did some mprotect() of some
> >> sorts that caused the original VMA to be split?
> >
> > Let me try to answer this one for Mike.. We have two callers in tcp
> > zerocopy code for this function:
> >
> > tcp_zerocopy_vm_insert_batch_error[2095] zap_page_range(vma, *address, maybe_zap_len);
> > tcp_zerocopy_receive[2237] zap_page_range(vma, address, total_bytes_to_map);
> >
> > Both of them take the mmap lock for read, so firstly mprotect is not
> > possible.
> >
> > The 1st call has:
> >
> > mmap_read_lock(current->mm);
> >
> > vma = vma_lookup(current->mm, address);
> > if (!vma || vma->vm_ops != &tcp_vm_ops) {
> > mmap_read_unlock(current->mm);
> > return -EINVAL;
> > }
> > vma_len = min_t(unsigned long, zc->length, vma->vm_end - address);
> > avail_len = min_t(u32, vma_len, inq);
> > total_bytes_to_map = avail_len & ~(PAGE_SIZE - 1);
> > if (total_bytes_to_map) {
> > if (!(zc->flags & TCP_RECEIVE_ZEROCOPY_FLAG_TLB_CLEAN_HINT))
> > zap_page_range(vma, address, total_bytes_to_map);
> >
> > Here total_bytes_to_map comes from avail_len <--- vma_len, which is a min()
> > of the rest vma range. So total_bytes_to_map will never go beyond the vma.
> >
> > The 2nd call uses maybe_zap_len as len, we need to look two layers of the
> > callers, but ultimately it's something smaller than total_bytes_to_map we
> > discussed. Hopefully it proves 100% safety on tcp zerocopy.
>
> Thanks Peter for the detailed explanation.
>
> I had another look at the code and indeed it should not break. I am not sure
> whether users who zero-copy receive and mprotect() part of the memory would
> not be surprised, but I guess that’s a different story, which I should
> further study at some point.
I did audit all calling sites and am fairly certain passed ranges are within
a single vma. Because of this, Peter suggested removing zap_page_range. If
there is concern, we can just fix up the mmu notifiers in zap_page_range and
leave it. This is what is done in the patch which is currently in
mm-hotfixes-unstable.
--
Mike Kravetz
next prev parent reply other threads:[~2022-11-10 22:20 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-08 1:19 [PATCH v8 0/2] hugetlb MADV_DONTNEED fix and zap_page_range cleanup Mike Kravetz
2022-11-08 1:19 ` [PATCH v8 1/2] hugetlb: don't delete vma_lock in hugetlb MADV_DONTNEED processing Mike Kravetz
2022-11-10 20:56 ` Peter Xu
2022-11-10 21:55 ` Mike Kravetz
2022-11-10 20:59 ` Nadav Amit
2022-11-10 21:48 ` Mike Kravetz
2022-11-10 22:07 ` Peter Xu
2022-11-10 22:22 ` Nadav Amit
2022-11-10 22:31 ` Mike Kravetz
2022-11-10 21:07 ` Peter Xu
2022-11-08 1:19 ` [PATCH v8 2/2] mm: remove zap_page_range and change callers to use zap_vma_range Mike Kravetz
2022-11-10 21:09 ` Nadav Amit
2022-11-10 21:27 ` Peter Xu
2022-11-10 22:02 ` Nadav Amit
2022-11-10 22:17 ` Mike Kravetz [this message]
2022-11-10 19:46 ` [PATCH v8 0/2] hugetlb MADV_DONTNEED fix and zap_page_range cleanup Mike Kravetz
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