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Subject: Re: Sealed memfd & no-fault mmap
From:   Ming Lin <minggr@gmail.com>
To:     Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Simon Ser <contact@emersion.fr>
Cc:     Peter Xu <peterx@redhat.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Herrmann <dh.herrmann@gmail.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Greg Kroah-Hartman <greg@kroah.com>,
        "tytso@mit.edu" <tytso@mit.edu>
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Message-ID: <e7454046-c071-888d-f673-276f9c24d9d3@gmail.com>
Date:   Mon, 31 May 2021 14:13:28 -0700
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On 5/29/2021 4:36 PM, Ming Lin wrote:
> On 5/29/2021 1:15 PM, Hugh Dickins wrote:
> 
>>
>> I believe the correct behaviour would be to unmap the nofault page
>> then, allowing the proper page to be faulted in after.  That is
>> certainly doable (the old mm/filemap_xip.c used to do so), but might
>> get into some awkward race territory, with filesystem dependence
>> (reminiscent of hole punch, in reverse).  shmem could operate that
>> way, and be the better for it: but I wouldn't want to add that,
>> without also cleaning away all the shmem_recalc_inode() stuff.

OK, I borrowed code from filemap_xip.c and implemented this behavior.

Simon,

Before I send out the patches for review, would you mind have a quick test?
https://github.com/minggr/linux, branch shmem_no_sigbus

In Wayland compositors, you only need to pass in MAP_NOSIGBUS in mmap().
For example,

//fd should be received from Wayland compositors client
#define MAP_NOSIGBUS    0x200000
addr = mmap(NULL, size, PROT_READ, MAP_SHARED|MAP_NOSIGBUS, fd, offset)

Thanks,
Ming