Re: Sealed memfd & no-fault mmap

[Ming Lin]

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