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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ming Lin <minggr@gmail.com>
Cc: Hugh Dickins <hughd@google.com>, Simon Ser <contact@emersion.fr>,
	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>
Subject: Re: Sealed memfd & no-fault mmap
Date: Mon, 31 May 2021 20:24:07 -1000	[thread overview]
Message-ID: <CAHk-=whce3vmj+g7jcE0rasoDavJutxno3ZZrvvWYQywWXH31Q@mail.gmail.com> (raw)
In-Reply-To: <e7454046-c071-888d-f673-276f9c24d9d3@gmail.com>

On Mon, May 31, 2021 at 11:13 AM Ming Lin <minggr@gmail.com> wrote:
>
> OK, I borrowed code from filemap_xip.c and implemented this behavior.

I think that "unmap page" is too complicated and fragile.

The only page that could possibly validly be unmapped is a stale zero
page, but that code in shmem_unmap_nofault_page() seems to try to
handle other cases too (ie that whole page_remove_rmap() - afaik a
zero page has no rmap).

I get the feeling that the simpler thing to do is to just say "if you
use MAP_NOSIGBUS, and you access pages that don't have a backing
store, you will get zero pages, and they will NOT BE SYNCHRONIZED with
the backing store possibly later being updated".

IOW, just document the fact that a MAP_NOSIGBUS mapping isn't coherent
wrt shmem contents that are expanded and filled in later.

Don't try to "fix" it - because any user that uses MAP_NOSIGBUS had
better just accept that it's not compatible with expanding the shmem
backing store later.

Keep it simple and stupid. Hmm?

                  Linus


  reply	other threads:[~2021-06-01  6:31 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-27  8:24 Sealed memfd & no-fault mmap Simon Ser
2021-04-27 16:51 ` Linus Torvalds
2021-04-29 15:48   ` Kirill A. Shutemov
2021-04-29 18:38     ` Peter Xu
2021-05-04  9:29       ` Simon Ser
2021-05-04 16:08         ` Linus Torvalds
2021-05-05 10:21           ` Simon Ser
2021-05-05 18:42             ` Linus Torvalds
2021-05-28 17:07               ` Lin, Ming
2021-05-29  1:03                 ` Linus Torvalds
2021-05-29  7:31                   ` Lin, Ming
2021-05-29 15:44                     ` Linus Torvalds
2021-05-29 20:15                       ` Hugh Dickins
2021-05-29 23:36                         ` Ming Lin
2021-05-31 21:13                           ` Ming Lin
2021-06-01  6:24                             ` Linus Torvalds [this message]
2021-06-01  7:08                               ` Ming Lin
2021-06-03 13:01                                 ` Simon Ser
2021-06-03 20:07                                   ` Ming Lin
2021-06-03 20:49                                     ` Simon Ser
2021-06-03 13:14                         ` Simon Ser
2021-06-03 13:57                           ` Matthew Wilcox
2021-06-03 14:48                             ` Simon Ser

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