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Message-ID: <20201127122224.GX4327@casper.infradead.org>
Date: Fri, 27 Nov 2020 12:22:24 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [PATCH] mm: Don't fault around userfaultfd-registered regions on
reads
On Thu, Nov 26, 2020 at 05:23:59PM -0500, Peter Xu wrote:
> For missing mode uffds, fault around does not help because if the page cache
> existed, then the page should be there already. If the page cache is not
> there, nothing else we can do, either. If the fault-around code is destined to
> be helpless for userfault-missing vmas, then ideally we can skip it.
But it might have been faulted into the cache by another task, so skipping
it is bad.
> For wr-protected mode uffds, errornously fault in those pages around could lead
> to threads accessing the pages without uffd server's awareness. For example,
> when punching holes on uffd-wp registered shmem regions, we'll first try to
> unmap all the pages before evicting the page cache but without locking the
> page (please refer to shmem_fallocate(), where unmap_mapping_range() is called
> before shmem_truncate_range()). When fault-around happens near a hole being
> punched, we might errornously fault in the "holes" right before it will be
> punched. Then there's a small window before the page cache was finally
> dropped, and after the page will be writable again (NOTE: the uffd-wp protect
> information is totally lost due to the pre-unmap in shmem_fallocate(), so the
> page can be writable within the small window). That's severe data loss.
Sounds like you have a missing page_mkwrite implementation.
> This patch comes from debugging a data loss issue when working on the uffd-wp
> support on shmem/hugetlbfs. I posted this out for early review and comments,
> but also because it should already start to benefit missing mode userfaultfd to
> avoid trying to fault around on reads.
A measurable difference?
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