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From: Greg KH <>
To: Suren Baghdasaryan <>
Cc:,,,,,,,,,,,,,, Christoph Hellwig <>,
	Oleg Nesterov <>,
	Kirill Shutemov <>, Jan Kara <>,
	Matthew Wilcox <>
Subject: Re: [PATCH 1/1] gup: document and work around "COW can break either way" issue
Date: Fri, 23 Apr 2021 17:05:21 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, Apr 21, 2021 at 03:56:13PM -0700, Suren Baghdasaryan wrote:
> From: Linus Torvalds <>
> commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.
> Doing a "get_user_pages()" on a copy-on-write page for reading can be
> ambiguous: the page can be COW'ed at any time afterwards, and the
> direction of a COW event isn't defined.
> Yes, whoever writes to it will generally do the COW, but if the thread
> that did the get_user_pages() unmapped the page before the write (and
> that could happen due to memory pressure in addition to any outright
> action), the writer could also just take over the old page instead.
> End result: the get_user_pages() call might result in a page pointer
> that is no longer associated with the original VM, and is associated
> with - and controlled by - another VM having taken it over instead.
> So when doing a get_user_pages() on a COW mapping, the only really safe
> thing to do would be to break the COW when getting the page, even when
> only getting it for reading.
> At the same time, some users simply don't even care.
> For example, the perf code wants to look up the page not because it
> cares about the page, but because the code simply wants to look up the
> physical address of the access for informational purposes, and doesn't
> really care about races when a page might be unmapped and remapped
> elsewhere.
> This adds logic to force a COW event by setting FOLL_WRITE on any
> copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page
> pointer as a result.
> The current semantics end up being:
>  - __get_user_pages_fast(): no change. If you don't ask for a write,
>    you won't break COW. You'd better know what you're doing.
>  - get_user_pages_fast(): the fast-case "look it up in the page tables
>    without anything getting mmap_sem" now refuses to follow a read-only
>    page, since it might need COW breaking.  Which happens in the slow
>    path - the fast path doesn't know if the memory might be COW or not.
>  - get_user_pages() (including the slow-path fallback for gup_fast()):
>    for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with
>    very similar semantics to FOLL_FORCE.
> If it turns out that we want finer granularity (ie "only break COW when
> it might actually matter" - things like the zero page are special and
> don't need to be broken) we might need to push these semantics deeper
> into the lookup fault path.  So if people care enough, it's possible
> that we might end up adding a new internal FOLL_BREAK_COW flag to go
> with the internal FOLL_COW flag we already have for tracking "I had a
> COW".
> Alternatively, if it turns out that different callers might want to
> explicitly control the forced COW break behavior, we might even want to
> make such a flag visible to the users of get_user_pages() instead of
> using the above default semantics.
> But for now, this is mostly commentary on the issue (this commit message
> being a lot bigger than the patch, and that patch in turn is almost all
> comments), with that minimal "enable COW breaking early" logic using the
> existing FOLL_WRITE behavior.
> [ It might be worth noting that we've always had this ambiguity, and it
>   could arguably be seen as a user-space issue.
>   You only get private COW mappings that could break either way in
>   situations where user space is doing cooperative things (ie fork()
>   before an execve() etc), but it _is_ surprising and very subtle, and
>   fork() is supposed to give you independent address spaces.
>   So let's treat this as a kernel issue and make the semantics of
>   get_user_pages() easier to understand. Note that obviously a true
>   shared mapping will still get a page that can change under us, so this
>   does _not_ mean that get_user_pages() somehow returns any "stable"
>   page ]
> [surenb: backport notes]
> Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_pgd_range.
> Removed FOLL_PIN usage in should_force_cow_break since it's missing in
> the earlier kernels.
> Reported-by: Jann Horn <>
> Tested-by: Christoph Hellwig <>
> Acked-by: Oleg Nesterov <>
> Acked-by: Kirill Shutemov <>
> Acked-by: Jan Kara <>
> Cc: Andrea Arcangeli <>
> Cc: Matthew Wilcox <>
> Signed-off-by: Linus Torvalds <>
> [surenb: backport to 4.19 kernel]
> Cc: # 4.19.x
> Signed-off-by: Suren Baghdasaryan <>
> ---
>  mm/gup.c         | 44 ++++++++++++++++++++++++++++++++++++++------
>  mm/huge_memory.c |  7 +++----
>  2 files changed, 41 insertions(+), 10 deletions(-)

Thanks for these backports, I've now queued them up.

greg k-h

  reply	other threads:[~2021-04-23 15:05 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-21 22:56 Suren Baghdasaryan
2021-04-23 15:05 ` Greg KH [this message]
2021-04-21 22:57 Suren Baghdasaryan
2021-10-12  1:52 Suren Baghdasaryan
2021-10-12  1:55 ` Suren Baghdasaryan
2021-10-12  5:45   ` Greg Kroah-Hartman
2021-10-12  8:06 ` Jan Kara
2021-10-12  8:14   ` Vlastimil Babka
2021-10-12  8:42     ` Greg KH
2021-10-12 18:57       ` Thadeu Lima de Souza Cascardo
2021-10-13  8:56         ` Greg KH
2021-10-12 10:06 ` Greg KH
2021-10-12 16:16   ` Suren Baghdasaryan
2021-10-13 21:58 ` John Hubbard
2021-10-12  1:53 Suren Baghdasaryan

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