From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>,
Andrea Arcangeli <aarcange@redhat.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Borislav Petkov <bp@suse.de>
Subject: Re: [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults
Date: Wed, 22 May 2019 21:43:24 +0200 [thread overview]
Message-ID: <20190522194322.5k52docwgp5zkdcj@linutronix.de> (raw)
In-Reply-To: <20190522122113.a2edc8aba32f0fad189bae21@linux-foundation.org>
On 2019-05-22 12:21:13 [-0700], Andrew Morton wrote:
> On Tue, 14 May 2019 17:29:55 +0300 Mike Rapoport <rppt@linux.ibm.com> wrote:
>
> > When get_user_pages*() is called with pages = NULL, the processing of
> > VM_FAULT_RETRY terminates early without actually retrying to fault-in all
> > the pages.
> >
> > If the pages in the requested range belong to a VMA that has userfaultfd
> > registered, handle_userfault() returns VM_FAULT_RETRY *after* user space
> > has populated the page, but for the gup pre-fault case there's no actual
> > retry and the caller will get no pages although they are present.
> >
> > This issue was uncovered when running post-copy memory restore in CRIU
> > after commit d9c9ce34ed5c ("x86/fpu: Fault-in user stack if
> > copy_fpstate_to_sigframe() fails").
> >
> > After this change, the copying of FPU state to the sigframe switched from
> > copy_to_user() variants which caused a real page fault to get_user_pages()
> > with pages parameter set to NULL.
>
> You're saying that argument buf_fx in copy_fpstate_to_sigframe() is NULL?
buf_fx is user stack pointer and it should not be NULL.
> If so was that expected by the (now cc'ed) developers of
> d9c9ce34ed5c8923 ("x86/fpu: Fault-in user stack if
> copy_fpstate_to_sigframe() fails")?
>
> It seems rather odd. copy_fpregs_to_sigframe() doesn't look like it's
> expecting a NULL argument.
exactly, this is not expected.
> Also, I wonder if copy_fpstate_to_sigframe() would be better using
> fault_in_pages_writeable() rather than get_user_pages_unlocked(). That
> seems like it operates at a more suitable level and I guess it will fix
> this issue also.
It looks, like fault_in_pages_writeable() would work. If this is the
recommendation from the MM department than I can switch to that.
> > In post-copy mode of CRIU, the destination memory is managed with
> > userfaultfd and lack of the retry for pre-fault case in get_user_pages()
> > causes a crash of the restored process.
> >
> > Making the pre-fault behavior of get_user_pages() the same as the "normal"
> > one fixes the issue.
>
> Should this be backported into -stable trees?
Sebastian
next prev parent reply other threads:[~2019-05-22 19:43 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-26 17:33 [PATCH] x86/fpu: Use fault_in_pages_writeable() for pre-faulting Sebastian Andrzej Siewior
2019-05-26 17:35 ` Sebastian Andrzej Siewior
2019-05-26 19:25 ` Hugh Dickins
2019-05-28 11:54 ` My emacs problem -- was " Pavel Machek
2019-05-29 4:18 ` Andrew Morton
2019-05-29 7:25 ` [PATCH v2] " Sebastian Andrzej Siewior
2019-05-14 14:29 ` [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults Mike Rapoport
2019-05-16 16:25 ` Andrei Vagin
2019-05-21 15:53 ` Mike Rapoport
2019-05-22 19:21 ` Andrew Morton
2019-05-22 19:43 ` Sebastian Andrzej Siewior [this message]
2019-05-24 22:22 ` Hugh Dickins
2019-05-25 8:45 ` Sebastian Andrzej Siewior
2019-05-25 18:09 ` Hugh Dickins
2019-05-26 19:36 ` Sebastian Andrzej Siewior
2019-05-26 20:17 ` Hugh Dickins
2019-05-26 17:25 ` Pavel Machek
2019-05-22 20:38 ` Mike Rapoport
2019-05-22 21:18 ` Andrew Morton
2019-06-22 17:51 ` Andrea Arcangeli
2019-06-06 17:25 ` [tip:x86/urgent] x86/fpu: Use fault_in_pages_writeable() for pre-faulting tip-bot for Hugh Dickins
2019-05-29 21:29 ` [PATCH v2] " Chris Wilson
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