From: Jerome Glisse <jglisse@redhat.com>
To: John Hubbard <jhubbard@nvidia.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org,
Evgeny Baskakov <ebaskakov@nvidia.com>,
Ralph Campbell <rcampbell@nvidia.com>,
Mark Hairgrove <mhairgrove@nvidia.com>
Subject: Re: [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit
Date: Wed, 21 Mar 2018 14:12:36 -0400 [thread overview]
Message-ID: <20180321181235.GF3214@redhat.com> (raw)
In-Reply-To: <55b8cf9f-2a81-19f3-ff4f-70d5a411baaa@nvidia.com>
On Tue, Mar 20, 2018 at 09:24:41PM -0700, John Hubbard wrote:
> On 03/19/2018 07:00 PM, jglisse@redhat.com wrote:
> > From: Jerome Glisse <jglisse@redhat.com>
> >
> > This code was lost in translation at one point. This properly call
> > mmu_notifier_unregister_no_release() once last user is gone. This
> > fix the zombie mm_struct as without this patch we do not drop the
> > refcount we have on it.
> >
> > Signed-off-by: Jerome Glisse <jglisse@redhat.com>
> > Cc: Evgeny Baskakov <ebaskakov@nvidia.com>
> > Cc: Ralph Campbell <rcampbell@nvidia.com>
> > Cc: Mark Hairgrove <mhairgrove@nvidia.com>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > ---
> > mm/hmm.c | 19 +++++++++++++++++++
> > 1 file changed, 19 insertions(+)
> >
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index 6088fa6ed137..667944630dc9 100644
> > --- a/mm/hmm.c
> > +++ b/mm/hmm.c
> > @@ -244,10 +244,29 @@ EXPORT_SYMBOL(hmm_mirror_register);
> > void hmm_mirror_unregister(struct hmm_mirror *mirror)
> > {
> > struct hmm *hmm = mirror->hmm;
> > + struct mm_struct *mm = NULL;
> > + bool unregister = false;
> >
> > down_write(&hmm->mirrors_sem);
> > list_del_init(&mirror->list);
> > + unregister = list_empty(&hmm->mirrors);
>
> Hi Jerome,
>
> This first minor point may be irrelevant, depending on how you fix
> the other problem below, but: tiny naming idea: rename unregister
> to either "should_unregister", or "mirror_snapshot_empty"...the
> latter helps show that this is stale information, once the lock is
> dropped.
First name make sense, second doesn't (at least to me), mirror is dead
at this point, it does not have any implication respective to snapshot
(the mm might still be very well alive and in active use).
> > up_write(&hmm->mirrors_sem);
> > +
> > + if (!unregister)
> > + return;
>
> Whee, here I am, lock-free, in the middle of a race condition
> window. :) Right here, someone (hmm_mirror_register) could be adding
> another mirror.
>
> It's not immediately clear to me what the best solution is.
> I'd be happier if we didn't have to drop one lock and take
> another like this, but if we do, then maybe rechecking that
> the list hasn't changed...safely, somehow, is a way forward here.
>
First i want to stress this is very unlikely race, it can only happens
if one hmm_mirror unregister and is last one while another new one try
to register. Highly unlikely but i am sending a v2 which fix that any-
way better safe than sorry. It makes the register side a bit ugly.
Cheers,
Jerome
next prev parent reply other threads:[~2018-03-21 18:12 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-20 2:00 [PATCH 00/15] hmm: fixes and documentations v3 jglisse
2018-03-20 2:00 ` [PATCH 01/15] mm/hmm: documentation editorial update to HMM documentation jglisse
2018-03-20 2:00 ` [PATCH 02/15] mm/hmm: fix header file if/else/endif maze v2 jglisse
2018-03-20 2:00 ` [PATCH 03/15] mm/hmm: HMM should have a callback before MM is destroyed v2 jglisse
2018-03-21 4:14 ` John Hubbard
2018-03-21 18:03 ` Jerome Glisse
2018-03-21 22:16 ` John Hubbard
2018-03-21 22:46 ` Jerome Glisse
2018-03-21 23:10 ` John Hubbard
2018-03-21 23:37 ` Jerome Glisse
2018-03-22 0:11 ` John Hubbard
2018-03-22 1:32 ` Jerome Glisse
2018-03-22 1:28 ` [PATCH 03/15] mm/hmm: HMM should have a callback before MM is destroyed v3 jglisse
2018-03-22 6:58 ` John Hubbard
2018-03-20 2:00 ` [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit jglisse
2018-03-21 4:24 ` John Hubbard
2018-03-21 18:12 ` Jerome Glisse [this message]
2018-03-21 18:16 ` [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v2 jglisse
2018-03-21 23:22 ` John Hubbard
2018-03-21 23:41 ` Jerome Glisse
2018-03-22 22:47 ` John Hubbard
2018-03-22 23:37 ` Jerome Glisse
2018-03-23 0:13 ` John Hubbard
2018-03-23 0:50 ` Jerome Glisse
2018-03-23 0:56 ` John Hubbard
2018-03-22 1:30 ` [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v3 jglisse
2018-03-22 22:36 ` Andrew Morton
2018-03-20 2:00 ` [PATCH 05/15] mm/hmm: hmm_pfns_bad() was accessing wrong struct jglisse
2018-03-20 2:00 ` [PATCH 06/15] mm/hmm: use struct for hmm_vma_fault(), hmm_vma_get_pfns() parameters v2 jglisse
2018-03-20 2:00 ` [PATCH 07/15] mm/hmm: remove HMM_PFN_READ flag and ignore peculiar architecture v2 jglisse
2018-03-20 2:00 ` [PATCH 08/15] mm/hmm: use uint64_t for HMM pfn instead of defining hmm_pfn_t to ulong v2 jglisse
2018-03-20 2:00 ` [PATCH 09/15] mm/hmm: cleanup special vma handling (VM_SPECIAL) jglisse
2018-03-20 2:00 ` [PATCH 10/15] mm/hmm: do not differentiate between empty entry or missing directory v2 jglisse
2018-03-21 5:24 ` John Hubbard
2018-03-21 14:48 ` Jerome Glisse
2018-03-21 23:16 ` John Hubbard
2018-03-20 2:00 ` [PATCH 11/15] mm/hmm: rename HMM_PFN_DEVICE_UNADDRESSABLE to HMM_PFN_DEVICE_PRIVATE jglisse
2018-03-20 2:00 ` [PATCH 12/15] mm/hmm: move hmm_pfns_clear() closer to where it is use jglisse
2018-03-20 2:00 ` [PATCH 13/15] mm/hmm: factor out pte and pmd handling to simplify hmm_vma_walk_pmd() jglisse
2018-03-21 5:07 ` John Hubbard
2018-03-21 15:08 ` Jerome Glisse
2018-03-21 22:36 ` John Hubbard
2018-03-20 2:00 ` [PATCH 14/15] mm/hmm: change hmm_vma_fault() to allow write fault on page basis jglisse
2018-03-20 2:00 ` [PATCH 15/15] mm/hmm: use device driver encoding for HMM pfn v2 jglisse
2018-03-21 4:39 ` John Hubbard
2018-03-21 15:52 ` Jerome Glisse
2018-03-21 23:19 ` John Hubbard
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180321181235.GF3214@redhat.com \
--to=jglisse@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=ebaskakov@nvidia.com \
--cc=jhubbard@nvidia.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhairgrove@nvidia.com \
--cc=rcampbell@nvidia.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).