From: Andrew Morton <akpm@linux-foundation.org>
To: Henry Burns <henryburns@google.com>
Cc: Shakeel Butt <shakeelb@google.com>,
Vitaly Wool <vitalywool@gmail.com>,
Vitaly Vul <vitaly.vul@sony.com>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
Xidong Wang <wangxidong_97@163.com>,
Jonathan Adams <jwadams@google.com>,
Linux MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()
Date: Tue, 2 Jul 2019 14:19:30 -0700 [thread overview]
Message-ID: <20190702141930.e31bf1c07a77514d976ef6e2@linux-foundation.org> (raw)
In-Reply-To: <CAGQXPTjU0xAWCLTWej8DdZ5TbH91m8GzeiCh5pMJLQajtUGu_g@mail.gmail.com>
On Mon, 1 Jul 2019 18:16:30 -0700 Henry Burns <henryburns@google.com> wrote:
> Cc: Vitaly Wool <vitalywool@gmail.com>, Vitaly Vul <vitaly.vul@sony.com>
Are these the same person?
> Subject: Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()
> Date: Mon, 1 Jul 2019 18:16:30 -0700
>
> On Mon, Jul 1, 2019 at 6:00 PM Shakeel Butt <shakeelb@google.com> wrote:
> >
> > On Mon, Jul 1, 2019 at 5:51 PM Henry Burns <henryburns@google.com> wrote:
> > >
> > > __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> > > lock the page. Following zsmalloc.c's example we call trylock_page() and
> > > unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> > > passed in locked, as documentation.
The changelog still doesn't mention that this bug triggers a
VM_BUG_ON_PAGE(). It should do so. I did this:
: __SetPageMovable() expects its page to be locked, but z3fold.c doesn't
: lock the page. This triggers the VM_BUG_ON_PAGE(!PageLocked(page), page)
: in __SetPageMovable().
:
: Following zsmalloc.c's example we call trylock_page() and unlock_page().
: Also make z3fold_page_migrate() assert that newpage is passed in locked,
: as per the documentation.
I'll add a cc:stable to this fix.
> > > Signed-off-by: Henry Burns <henryburns@google.com>
> > > Suggested-by: Vitaly Wool <vitalywool@gmail.com>
> > > ---
> > > Changelog since v1:
> > > - Added an if statement around WARN_ON(trylock_page(page)) to avoid
> > > unlocking a page locked by a someone else.
> > >
> > > mm/z3fold.c | 6 +++++-
> > > 1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/z3fold.c b/mm/z3fold.c
> > > index e174d1549734..6341435b9610 100644
> > > --- a/mm/z3fold.c
> > > +++ b/mm/z3fold.c
> > > @@ -918,7 +918,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
> > > set_bit(PAGE_HEADLESS, &page->private);
> > > goto headless;
> > > }
> > > - __SetPageMovable(page, pool->inode->i_mapping);
> > > + if (!WARN_ON(!trylock_page(page))) {
> > > + __SetPageMovable(page, pool->inode->i_mapping);
> > > + unlock_page(page);
> > > + }
> >
> > Can you please comment why lock_page() is not used here?
Shakeel asked "please comment" (ie, please add a code comment), not
"please comment on". Subtle ;)
> Since z3fold_alloc can be called in atomic or non atomic context,
> calling lock_page() could trigger a number of
> warnings about might_sleep() being called in atomic context. WARN_ON
> should avoid the problem described
> above as well, and in any weird condition where someone else has the
> page lock, we can avoid calling
> __SetPageMovable().
I think this will suffice:
--- a/mm/z3fold.c~mm-z3foldc-lock-z3fold-page-before-__setpagemovable-fix
+++ a/mm/z3fold.c
@@ -919,6 +919,9 @@ retry:
set_bit(PAGE_HEADLESS, &page->private);
goto headless;
}
+ /*
+ * z3fold_alloc() can be called from atomic contexts, hence the trylock
+ */
if (!WARN_ON(!trylock_page(page))) {
__SetPageMovable(page, pool->inode->i_mapping);
unlock_page(page);
However this code would be more effective if z3fold_alloc() were to be
told when it is running in non-atomic context so it can perform a
sleeping lock_page() in that case. That's an improvement to consider
for later, please.
next prev parent reply other threads:[~2019-07-02 21:19 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-02 0:51 [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable() Henry Burns
2019-07-02 1:00 ` Shakeel Butt
2019-07-02 1:16 ` Henry Burns
2019-07-02 21:19 ` Andrew Morton [this message]
2019-07-02 22:17 ` Henry Burns
2019-07-02 22:24 ` Andrew Morton
2019-07-03 5:53 ` Vitaly Wool
2019-07-03 16:39 ` Henry Burns
2019-07-03 5:54 ` Vitaly Wool
2019-07-02 9:53 ` Vitaly Wool
2019-07-02 18:58 ` David Rientjes
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=20190702141930.e31bf1c07a77514d976ef6e2@linux-foundation.org \
--to=akpm@linux-foundation.org \
--cc=henryburns@google.com \
--cc=jwadams@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=rppt@linux.vnet.ibm.com \
--cc=shakeelb@google.com \
--cc=vitaly.vul@sony.com \
--cc=vitalywool@gmail.com \
--cc=wangxidong_97@163.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).