linux-kernel - Re: [PATCH v7 6/7] mm/madvise: employ mmget_still

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Message-ID: <20200306130303.kztv64f52qknxb6k@butterfly.localdomain>
Date:   Fri, 6 Mar 2020 14:03:03 +0100
From:   Oleksandr Natalenko <oleksandr@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, linux-api@...r.kernel.org,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Dias <joaodias@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jann Horn <jannh@...gle.com>,
        alexander.h.duyck@...ux.intel.com, sj38.park@...il.com
Subject: Re: [PATCH v7 6/7] mm/madvise: employ mmget_still_valid for write
 lock

Hello.

On Fri, Mar 06, 2020 at 01:52:07PM +0100, Vlastimil Babka wrote:
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index e794367f681e..e77c6c1fad34 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1118,6 +1118,8 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
> >  	if (write) {
> >  		if (down_write_killable(&mm->mmap_sem))
> >  			return -EINTR;
> > +		if (current->mm != mm && !mmget_still_valid(mm))
> > +			goto skip_mm;
> 
> This will return 0, is that correct? Shoudln't there be a similar error e.g. as
> when finding the task by pid fails (-ESRCH ?), because IIUC the task here is
> going away and dumping the core?

Yeah.

Something like this then:

===
diff --git a/mm/madvise.c b/mm/madvise.c
index 48d1da08c160..7ed2f4d13924 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1122,6 +1122,10 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
 	if (write) {
 		if (down_write_killable(&mm->mmap_sem))
 			return -EINTR;
+		if (current->mm != mm && !mmget_still_valid(mm)) {
+			error = -ESRCH;
+			goto skip_mm;
+		}
 	} else {
 		down_read(&mm->mmap_sem);
 	}
@@ -1173,6 +1177,7 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
 	}
 out:
 	blk_finish_plug(&plug);
+skip_mm:
 	if (write)
 		up_write(&mm->mmap_sem);
 	else

===

?

> 
> >  	} else {
> >  		down_read(&mm->mmap_sem);
> >  	}
> > @@ -1169,6 +1171,7 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
> >  	}
> >  out:
> >  	blk_finish_plug(&plug);
> > +skip_mm:
> >  	if (write)
> >  		up_write(&mm->mmap_sem);
> >  	else
> > 
> 

-- 
  Best regards,
    Oleksandr Natalenko (post-factum)
    Principal Software Maintenance Engineer