linux-kernel - Re: [PATCH v2 1/5] mm: factor out madvise's core functionality

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Message-ID: <20200117181413.GC140922@google.com>
Date:   Fri, 17 Jan 2020 10:14:13 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, linux-api@...r.kernel.org,
        oleksandr@...hat.com, 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>, christian.brauner@...ntu.com,
        sjpark@...zon.de, "Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v2 1/5] mm: factor out madvise's core functionality

On Fri, Jan 17, 2020 at 01:02:34PM +0300, Kirill Tkhai wrote:
> On 17.01.2020 02:59, Minchan Kim wrote:
> > This patch factor out madvise's core functionality so that upcoming
> > patch can reuse it without duplication. It shouldn't change any behavior.
> > 
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > ---
> >  mm/madvise.c | 194 +++++++++++++++++++++++++++++----------------------
> >  1 file changed, 111 insertions(+), 83 deletions(-)
> > 
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index bcdb6a042787..0c901de531e4 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -35,6 +35,7 @@
> >  struct madvise_walk_private {
> >  	struct mmu_gather *tlb;
> >  	bool pageout;
> > +	struct task_struct *task;
> >  };
> >  
> >  /*
> > @@ -306,12 +307,13 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >  	bool pageout = private->pageout;
> >  	struct mm_struct *mm = tlb->mm;
> >  	struct vm_area_struct *vma = walk->vma;
> > +	struct task_struct *task = private->task;
> >  	pte_t *orig_pte, *pte, ptent;
> >  	spinlock_t *ptl;
> >  	struct page *page = NULL;
> >  	LIST_HEAD(page_list);
> >  
> > -	if (fatal_signal_pending(current))
> > +	if (fatal_signal_pending(task))
> >  		return -EINTR;
> 
> This EINTR may confuse userspace. Users will think the syscall was interrupted,
> and it may be restarted, but this is not true.

madvise_[pageout|cold] doesn't propagate the error to userspace.

> 
> What we care here? Current task received fatal signal, while walk_page_range(..&cold_walk_ops..)
> is a long cycle. So, this check allows to break the cycle faster.
> 
> Iteration over remote task's mm may also be long, and we still may need to break
> it if current received a signal.
> 
> So, we'd better left fatal_signal_pending(current) here.
> 
> Maybe we need both tasks fatal_signal_pending() checks and different retvals here,
> but it's up to you.

Yub, let's check both processes here to bail out.

Thanks for the review!