Re: mlockall(MCL_CURRENT) blocking infinitely

From: Robert Stupp <snazy@gmx.de>
To: Josef Bacik <josef@toxicpanda.com>, Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Michal Hocko <mhocko@kernel.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Randy Dunlap <rdunlap@infradead.org>,
	linux-kernel@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Potyra, Stefan" <Stefan.Potyra@elektrobit.com>
Subject: Re: mlockall(MCL_CURRENT) blocking infinitely
Date: Wed, 06 Nov 2019 17:39:35 +0100	[thread overview]
Message-ID: <9f6b707c69ceb34e3916b1d47f2e2fa6a4f025ab.camel@gmx.de> (raw)
In-Reply-To: <20191106151429.swqtq2dt4uelhjzn@macbook-pro-91.dhcp.thefacebook.com>

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On Wed, 2019-11-06 at 10:14 -0500, Josef Bacik wrote:
> On Wed, Nov 06, 2019 at 04:05:24PM +0100, Jan Kara wrote:
> > On Wed 06-11-19 09:56:09, Josef Bacik wrote:
> > I don't think this will work. AFAICS faultin_page() just checks
> > whether
> > 'nonblocking' is != NULL but doesn't ever look at its value...
> > Honestly the
> > whole interface is rather weird like lots of things around gup().
> >
>
> Oh what the hell, yeah this is super bonkers.  The whole fault path
> probably
> should be cleaned up to handle retry better.  This will do the trick
> I think?

I tried the patch, and it seems to fix the `mlockall(MCL_CURRENT)`
issue for "my test.c".
However, shutdown & reboot are still broken - i.e. the console says
"Reached target reboot" and "hangs forever". Shutdown & reboot work
with __get_user_pages_locked(). No clue what the difference there is.

Anyway, I've captured three smaps outputs: from 5.0.21, from
5.3.9+"nonblocking patch", from 5.3.9+"__get_user_pages_locked". All
three look okay to me - although I don't completely understand why some
areas are not locked (-> "Locked: 0kB") - but "Locked" is always equal
to "Pss", so I assume that's fine?

>
> Josef
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 8f236a335ae9..2468789298e6 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -628,7 +628,7 @@ static int faultin_page(struct task_struct *tsk,
> struct vm_area_struct *vma,
>  		fault_flags |= FAULT_FLAG_WRITE;
>  	if (*flags & FOLL_REMOTE)
>  		fault_flags |= FAULT_FLAG_REMOTE;
> -	if (nonblocking)
> +	if (nonblocking && *nonblocking != 0)
>  		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
>  	if (*flags & FOLL_NOWAIT)
>  		fault_flags |= FAULT_FLAG_ALLOW_RETRY |
> FAULT_FLAG_RETRY_NOWAIT;
> @@ -1237,6 +1237,7 @@ int __mm_populate(unsigned long start, unsigned
> long len, int ignore_errors)
>  	unsigned long end, nstart, nend;
>  	struct vm_area_struct *vma = NULL;
>  	int locked = 0;
> +	int nonblocking = 1;
>  	long ret = 0;
>
>  	end = start + len;
> @@ -1268,7 +1269,7 @@ int __mm_populate(unsigned long start, unsigned
> long len, int ignore_errors)
>  		 * double checks the vma flags, so that it won't mlock
> pages
>  		 * if the vma was already munlocked.
>  		 */
> -		ret = populate_vma_page_range(vma, nstart, nend,
> &locked);
> +		ret = populate_vma_page_range(vma, nstart, nend,
> &nonblocking);
>  		if (ret < 0) {
>  			if (ignore_errors) {
>  				ret = 0;
> @@ -1276,6 +1277,14 @@ int __mm_populate(unsigned long start,
> unsigned long len, int ignore_errors)
>  			}
>  			break;
>  		}
> +
> +		/*
> +		 * We dropped the mmap_sem, so we need to re-lock, and
> the next
> +		 * loop around we won't drop because nonblocking is now
> 0.
> +		 */
> +		if (!nonblocking)
> +			locked = 0;
> +
>  		nend = nstart + ret * PAGE_SIZE;
>  		ret = 0;
>  	}

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