From: Jan Kara <jack@suse.cz>
To: Josef Bacik <josef@toxicpanda.com>
Cc: Jan Kara <jack@suse.cz>,
snazy@snazy.de, 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, 6 Nov 2019 16:25:03 +0100 [thread overview]
Message-ID: <20191106152503.GA12685@quack2.suse.cz> (raw)
In-Reply-To: <20191106151429.swqtq2dt4uelhjzn@macbook-pro-91.dhcp.thefacebook.com>
On Wed 06-11-19 10:14:31, 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:
> > > On Wed, Nov 06, 2019 at 02:45:43PM +0100, Robert Stupp wrote:
> > > > On Wed, 2019-11-06 at 13:03 +0100, Jan Kara wrote:
> > > > > On Tue 05-11-19 13:22:11, Johannes Weiner wrote:
> > > > > > What I don't quite understand yet is why the fault path doesn't
> > > > > > make
> > > > > > progress eventually. We must drop the mmap_sem without changing the
> > > > > > state in any way. How can we keep looping on the same page?
> > > > >
> > > > > That may be a slight suboptimality with Josef's patches. If the page
> > > > > is marked as PageReadahead, we always drop mmap_sem if we can and
> > > > > start
> > > > > readahead without checking whether that makes sense or not in
> > > > > do_async_mmap_readahead(). OTOH page_cache_async_readahead() then
> > > > > clears
> > > > > PageReadahead so the only way how I can see we could loop like this
> > > > > is when
> > > > > file->ra->ra_pages is 0. Not sure if that's what's happening through.
> > > > > We'd
> > > > > need to find which of the paths in filemap_fault() calls
> > > > > maybe_unlock_mmap_for_io() to tell more.
> > > >
> > > > Yes, ra_pages==0
> > > > Attached the dmesg + smaps outputs
> > > >
> > > >
> > >
> > > Ah ok I see what's happening, __get_user_pages() returns 0 if we get an EBUSY
> > > from faultin_page, and then __mm_populate does nend = nstart + ret * PAGE_SIZE,
> > > which just leaves us where we are.
> > >
> > > We need to handle the non-blocking and the locking separately in __mm_populate
> > > so we know what's going on. Jan's fix for the readahead thing is definitely
> > > valid as well, but this will keep us from looping forever in other retry cases.
> >
> > 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?
Yes, this should do it. But then I'm not sure if using
__get_user_pages_locked() instead of __get_user_pages() in
populate_vma_page_range() isn't a better option. I guess I'll defer that
decision to mm people to pick their poison ;)...
Honza
> 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;
> }
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2019-11-06 15:25 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <4576b336-66e6-e2bb-cd6a-51300ed74ab8@snazy.de>
2019-10-24 23:34 ` mlockall(MCL_CURRENT) blocking infinitely Randy Dunlap
2019-10-25 9:21 ` Michal Hocko
2019-10-25 11:02 ` Robert Stupp
2019-10-25 11:46 ` Michal Hocko
2019-10-25 11:50 ` Michal Hocko
2019-10-25 11:59 ` Robert Stupp
2019-10-25 13:19 ` Robert Stupp
2019-10-25 11:55 ` Robert Stupp
2019-10-25 12:05 ` Michal Hocko
2019-10-25 12:11 ` Michal Hocko
2019-10-25 13:10 ` Robert Stupp
2019-10-25 13:27 ` Michal Hocko
2019-10-25 13:45 ` Robert Stupp
2019-10-25 13:57 ` Michal Hocko
2019-10-25 14:00 ` Michal Hocko
2019-10-25 15:58 ` Robert Stupp
2019-11-05 13:23 ` Robert Stupp
2019-11-05 15:28 ` Vlastimil Babka
2019-11-05 18:22 ` Johannes Weiner
2019-11-05 20:05 ` Robert Stupp
2019-11-06 10:25 ` Robert Stupp
2019-11-06 11:26 ` Robert Stupp
2019-11-06 12:04 ` Jan Kara
2019-11-06 12:24 ` Robert Stupp
2019-11-06 12:03 ` Jan Kara
2019-11-06 13:45 ` Robert Stupp
2019-11-06 14:35 ` Jan Kara
2019-11-06 15:32 ` Robert Stupp
2019-11-06 14:38 ` Jan Kara
2019-11-06 14:56 ` Josef Bacik
2019-11-06 15:05 ` Jan Kara
2019-11-06 15:14 ` Josef Bacik
2019-11-06 15:25 ` Jan Kara [this message]
2019-11-06 16:39 ` Robert Stupp
2019-11-06 17:03 ` Robert Stupp
2019-11-06 17:25 ` Jan Kara
2019-11-07 8:08 ` Robert Stupp
2019-11-20 12:42 ` Robert Stupp
2019-10-25 13:55 ` Robert Stupp
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