Re: mlockall(MCL_CURRENT) blocking infinitely

[Jan Kara]

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().

								Honza

> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 8f236a335ae9..ac625805d569 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -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@xxxxxxxx>
SUSE Labs, CR