Re: [PATCH] mm: fix long time stall from mm_populate

[Minchan Kim]

On Wed, Feb 12, 2020 at 02:24:35PM -0800, Andrew Morton wrote:
> On Wed, 12 Feb 2020 11:53:22 -0800 Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 
> > > That's definitely wrong.  It'll clear PageReclaim and then pretend it did
> > > nothing wrong.
> > > 
> > > 	return !PageWriteback(page) ||
> > > 		test_and_clear_bit(PG_reclaim, &page->flags);
> > > 
> > 
> > Much better, Thanks for the review, Matthew!
> > If there is no objection, I will send two patches to Andrew.
> > One is PageReadahead strict, the other is limit retry from mm_populate.
> 
> With much more detailed changelogs, please!
> 
> This all seems rather screwy.  if a page is under writeback then it is
> uptodate and we should be able to fault it in immediately.

Hi Andrew,

This description in cover-letter will work? If so, I will add each part
below in each patch.

Subject: [PATCH 0/3] fixing mm_populate long stall

I got several reports major page fault takes several seconds sometime.
When I review drop mmap_sem in page fault hanlder, I found several bugs.

   CPU 1							CPU 2
mm_populate
 for ()
   ..
   ret = populate_vma_page_range
     __get_user_pages
       faultin_page
         handle_mm_fault
	   filemap_fault
	     do_async_mmap_readahead
	     						shrink_page_list
							  pageout
							    SetPageReclaim(=SetPageReadahead)
							      writepage
							        SetPageWriteback
	       if (PageReadahead(page))
	         maybe_unlock_mmap_for_io
		   up_read(mmap_sem)
		 page_cache_async_readahead()
		   if (PageWriteback(page))
		     return;

    here, since ret from populate_vma_page_range is zero,
    the loop continue to run with same address with previous
    iteration. It will repeat the loop until the page's
    writeout is done(ie, PG_writeback or PG_reclaim clear).

We could fix the above specific case via adding PageWriteback. IOW,

   ret = populate_vma_page_range
   	   ...
	   ...
	   filemap_fault
	     do_async_mmap_readahead
	       if (!PageWriteback(page) && PageReadahead(page))
	         maybe_unlock_mmap_for_io
		   up_read(mmap_sem)
		 page_cache_async_readahead()
		   if (PageWriteback(page))
		     return;

That's a thing [3/3] is fixing here. Even though it could fix the
problem effectively, it has still livelock problem theoretically
because the page of faulty address could be reclaimed and then
allocated/become readahead marker on other CPUs during faulty
process is retrying in mm_populate's loop. [2/3] is fixing the
such livelock via limiting retry count.
There is another hole for the livelock or hang of the process as well
as ageWriteback - ra_pages.

mm_populate
 for ()
   ..
   ret = populate_vma_page_range
     __get_user_pages
       faultin_page
         handle_mm_fault
	   filemap_fault
	     do_async_mmap_readahead
	       if (PageReadahead(page))
	         maybe_unlock_mmap_for_io
		   up_read(mmap_sem)
		 page_cache_async_readahead()
		   if (!ra->ra_pages)
		     return;

It will repeat the loop until ra->ra_pages become non-zero.
[1/3] is fixing the problem.

Jan Kara (1):
  mm: Don't bother dropping mmap_sem for zero size readahead

Minchan Kim (2):
  mm: fix long time stall from mm_populate
  mm: make PageReadahead more strict

 include/linux/page-flags.h | 28 ++++++++++++++++++++++++++--
 mm/filemap.c               |  2 +-
 mm/gup.c                   |  9 +++++++--
 mm/readahead.c             |  6 ------
 4 files changed, 34 insertions(+), 11 deletions(-)

-- 
2.25.0.225.g125e21ebc7-goog