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

[Minchan Kim <minchan@kernel.org>]

On Mon, Feb 10, 2020 at 07:54:12PM -0800, Matthew Wilcox wrote:
> On Mon, Feb 10, 2020 at 07:50:04PM -0800, Minchan Kim wrote:
> > On Mon, Feb 10, 2020 at 05:10:21PM -0800, Matthew Wilcox wrote:
> > > On Mon, Feb 10, 2020 at 04:19:58PM -0800, Minchan Kim wrote:
> > > >       filemap_fault
> > > >         find a page form page(PG_uptodate|PG_readahead|PG_writeback)
> > > 
> > > Uh ... That shouldn't be possible.
> > 
> > Please see shrink_page_list. Vmscan uses PG_reclaim to accelerate
> > page reclaim when the writeback is done so the page will have both
> > flags at the same time and the PG reclaim could be regarded as
> > PG_readahead in fault conext.
> 
> What part of fault context can make that mistake?  The snippet I quoted
> below is from page_cache_async_readahead() where it will clearly not
> make that mistake.  There's a lot of code here; please don't presume I
> know all the areas you're talking about.

Sorry about being not clear. I am saying  filemap_fault ->
do_async_mmap_readahead

Let's assume the page is hit in page cache and vmf->flags is !FAULT_FLAG
TRIED so it calls do_async_mmap_readahead. Since the page has PG_reclaim
and PG_writeback by shrink_page_list, it goes to 

do_async_mmap_readahead
  if (PageReadahead(page))
    fpin = maybe_unlock_mmap_for_io();
    page_cache_async_readahead
      if (PageWriteback(page))
        return;
      ClearPageReadahead(page); <- doesn't reach here until the writeback is clear
      
So, mm_populate will repeat the loop until the writeback is done.
It's my just theory but didn't comfirm it by the testing.
If I miss something clear, let me know it.

Thanks!

> 
> > > 
> > >         /*
> > >          * Same bit is used for PG_readahead and PG_reclaim.
> > >          */
> > >         if (PageWriteback(page))
> > >                 return;
> > > 
> > >         ClearPageReadahead(page);