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

[Matthew Wilcox]

On Mon, Feb 10, 2020 at 08:25:36PM -0800, Minchan Kim wrote:
> 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.

Ah!  Surely the right way to fix this is ...

+++ b/mm/filemap.c
@@ -2420,7 +2420,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
                return fpin;
        if (ra->mmap_miss > 0)
                ra->mmap_miss--;
-       if (PageReadahead(page)) {
+       if (!PageWriteback(page) && PageReadahead(page)) {
                fpin = maybe_unlock_mmap_for_io(vmf, fpin);
                page_cache_async_readahead(mapping, ra, file,
                                           page, offset, ra->ra_pages);