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

[Matthew Wilcox]

On Tue, Feb 11, 2020 at 08:34:04AM -0800, Minchan Kim wrote:
> On Tue, Feb 11, 2020 at 04:23:23AM -0800, Matthew Wilcox wrote:
> > 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 ...
> 
> I'm not sure it's right fix. Actually, I wanted to remove PageWriteback check
> in page_cache_async_readahead because I don't see corelation. Why couldn't we
> do readahead if the marker page is PG_readahead|PG_writeback design PoV?
> Only reason I can think of is it makes *a page* will be delayed for freeing
> since we removed PG_reclaim bit, which would be over-optimization for me.

You're confused.  Because we have a shortage of bits in the page flags,
we use the same bit for both PageReadahead and PageReclaim.  That doesn't
mean that a page marked as PageReclaim should be treated as PageReadahead.

> Other concern is isn't it's racy? IOW, page was !PG_writeback at the check below
> in your snippet but it was under PG_writeback in page_cache_async_readahead and
> then the IO was done before refault reaching the code again. It could be repeated
> *theoretically* even though it's very hard to happen in real practice.
> Thus, I think it would be better to remove PageWriteback check from
> page_cache_async_readahead if we really want to go the approach.

PageReclaim is always cleared before PageWriteback.  eg here:

void end_page_writeback(struct page *page)
...
        if (PageReclaim(page)) {
                ClearPageReclaim(page);
                rotate_reclaimable_page(page);
        }

        if (!test_clear_page_writeback(page))
                BUG();

so if PageWriteback is clear, PageReclaim must already be observable as clear.