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

[Yang Shi]

On Tue, Feb 11, 2020 at 9:28 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> 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.

Not sure if the below race in vmscan matters or not.

               if (PageWriteback(page)) {
                      [snip]
                        /* Case 2 above */
                        } else if (writeback_throttling_sane(sc) ||
                            !PageReclaim(page) || !may_enter_fs) {
                                /*
                                 * This is slightly racy - end_page_writeback()
                                 * might have just cleared PageReclaim, then
                                 * setting PageReclaim here end up interpreted
                                 * as PageReadahead - but that does not matter
                                 * enough to care.  What we do want is for this
                                 * page to have PageReclaim set next time memcg
                                 * reclaim reaches the tests above, so it will
                                 * then wait_on_page_writeback() to avoid OOM;
                                 * and it's also appropriate in global reclaim.
                                 */
                                SetPageReclaim(page);
                                stat->nr_writeback++;
                                goto activate_locked;


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