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SubjectRe: [PATCH] mm: fix long time stall from mm_populate
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);

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