linux-kernel - Re: [PATCH] mm: fix long time stall from mm

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Message-ID: <20200211122323.GS8731@bombadil.infradead.org>
Date:   Tue, 11 Feb 2020 04:23:23 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        Josef Bacik <josef@...icpanda.com>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fix long time stall from mm_populate

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);