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SubjectRe: [PATCH] mm: fix long time stall from mm_populate
On Wed, Feb 12, 2020 at 02:24:35PM -0800, Andrew Morton wrote:
> On Wed, 12 Feb 2020 11:53:22 -0800 Minchan Kim <minchan@kernel.org> wrote:
>
> > > That's definitely wrong. It'll clear PageReclaim and then pretend it did
> > > nothing wrong.
> > >
> > > return !PageWriteback(page) ||
> > > test_and_clear_bit(PG_reclaim, &page->flags);
> > >
> >
> > Much better, Thanks for the review, Matthew!
> > If there is no objection, I will send two patches to Andrew.
> > One is PageReadahead strict, the other is limit retry from mm_populate.
>
> With much more detailed changelogs, please!
>
> This all seems rather screwy. if a page is under writeback then it is
> uptodate and we should be able to fault it in immediately.

Hi Andrew,

This description in cover-letter will work? If so, I will add each part
below in each patch.

Subject: [PATCH 0/3] fixing mm_populate long stall

I got several reports major page fault takes several seconds sometime.
When I review drop mmap_sem in page fault hanlder, I found several bugs.

CPU 1 CPU 2
mm_populate
for ()
..
ret = populate_vma_page_range
__get_user_pages
faultin_page
handle_mm_fault
filemap_fault
do_async_mmap_readahead
shrink_page_list
pageout
SetPageReclaim(=SetPageReadahead)
writepage
SetPageWriteback
if (PageReadahead(page))
maybe_unlock_mmap_for_io
up_read(mmap_sem)
page_cache_async_readahead()
if (PageWriteback(page))
return;

here, since ret from populate_vma_page_range is zero,
the loop continue to run with same address with previous
iteration. It will repeat the loop until the page's
writeout is done(ie, PG_writeback or PG_reclaim clear).

We could fix the above specific case via adding PageWriteback. IOW,

ret = populate_vma_page_range
...
...
filemap_fault
do_async_mmap_readahead
if (!PageWriteback(page) && PageReadahead(page))
maybe_unlock_mmap_for_io
up_read(mmap_sem)
page_cache_async_readahead()
if (PageWriteback(page))
return;

That's a thing [3/3] is fixing here. Even though it could fix the
problem effectively, it has still livelock problem theoretically
because the page of faulty address could be reclaimed and then
allocated/become readahead marker on other CPUs during faulty
process is retrying in mm_populate's loop. [2/3] is fixing the
such livelock via limiting retry count.
There is another hole for the livelock or hang of the process as well
as ageWriteback - ra_pages.

mm_populate
for ()
..
ret = populate_vma_page_range
__get_user_pages
faultin_page
handle_mm_fault
filemap_fault
do_async_mmap_readahead
if (PageReadahead(page))
maybe_unlock_mmap_for_io
up_read(mmap_sem)
page_cache_async_readahead()
if (!ra->ra_pages)
return;

It will repeat the loop until ra->ra_pages become non-zero.
[1/3] is fixing the problem.

Jan Kara (1):
mm: Don't bother dropping mmap_sem for zero size readahead

Minchan Kim (2):
mm: fix long time stall from mm_populate
mm: make PageReadahead more strict

include/linux/page-flags.h | 28 ++++++++++++++++++++++++++--
mm/filemap.c | 2 +-
mm/gup.c | 9 +++++++--
mm/readahead.c | 6 ------
4 files changed, 34 insertions(+), 11 deletions(-)

--
2.25.0.225.g125e21ebc7-goog

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