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;
>
>
