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Tue, 11 Feb 2020 10:15:14 -0800 (PST) MIME-Version: 1.0 References: <20200211001958.170261-1-minchan@kernel.org> <20200211011021.GP8731@bombadil.infradead.org> <20200211035004.GA242563@google.com> <20200211035412.GR8731@bombadil.infradead.org> <20200211042536.GB242563@google.com> <20200211122323.GS8731@bombadil.infradead.org> <20200211163404.GC242563@google.com> <20200211172803.GA7778@bombadil.infradead.org> In-Reply-To: <20200211172803.GA7778@bombadil.infradead.org> From: Yang Shi Date: Tue, 11 Feb 2020 10:14:58 -0800 Message-ID: Subject: Re: [PATCH] mm: fix long time stall from mm_populate To: Matthew Wilcox Cc: Minchan Kim , Andrew Morton , linux-mm , Josef Bacik , Johannes Weiner , Jan Kara , LKML Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Feb 11, 2020 at 9:28 AM Matthew Wilcox 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; > >