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Wed, 12 Feb 2020 09:40:16 -0800 (PST) Date: Wed, 12 Feb 2020 09:40:15 -0800 From: Minchan Kim To: Jan Kara Cc: Matthew Wilcox , Andrew Morton , linux-mm , Josef Bacik , Johannes Weiner , LKML Subject: Re: [PATCH] mm: fix long time stall from mm_populate Message-ID: <20200212174015.GB93795@google.com> 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> <20200211175731.GA185752@google.com> <20200212101804.GD25573@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200212101804.GD25573@quack2.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Jan, On Wed, Feb 12, 2020 at 11:18:04AM +0100, Jan Kara wrote: > On Tue 11-02-20 09:57:31, Minchan Kim wrote: > > On Tue, Feb 11, 2020 at 09:28:03AM -0800, 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. > > > > My point is why we couldn't do readahead if the marker page is under PG_writeback. > > Well, as far as I'm reading the code, this shouldn't usually happen - > PageReadahead is set on a page that the preread into memory. Once it is > used for the first time (either by page fault or normal read), readahead > logic triggers starting further readahead and PageReadahead gets cleared. > > What could happen though is that the page gets written into (say just a few > bytes). That would keep PageReadahead set although the page will become > dirty and can later be written back. I don't find not triggering writeback in > this case too serious though since it should be very rare. Please see pageout in vmscan.c which set PG_reclaim righ before calling writepage. Since PG_reclaim and PG_readahead shares the same bit of the page flags, do_async_mmap_readahead will decipher the PG_reclaim as PageReadahead so it releases mmap but page_cache_async_readahead just returns by PageWriteback check. It will be repeated until the writeback is done. > > So I'd be OK with the change Matthew suggested although I'd prefer if we > had this magic "!PageWriteback && PageReadahead" test in some helper > function (like page_should_trigger_readahead()?) with a good comment explaining > the details. How about this? diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 1bf83c8fcaa7..d07d602476df 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -363,8 +363,28 @@ PAGEFLAG(MappedToDisk, mappedtodisk, PF_NO_TAIL) /* PG_readahead is only used for reads; PG_reclaim is only for writes */ PAGEFLAG(Reclaim, reclaim, PF_NO_TAIL) TESTCLEARFLAG(Reclaim, reclaim, PF_NO_TAIL) -PAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND) - TESTCLEARFLAG(Readahead, reclaim, PF_NO_COMPOUND) + +SETPAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND) +CLEARPAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND) + +/* + * Since PG_readahead is shared with PG_reclaim of the page flags, + * PageReadahead should double check whether it's readahead marker + * or PG_reclaim. It could be done by PageWriteback check because + * PG_reclaim is always with PG_writeback. + */ +static inline int PageReadahead(struct page *page) +{ + VM_BUG_ON_PGFLAGS(PageCompound(page), page); + return test_bit(PG_reclaim, &(page)->flags) && !PageWriteback(page); +} + +static inline int TestClearPageReadahead(struct page *page) +{ + VM_BUG_ON_PGFLAGS(PageCompound(page), page); + + return test_and_clear_bit(PG_reclaim, &page->flags) && !PageWriteback(page); +} #ifdef CONFIG_HIGHMEM /* diff --git a/mm/readahead.c b/mm/readahead.c index 2fe72cd29b47..85b15e5a1d7b 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -553,12 +553,6 @@ page_cache_async_readahead(struct address_space *mapping, if (!ra->ra_pages) return; - /* - * Same bit is used for PG_readahead and PG_reclaim. - */ - if (PageWriteback(page)) - return; - ClearPageReadahead(page); /* -- 2.25.0.225.g125e21ebc7-goog > > > It was there for a long time and you were adding one more so I was curious what's > > reasoning comes from. Let me find why PageWriteback check in > > page_cache_async_readahead from the beginning. > > > > fe3cba17c4947, mm: share PG_readahead and PG_reclaim > > > > The reason comes from the description > > > > b) clear PG_readahead => implicit clear of PG_reclaim > > one(and only one) page will not be reclaimed in time > > it can be avoided by checking PageWriteback(page) in readahead first > > > > The goal was to avoid delay freeing of the page by clearing PG_reclaim. > > I'm saying that I feel it's over optimization. IOW, it would be okay to > > lose a page to be accelerated reclaim. > > > > > > > > > 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. > > > > > > > I'm saying live lock siutation below. > > It would be hard to trigger since IO is very slow but isn't it possible > > theoretically? > > > > > > CPU 1 CPU 2 > > mm_populate > > 1st trial > > __get_user_pages > > handle_mm_fault > > filemap_fault > > do_async_mmap_readahead > > if (!PageWriteback(page) && PageReadahead(page)) { > > fpin = maybe_unlock_mmap_for_io > > page_cache_async_readahead > > set_page_writeback here > > if (PageWriteback(page)) > > return; <- hit > > > > writeback completed and reclaimed the page > > .. > > ondemand readahead allocates new page and mark it to PG_readahead > > 2nd trial > > __get_user_pages > > handle_mm_fault > > filemap_fault > > do_async_mmap_readahead > > if (!PageWriteback(page) && PageReadahead(page)) { > > fpin = maybe_unlock_mmap_for_io > > page_cache_async_readahead > > set_page_writeback here > > if (PageWriteback(page)) > > return; <- hit > > > > writeback completed and reclaimed the page > > .. > > ondemand readahead allocates new page and mark it to PG_readahead > > > > 3rd trial > > .. > > > > > > Let's consider ra_pages, too as I mentioned. Isn't it another hole to make > > such live lock if other task suddenly reset it to zero? > > > > void page_cache_async_readahead(..) > > { > > /* no read-ahead */ > > if (!ra->ra_pages) > > return; > > So this is definitively a bug which was already reported previously. I've > just sent out a patch to fix this which has somehow fallen through the > cracks. > > Now I agree that regardless of this fix and the fix Matthew has proposed, > mm_populate() would benefit from being more robust like you suggested so > I'll check that separately (but I'm no expert in that area). True because I think we couldn't prevent live lock as I wrote above. Thanks for the review, Jan!