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Wong" To: Brian Foster Cc: Dave Chinner , Matthew Wilcox , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , linux-mm@kvack.org Subject: Re: Multi-page folio issues in 5.19-rc4 (was [PATCH v3 25/25] xfs: Support large folios) Message-ID: References: <20220628073120.GI227878@dread.disaster.area> <20220628221757.GJ227878@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-7.5 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_PASS,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 01, 2022 at 12:03:23PM -0400, Brian Foster wrote: > On Wed, Jun 29, 2022 at 01:22:06PM -0700, Darrick J. Wong wrote: > > On Wed, Jun 29, 2022 at 08:57:30AM -0400, Brian Foster wrote: > > > On Tue, Jun 28, 2022 at 04:21:55PM -0700, Darrick J. Wong wrote: > > > > On Wed, Jun 29, 2022 at 08:17:57AM +1000, Dave Chinner wrote: > > > > > On Tue, Jun 28, 2022 at 02:18:24PM +0100, Matthew Wilcox wrote: > > > > > > On Tue, Jun 28, 2022 at 12:31:55PM +0100, Matthew Wilcox wrote: > > > > > > > On Tue, Jun 28, 2022 at 12:27:40PM +0100, Matthew Wilcox wrote: > > > > > > > > On Tue, Jun 28, 2022 at 05:31:20PM +1000, Dave Chinner wrote: > > > > > > > > > So using this technique, I've discovered that there's a dirty page > > > > > > > > > accounting leak that eventually results in fsx hanging in > > > > > > > > > balance_dirty_pages(). > > > > > > > > > > > > > > > > Alas, I think this is only an accounting error, and not related to > > > > > > > > the problem(s) that Darrick & Zorro are seeing. I think what you're > > > > > > > > seeing is dirty pages being dropped at truncation without the > > > > > > > > appropriate accounting. ie this should be the fix: > > > > > > > > > > > > > > Argh, try one that actually compiles. > > > > > > > > > > > > ... that one's going to underflow the accounting. Maybe I shouldn't > > > > > > be writing code at 6am? > > > > > > > > > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > > > > > index f7248002dad9..4eec6ee83e44 100644 > > > > > > --- a/mm/huge_memory.c > > > > > > +++ b/mm/huge_memory.c > > > > > > @@ -18,6 +18,7 @@ > > > > > > #include > > > > > > #include > > > > > > #include > > > > > > +#include > > > > > > #include > > > > > > #include > > > > > > #include > > > > > > @@ -2439,11 +2440,15 @@ static void __split_huge_page(struct page *page, struct list_head *list, > > > > > > __split_huge_page_tail(head, i, lruvec, list); > > > > > > /* Some pages can be beyond EOF: drop them from page cache */ > > > > > > if (head[i].index >= end) { > > > > > > - ClearPageDirty(head + i); > > > > > > - __delete_from_page_cache(head + i, NULL); > > > > > > + struct folio *tail = page_folio(head + i); > > > > > > + > > > > > > if (shmem_mapping(head->mapping)) > > > > > > shmem_uncharge(head->mapping->host, 1); > > > > > > - put_page(head + i); > > > > > > + else if (folio_test_clear_dirty(tail)) > > > > > > + folio_account_cleaned(tail, > > > > > > + inode_to_wb(folio->mapping->host)); > > > > > > + __filemap_remove_folio(tail, NULL); > > > > > > + folio_put(tail); > > > > > > } else if (!PageAnon(page)) { > > > > > > __xa_store(&head->mapping->i_pages, head[i].index, > > > > > > head + i, 0); > > > > > > > > > > > > > > > > Yup, that fixes the leak. > > > > > > > > > > Tested-by: Dave Chinner > > > > > > > > Four hours of generic/522 running is long enough to conclude that this > > > > is likely the fix for my problem and migrate long soak testing to my > > > > main g/522 rig and: > > > > > > > > Tested-by: Darrick J. Wong > > > > > > > > > > Just based on Willy's earlier comment.. what I would probably be a > > > little careful/curious about here is whether the accounting fix leads to > > > an indirect behavior change that does impact reproducibility of the > > > corruption problem. For example, does artificially escalated dirty page > > > tracking lead to increased reclaim/writeback activity than might > > > otherwise occur, and thus contend with the fs workload? Clearly it has > > > some impact based on Dave's balance_dirty_pages() problem reproducer, > > > but I don't know if it extends beyond that off the top of my head. That > > > might make some sense if the workload is fsx, since that doesn't > > > typically stress cache/memory usage the way a large fsstress workload or > > > something might. > > > > > > So for example, interesting questions might be... Do your corruption > > > events happen to correspond with dirty page accounting crossing some > > > threshold based on available memory in your test environment? Does > > > reducing available memory affect reproducibility? Etc. > > > > Yeah, I wonder that too now. I managed to trace generic/522 a couple of > > times before willy's patch dropped. From what I could tell, a large > > folio X would get page P assigned to the fsx file's page cache to cover > > range R, dirtied, and written to disk. At some point later, we'd > > reflink into part of the file range adjacent to P, but not P itself. > > I /think/ that should have caused the whole folio to get invalidated? > > > > Then some more things happened (none of which dirtied R, according to > > fsx) and then suddenly writeback would trigger on some page (don't know > > which) that would write to the disk blocks backing R. I'm fairly sure > > that's where the incorrect disk contents came from. > > > > Next, we'd reflink part of the file range including R into a different > > part of the file (call it R2). fsx would read R2, bringing a new page > > into cache, and it wouldn't match the fsxgood buffer, leading to fsx > > aborting. > > > > After a umount/mount cycle, reading R and R2 would both reveal the > > incorrect contents that had caused fsx to abort. > > > > FWIW, I hadn't been able to reproduce this in my default environment to > this point. With the memory leak issue in the light, I was eventually > able to by reducing dirty_bytes to something the system would be more > likely to hit sooner (i.e. 16-32MB), but I also see stalling behavior > and whatnot due to the leak that requires backing off from the specified > dirty limit every so often. > > If I apply the accounting patch to avoid the leak and set > dirty_background_bytes to something notably aggressive (1kB), the test > survived 100 iterations or so before I stopped it. If I then set > dirty_bytes to something similarly aggressive (1MB), I hit the failure > on the next iteration (assuming it's the same problem). It's spinning > again at ~25 or so iterations without a failure so far, so I'd have to > wait and see how reliable the reproducer really is. Though if it doesn't > reoccur soonish, perhaps I'll try reducing dirty_bytes a bit more... > > My suspicion based on these characteristics would be that the blocking > limit triggers more aggressive reclaim/invalidation, and thus helps > detect the problem sooner. If reflink is involved purely as a cache > invalidation step (i.e. so a subsequent read will hit the disk and > detect a cache inconsistency), then it might be interesting to see if it > can still be reproduced without reflink operations enabled but instead > with some combination of the -f/-X fsx flags to perform more flush > invals and on-disk data checks.. Hm. I didn't try -f or lowering dirty_bytes, but with the reflink operations disabled, g522 ran for 3 hours before I gave up and killed it. I would've thought that the fallocate zero/collapse/insert range functions (which use the same flush/unmap helper) would have sufficed to make the problem happen, but ... it didn't. I think I'll try changing dirty_bytes next, to see if I can reproduce the problem that way. I'm not surprised that you had to set dirty_bytes to 1MB, since 522 is only ever creating a 600K file anyway. (Hopefully willy will be back next week to help us shed some light on this.) --D > Brian > > > Unfortunately the second ftrace attempt ate some trace data, so I was > > unable to figure out if the same thing happened again. > > > > At this point I really need to get on reviewing patches for 5.20, so > > I'll try to keep poking at this (examining the trace data requires a lot > > of concentration which isn't really possible while sawzall construction > > is going on at home) but at worst I can ask Linus to merge a patch for > > 5.19 final that makes setting mapping_set_large_folio a > > Kconfig/CONFIG_XFS_DEBUG option. > > > > --D > > > > > > > > Brian > > > > > > > --D > > > > > > > > > Cheers, > > > > > > > > > > Dave. > > > > > -- > > > > > Dave Chinner > > > > > david@fromorbit.com > > > > > > > > > >