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Message-ID: <ZPendrb8gSbAC6fM@casper.infradead.org> Date: Tue, 5 Sep 2023 23:11:02 +0100 From: Matthew Wilcox <willy@...radead.org> To: Theodore Ts'o <tytso@....edu> Cc: Zorro Lang <zlang@...nel.org>, linux-ext4@...r.kernel.org, fstests@...r.kernel.org, regressions@...ts.linux.dev, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: [fstests generic/388, 455, 475, 482 ...] Ext4 journal recovery test fails On Mon, Sep 04, 2023 at 02:08:19AM -0400, Theodore Ts'o wrote: > #regzbot introduced: 8147c4c4546f9f05ef03bb839b741473b28bb560 ^ > > OK, I've isolated the regression of generic/455 failing with ext4/1k > to this commit, which came in via the mm tree. Nothing seems > *obviously* wrong, but I'm not sure if there are any differences in > the semantics of the new folio functions such as kmap_local_folio, > offset_in_folio, set_folio_bh() which might be making a difference. Thanks for the cc, Let's see what we can do ... virt_to_folio() - For an order-0 page, there is no difference. offset_in_folio() - Ditto bh->b_page vs bh->b_folio - Ditto virt_to_folio() - Ditto folio_set_bh() - Ditto kmap_local_folio() vs kmap_atomic - Here, we have a difference. memcpy_from_folio() - Same difference as above. I suppose it must be this, and yet I cannot understand how it would make a difference. Perhaps you can help me? static inline void *kmap_atomic_prot(struct page *page, pgprot_t prot) { if (IS_ENABLED(CONFIG_PREEMPT_RT)) migrate_disable(); else preempt_disable(); pagefault_disable(); return __kmap_local_page_prot(page, prot); } vs static inline void *kmap_local_folio(struct folio *folio, size_t offset) { struct page *page = folio_page(folio, offset / PAGE_SIZE); return __kmap_local_page_prot(page, kmap_prot) + offset % PAGE_SIZE; } I don't believe that returning the address with the offset included is the problem here. It must be disabling preemption / migration. There's no chace this funcation accesses userspace (... is there?) so it can't be the pagefault_disable(). We can try splitting this up into tiny commits and figuring out which of them is the problem. I'll be back at work tomorrow and can look more deeply then. > Using kvm-xfstests[1] I bisected this via the command: > > % install-kconfig ; kbuild ; kvm-xfstests -c ext4/1k -C 10 generic/455 > > [1] https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md > > > And the bisection pointed me at this commit: > > commit 8147c4c4546f9f05ef03bb839b741473b28bb560 (refs/bisect/bad) > Author: Matthew Wilcox (Oracle) <willy@...radead.org> > AuthorDate: Thu Jul 13 04:55:11 2023 +0100 > Commit: Andrew Morton <akpm@...ux-foundation.org> > CommitDate: Fri Aug 18 10:12:30 2023 -0700 > > jbd2: use a folio in jbd2_journal_write_metadata_buffer() > > During the bisection, I treated a commit with 3+ failures as "bad", > and 0-2 commits as "good". Running generic/455 50 times to get a > sense of the failure, with the first bad commit (8147c4c4546f), I got: > > ext4/1k: 50 tests, 21 failures, 223 seconds > Flaky: generic/455: 42% (21/50) > Totals: 50 tests, 0 skipped, 21 failures, 0 errors, 223s > > While with the immediately preceding commit (07811230c3cd), I got: > > ext4/1k: 50 tests, 4 failures, 235 seconds > Flaky: generic/455: 8% (4/50) > Totals: 50 tests, 0 skipped, 4 failures, 0 errors, 235s > > > > Comparing these two commits (8147c4c4546f vs 07811230c3cd) using the > ext4 with a 4k block size, I get: > > ext4/4k: 50 tests, 2 failures, 365 seconds > Flaky: generic/455: 4% (2/50) > Totals: 50 tests, 0 skipped, 2 failures, 0 errors, 365s > > vs > > ext4/4k: 50 tests, 2 failures, 349 seconds > Flaky: generic/455: 4% (2/50) > Totals: 50 tests, 0 skipped, 2 failures, 0 errors, 349s > > So issue seems to be specifically with a sub-page size block size, > since ext4/4k doesn't show any issues, while ext4/1k does. I doubt I tried it with a 1kB block size, so I'll focus on that too.
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