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Thu, 30 May 2024 07:26:27 +0200 Message-ID: Date: Thu, 30 May 2024 14:56:19 +0930 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: 6.9/BUG: Bad page state in process kswapd0 pfn:d6e840 From: Qu Wenruo To: David Hildenbrand , Mikhail Gavrilov , Chris Mason , Josef Bacik , David Sterba Cc: Linux List Kernel Mailing , Linux Memory Management List , Matthew Wilcox , linux-btrfs References: <0672f0b7-36f5-4322-80e6-2da0f24c101b@redhat.com> <6b42ad9a-1f15-439a-8a42-34052fec017e@redhat.com> <162cb2a8-1b53-4e86-8d49-f4e09b3255a4@redhat.com> <209ff705-fe6e-4d6d-9d08-201afba7d74b@redhat.com> Content-Language: en-US Autocrypt: addr=quwenruo.btrfs@gmx.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNIlF1IFdlbnJ1byA8cXV3ZW5ydW8uYnRyZnNAZ214LmNvbT7CwJQEEwEIAD4CGwMFCwkI BwIGFQgJCgsCBBYCAwECHgECF4AWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCY00iVQUJDToH pgAKCRDCPZHzoSX+qNKACACkjDLzCvcFuDlgqCiS4ajHAo6twGra3uGgY2klo3S4JespWifr BLPPak74oOShqNZ8yWzB1Bkz1u93Ifx3c3H0r2vLWrImoP5eQdymVqMWmDAq+sV1Koyt8gXQ XPD2jQCrfR9nUuV1F3Z4Lgo+6I5LjuXBVEayFdz/VYK63+YLEAlSowCF72Lkz06TmaI0XMyj jgRNGM2MRgfxbprCcsgUypaDfmhY2nrhIzPUICURfp9t/65+/PLlV4nYs+DtSwPyNjkPX72+ LdyIdY+BqS8cZbPG5spCyJIlZonADojLDYQq4QnufARU51zyVjzTXMg5gAttDZwTH+8LbNI4 mm2YzsBNBFnVga8BCACqU+th4Esy/c8BnvliFAjAfpzhI1wH76FD1MJPmAhA3DnX5JDORcga CbPEwhLj1xlwTgpeT+QfDmGJ5B5BlrrQFZVE1fChEjiJvyiSAO4yQPkrPVYTI7Xj34FnscPj /IrRUUka68MlHxPtFnAHr25VIuOS41lmYKYNwPNLRz9Ik6DmeTG3WJO2BQRNvXA0pXrJH1fN GSsRb+pKEKHKtL1803x71zQxCwLh+zLP1iXHVM5j8gX9zqupigQR/Cel2XPS44zWcDW8r7B0 q1eW4Jrv0x19p4P923voqn+joIAostyNTUjCeSrUdKth9jcdlam9X2DziA/DHDFfS5eq4fEv ABEBAAHCwHwEGAEIACYCGwwWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCY00ibgUJDToHvwAK CRDCPZHzoSX+qK6vB/9yyZlsS+ijtsvwYDjGA2WhVhN07Xa5SBBvGCAycyGGzSMkOJcOtUUf tD+ADyrLbLuVSfRN1ke738UojphwkSFj4t9scG5A+U8GgOZtrlYOsY2+cG3R5vjoXUgXMP37 INfWh0KbJodf0G48xouesn08cbfUdlphSMXujCA8y5TcNyRuNv2q5Nizl8sKhUZzh4BascoK DChBuznBsucCTAGrwPgG4/ul6HnWE8DipMKvkV9ob1xJS2W4WJRPp6QdVrBWJ9cCdtpR6GbL iQi22uZXoSPv/0oUrGU+U5X4IvdnvT+8viPzszL5wXswJZfqfy8tmHM85yjObVdIG6AlnrrD In-Reply-To: Content-Type: text/plain; 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Is that >> BTRFS_BTREE_INODE_OBJECTID? >> >> Summarizing what we know so far: >> (1) Freeing an order-0 btrfs folio where folio->mapping >> =C2=A0=C2=A0=C2=A0=C2=A0 is still set >> (2) Triggered by kswapd and kcompactd; not triggered by other means of >> =C2=A0=C2=A0=C2=A0=C2=A0 page freeing so far > > From the implementation of filemap_migrate_folio() (and previous > migrate_page_moving_mapping()), it looks like the migration only involve= s: > > - Migrate the mapping > - Copy the page private value > - Copy the contents (if needed) > - Copy all the page flags > > The most recent touch on migration is from v6.0, which I do not believe > is the cause at all. > >> >> Possible theories: >> (A) folio->mapping not cleared when freeing the folio. But shouldn't >> =C2=A0=C2=A0=C2=A0=C2=A0 this also happen on other freeing paths? Or ar= e we simply lucky to >> =C2=A0=C2=A0=C2=A0=C2=A0 never trigger that for that folio? > > Yeah, in fact we never manually clean folio->mapping inside btrfs, thus > I'm not sure if it's the case. > >> (B) Messed-up refcounting: freeing a folio that is still in use (and >> =C2=A0=C2=A0=C2=A0=C2=A0 therefore has folio-> mapping still set) >> >> I was briefly wondering if large folio splitting could be involved. > > Although we have all the metadata support for larger folios, we do not > yet enable it. After some extra code digging and tons of trace_printk(), it indeed looks like btrfs is underflowing the folio ref count. During the lifespan of an extent buffer (btrfs' metadata), it should at least has 3 refs after attached to the address space: 1) folio_alloc() inside btrfs_alloc_folio_array() 2) folio_ref_add() inside __filemap_add_folio() 3) folio_add_lru() inside filemap_add_folio() Even if btrfs wants to release the folio of an eb, we only do: - Detach the folio::private - put_folio() So even if an eb got released, as long as it is not yet detached from filemap, its refcount should still be >=3D 2. Thus the warning is indeed correct, by somehow btrfs called extra put_folio() on the eb page which is already attached to the btree inode. I'll continue digging around the eb folio refs inside btrfs, meanwhile I will also test some extra checks for eb folios on their refcount. Thanks, Qu