From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 33F1240B383; Tue, 30 Jun 2026 12:28:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782822518; cv=none; b=OMddVGMjd33xJQ393BL8UOvb4GVOZdYlvMKs2TpqZwxNPA3ZNWfc7b91uX27k9N1QyR2k6HIrHYA0RkRZBgRLzZ/beH/3pyV95b/gBjAAqxmwy906R2LTJF9ZVtjRA2zlgtgsvNNG1QHWEoli72DMd+sZiSixI9MRTl6u4Nd+MY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782822518; c=relaxed/simple; bh=0m0RaBDGW+9+rDE7uYfNs/mB6EdTuUtIfUwrNB588rU=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Rz2XwR9X28ipY9GBPmvMYsmsQvrrfDra9b9rR3jDHWCfTa1vgEwww47xxCAyLlDDVHlgNLZFBsBP7IgoijpAnqlXdpNUTv71I4oKBeNZSzWYnyOZH86wfluzgNBVrao784Cd/bTcRyX0EEYc8zx4/scJiEMLf/Qq3GfAyboRI4s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HYnxVsz3; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="HYnxVsz3" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3B7DB1F000E9; Tue, 30 Jun 2026 12:28:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782822516; bh=8ePRxbyVbmmYLoDGDGNmJquf3XuqmjOsLFFsGxlIst8=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=HYnxVsz3zecpyH8x42mFPdL6rFQO1NJ2rtpDVZx5wapITaOSfO/NX5CF2DbghSooJ fxfM+i/5HeKm/Im4ximDqzb7NBiHbGSSWMN1i6XNSrBBmQt3sq/ctWhkUe+tNcAMvM 0BqiaMNievHRu/xnt8i8/SivEIShbJBvmZSTbVTiepmJBwrtmPkKz3dbWnMAMHZO5W fy1iYPo8Es9eSI9S4VP9HUh6MEszag5A2C+OTH97fMHvsmxXov2fMe0r6JdCK2HIr4 QFm/YEOkCRuRwe3aYbae16NjKaKzi2kDxRvQtAYGim1dXhK/6aWY2mYKcjVjslGm+/ 4xtqZjUw442VA== Message-ID: <2a2f3722ae7b822a48ec4ac3c641ecacc2ab052d.camel@kernel.org> Subject: Re: [PATCH] btrfs: don't let shrinker touch extent_maps that are being logged From: Jeff Layton To: Filipe Manana Cc: Chris Mason , David Sterba , Filipe Manana , Josef Bacik , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Tue, 30 Jun 2026 08:28:34 -0400 In-Reply-To: References: <20260629-btrfs-skip-logging-v1-1-4e3a28c1acaf@kernel.org> <5084d9c6b6a7025be156354194cf7d3fffebb0e2.camel@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Mon, 2026-06-29 at 16:47 +0100, Filipe Manana wrote: > On Mon, Jun 29, 2026 at 4:06=E2=80=AFPM Filipe Manana wrote: > >=20 > > On Mon, Jun 29, 2026 at 3:56=E2=80=AFPM Jeff Layton wrote: > > >=20 > > > On Mon, 2026-06-29 at 15:19 +0100, Filipe Manana wrote: > > > > On Mon, Jun 29, 2026 at 2:41=E2=80=AFPM Jeff Layton wrote: > > > > >=20 > > > > > The extent map shrinker can free an extent map that is still owne= d by an > > > > > in-flight fsync and still linked on the inode's modified_extents = list, > > > > > corrupting that list and eventually causing an RCU stall. > > > > >=20 > > > > > btrfs_scan_inode() currently skips EXTENT_FLAG_PINNED maps, then = calls > > > > > btrfs_remove_extent_mapping() followed by btrfs_free_extent_map()= : > > > > >=20 > > > > > if (em->flags & EXTENT_FLAG_PINNED) > > > > > goto next; > > > > > ... > > > > > btrfs_remove_extent_mapping(inode, em); > > > > > btrfs_free_extent_map(em); > > > > >=20 > > > > > But btrfs_remove_extent_mapping() deliberately does NOT unlink a = map that > > > > > has EXTENT_FLAG_LOGGING set: > > > > >=20 > > > > > if (!(em->flags & EXTENT_FLAG_LOGGING)) > > > > > list_del_init(&em->list); > > > > > remove_em(inode, em); > > > > >=20 > > > > > This sets up a UAF situation where a later fsync() can trip over = the > > > > > now-freed extent_map still on the modified_extents() list. > > > >=20 > > > > I don't see how that can happen, because fsync always takes the > > > > inode's i_mmap_lock (in write/exclusive mode), and btrfs_scan_inode= () > > > > takes the same lock too (in shared/read mode). > > > > EXTENT_FLAG_LOGGING is only set and cleared during fsync, so how ca= n > > > > the shrinker race with fsync? > > > >=20 > > >=20 > > > ...and yet, the vmcore shows that the shrinker hit an extent_map that > > > had LOGGING set while holding the i_mmap_lock. > >=20 > > Yes, but we need to understand why... > >=20 > > >=20 > > > I had hoped to add an assertion here that LOGGING couldn't be set on > > > the em's after taking the i_mmap_lock, but that's not viable: > > >=20 > > > The problem is that the i_mmap_lock coverage isn't complete. I had > > > Claude assist me with some analysis so take it with a hefty grain of > > > salt but the problem is that the em tree can contain em's from other > > > inodes where the i_mmap_lock is not held: > >=20 > > And that is the problem: an em tree, which belongs to an inode, should > > only have extent maps for that inode... > > An extent map can only belong to an inode. > >=20 > > So that's what needs investigation. The bug is elsewhere. > >=20 > > Thanks. > >=20 > > >=20 > > > -----------------------8<--------------------- > > >=20 > > > ...that inode is in progress, therefore no em in its tree should have= LOGGING set. Your invariant is the right one. > > >=20 > > > But it's violated, because the i_mmap_lock interlock is incomplete. = Two facts: > > >=20 > > > 1. tree-log.c never takes i_mmap_lock =E2=80=94 grep returns 0 occu= rrences. Only btrfs_sync_file takes it (file.c:1608/1610), and only > > > on the one inode being fsync'd. > > > 2. A single fsync logs many other inodes, and each can set LOGGING = on its own ems via btrfs_log_changed_extents(). >=20 > A single fsync can log other inodes yes, and without taking their > i_mmap_lock, but the inodes are logged in LOG_INODE_EXISTS mode, so > extents are not logged. > The only exception is for symlink inodes, but those are logged in full > mode, only have one extent (inlined), and always have > BTRFS_INODE_NEEDS_FULL_SYNC set, so we never end up in > btrfs_log_changed_extents() for symlinks. >=20 Thanks for the explanation. It sounds like we should wire some warnings into the shrinker around these invariants. Like, if LOGGING is set and the inode's not a symlink, do a WARN_ON_ONCE()? I'll plan to look into that as well. FWIW, internally at Meta, we're turning the WARN_ON() calls in btrfs_free_extent_map() into BUG() calls across a swath of machines in the hopes we can get a vmcore earlier. We'll keep you posted. Thanks for the help so far! > > > btrfs_log_inode() is called recursively on conflicting inodes, pare= nt directories, and referenced inodes: > > > tree-log.c:5982 btrfs_log_inode(di_inode, log_mode) # dir d= entries > > > tree-log.c:6339 btrfs_log_inode(inode, LOG_INODE_ALL) # confl= icting inodes > > > tree-log.c:6900 btrfs_log_inode(di_inode, log_mode) # new d= ir dentries > > > tree-log.c:7389 btrfs_log_inode(dir_inode, LOG_INODE_ALL) # log_a= ll_parents > > > tree-log.c:7488 btrfs_log_inode(inode, ...) # refer= enced inodes > > >=20 > > > 2. For all of these, btrfs_sync_file holds i_mmap_lock on the origi= nal fsync'd inode =E2=80=94 not on these side inodes. So a side > > > inode X gets LOGGING set on its ems while X's own i_mmap_lock is fr= ee. >=20 > Nop, see above. >=20 > > >=20 > > > So the shrinker can down_read_trylock(X->i_mmap_lock) successfully = (nobody holds it) and walk X's tree while X's ems are > > > LOGGING-flagged =E2=86=92 it removes+frees one =E2=86=92 corruption= . The interlock only protects the directly-fsync'd inode, not the inodes > > > logged as its dependencies. >=20 > Nop. Unless you can show an actual code path where while fsyncing one > inode we log another inode in LOG_INODE_ALL mode > Again, the exception is symlinks, but for those, the > BTRFS_INODE_NEEDS_FULL_SYNC flag is always set, so we never end up in > btrfs_log_changed_extents(). >=20 > > >=20 > > >=20 > > > > Also, there should be no () after modified_extents (it's not a func= tion name). > > > >=20 > > > >=20 > > >=20 > > > Sorry, early morning typo. Fixed in my local repo. > > >=20 > > > >=20 > > > > >=20 > > > > > Fix it by having the shrinker skip maps that are being logged, th= e same > > > > > way it skips pinned maps. Such a map is owned by the in-flight fs= ync and > > > > > will become reclaimable again once logging clears the flag. > > > > >=20 > > > > > Fixes: 956a17d9d050 ("btrfs: add a shrinker for extent maps") > > > > > Signed-off-by: Jeff Layton > > > > > --- > > > > > We've started hitting a number of these problems in our fleet. It > > > > > seems to mostly happen on ARM64 architecture, but there have been= some > > > > > WARN_ONs that popped on x86_64 too. > > > > > --- > > > > > fs/btrfs/extent_map.c | 8 +++++++- > > > > > 1 file changed, 7 insertions(+), 1 deletion(-) > > > > >=20 > > > > > diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c > > > > > index fce9c5cc0122..128f7800e101 100644 > > > > > --- a/fs/btrfs/extent_map.c > > > > > +++ b/fs/btrfs/extent_map.c > > > > > @@ -1166,7 +1166,13 @@ static long btrfs_scan_inode(struct btrfs_= inode *inode, struct btrfs_em_shrink_c > > > > > em =3D rb_entry(node, struct extent_map, rb_node)= ; > > > > > ctx->scanned++; > > > > >=20 > > > > > - if (em->flags & EXTENT_FLAG_PINNED) > > > > > + /* > > > > > + * Skip extent maps that are pinned or are being = logged. The > > > > > + * i_mmap_lock should prevent this from seeing LO= GGING on extent_maps > > > > > + * directly associated with inode, but em may be = associated with > > > > > + * other, dependent inodes and their locks are no= t held. > > > > > + */ > > > > > + if (em->flags & (EXTENT_FLAG_PINNED | EXTENT_FLAG= _LOGGING)) > > > > > goto next; > > > > >=20 > > > > > /* > > > > >=20 > > > > > --- > > > > > base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482 > > > > > change-id: 20260629-btrfs-skip-logging-3e31701d9647 > > > > >=20 > > > > > Best regards, > > > > > -- > > > > > Jeff Layton > > > > >=20 > > > > >=20 > > >=20 > > > -- > > > Jeff Layton --=20 Jeff Layton