Commit 6dbf7bb5 authored Sep 04, 2020 by Jan Kara Committed by Jens Axboe Sep 07, 2020

fs: Don't invalidate page buffers in block_write_full_page()



If block_write_full_page() is called for a page that is beyond current
inode size, it will truncate page buffers for the page and return 0.
This logic has been added in 2.5.62 in commit 81eb69062588 ("fix ext3
BUG due to race with truncate") in history.git tree to fix a problem
with ext3 in data=ordered mode. This particular problem doesn't exist
anymore because ext3 is long gone and ext4 handles ordered data
differently. Also normally buffers are invalidated by truncate code and
there's no need to specially handle this in ->writepage() code.

This invalidation of page buffers in block_write_full_page() is causing
issues to filesystems (e.g. ext4 or ocfs2) when block device is shrunk
under filesystem's hands and metadata buffers get discarded while being
tracked by the journalling layer. Although it is obviously "not
supported" it can cause kernel crashes like:

[ 7986.689400] BUG: unable to handle kernel NULL pointer dereference at
+0000000000000008
[ 7986.697197] PGD 0 P4D 0
[ 7986.699724] Oops: 0002 [#1] SMP PTI
[ 7986.703200] CPU: 4 PID: 203778 Comm: jbd2/dm-3-8 Kdump: loaded Tainted: G
+O     --------- -  - 4.18.0-147.5.0.5.h126.eulerosv2r9.x86_64 #1
[ 7986.716438] Hardware name: Huawei RH2288H V3/BC11HGSA0, BIOS 1.57 08/11/2015
[ 7986.723462] RIP: 0010:jbd2_journal_grab_journal_head+0x1b/0x40 [jbd2]
...
[ 7986.810150] Call Trace:
[ 7986.812595]  __jbd2_journal_insert_checkpoint+0x23/0x70 [jbd2]
[ 7986.818408]  jbd2_journal_commit_transaction+0x155f/0x1b60 [jbd2]
[ 7986.836467]  kjournald2+0xbd/0x270 [jbd2]

which is not great. The crash happens because bh->b_private is suddently
NULL although BH_JBD flag is still set (this is because
block_invalidatepage() cleared BH_Mapped flag and subsequent bh lookup
found buffer without BH_Mapped set, called init_page_buffers() which has
rewritten bh->b_private). So just remove the invalidation in
block_write_full_page().

Note that the buffer cache invalidation when block device changes size
is already careful to avoid similar problems by using
invalidate_mapping_pages() which skips busy buffers so it was only this
odd block_write_full_page() behavior that could tear down bdev buffers
under filesystem's hands.

Reported-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
CC: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent b445547e

fs/buffer.c

+0 −16

Original line number	Diff line number	Diff line
		@@ -2771,16 +2771,6 @@ int nobh_writepage(struct page page, get_block_t get_block,
		/* Is the page fully outside i_size? (truncate in progress) */
		offset = i_size & (PAGE_SIZE-1);
		if (page->index >= end_index+1 \|\| !offset) {
		/*
		* The page may have dirty, unmapped buffers. For example,
		* they may have been added in ext3_writepage(). Make them
		* freeable here, so the page does not leak.
		*/
		#if 0
		/* Not really sure about this - do we need this ? */
		if (page->mapping->a_ops->invalidatepage)
		page->mapping->a_ops->invalidatepage(page, offset);
		#endif
		unlock_page(page);
		return 0; /* don't care */
		}
		@@ -2975,12 +2965,6 @@ int block_write_full_page(struct page page, get_block_t get_block,
		/* Is the page fully outside i_size? (truncate in progress) */
		offset = i_size & (PAGE_SIZE-1);
		if (page->index >= end_index+1 \|\| !offset) {
		/*
		* The page may have dirty, unmapped buffers. For example,
		* they may have been added in ext3_writepage(). Make them
		* freeable here, so the page does not leak.
		*/
		do_invalidatepage(page, 0, PAGE_SIZE);
		unlock_page(page);
		return 0; /* don't care */
		}

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