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Date: Sat, 16 Jul 2022 11:00:46 +0800
From: Zhihao Cheng <chengzhihao1@...wei.com>
To: Jan Kara <jack@...e.cz>, Ted Tso <tytso@....edu>
CC: <linux-ext4@...r.kernel.org>,
Ritesh Harjani <ritesh.list@...il.com>,
<stable@...r.kernel.org>
Subject: Re: [PATCH 05/10] ext4: Fix race when reusing xattr blocks
在 2022/7/12 18:54, Jan Kara 写道:
Hi Jan, one little question confuses me:
> When ext4_xattr_block_set() decides to remove xattr block the following
> race can happen:
>
> CPU1 CPU2
> ext4_xattr_block_set() ext4_xattr_release_block()
> new_bh = ext4_xattr_block_cache_find()
>
> lock_buffer(bh);
> ref = le32_to_cpu(BHDR(bh)->h_refcount);
> if (ref == 1) {
> ...
> mb_cache_entry_delete();
> unlock_buffer(bh);
> ext4_free_blocks();
> ...
> ext4_forget(..., bh, ...);
> jbd2_journal_revoke(..., bh);
>
> ext4_journal_get_write_access(..., new_bh, ...)
> do_get_write_access()
> jbd2_journal_cancel_revoke(..., new_bh);
>
> Later the code in ext4_xattr_block_set() finds out the block got freed
> and cancels reusal of the block but the revoke stays canceled and so in
> case of block reuse and journal replay the filesystem can get corrupted.
> If the race works out slightly differently, we can also hit assertions
> in the jbd2 code.
>
> Fix the problem by making sure that once matching mbcache entry is
> found, code dropping the last xattr block reference (or trying to modify
> xattr block in place) waits until the mbcache entry reference is
> dropped. This way code trying to reuse xattr block is protected from
> someone trying to drop the last reference to xattr block.
>
> Reported-and-tested-by: Ritesh Harjani <ritesh.list@...il.com>
> CC: stable@...r.kernel.org
> Fixes: 82939d7999df ("ext4: convert to mbcache2")
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
> fs/ext4/xattr.c | 67 +++++++++++++++++++++++++++++++++----------------
> 1 file changed, 45 insertions(+), 22 deletions(-)
>
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index aadfae53d055..3a0928c8720e 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -439,9 +439,16 @@ static int ext4_xattr_inode_iget(struct inode *parent, unsigned long ea_ino,
> /* Remove entry from mbcache when EA inode is getting evicted */
> void ext4_evict_ea_inode(struct inode *inode)
> {
> - if (EA_INODE_CACHE(inode))
> - mb_cache_entry_delete(EA_INODE_CACHE(inode),
> - ext4_xattr_inode_get_hash(inode), inode->i_ino);
> + struct mb_cache_entry *oe;
> +
> + if (!EA_INODE_CACHE(inode))
> + return;
> + /* Wait for entry to get unused so that we can remove it */
> + while ((oe = mb_cache_entry_delete_or_get(EA_INODE_CACHE(inode),
> + ext4_xattr_inode_get_hash(inode), inode->i_ino))) {
> + mb_cache_entry_wait_unused(oe);
> + mb_cache_entry_put(EA_INODE_CACHE(inode), oe);
> + }
> }
>
> static int
> @@ -1229,6 +1236,7 @@ ext4_xattr_release_block(handle_t *handle, struct inode *inode,
> if (error)
> goto out;
>
> +retry_ref:
> lock_buffer(bh);
> hash = le32_to_cpu(BHDR(bh)->h_hash);
> ref = le32_to_cpu(BHDR(bh)->h_refcount);
> @@ -1238,9 +1246,18 @@ ext4_xattr_release_block(handle_t *handle, struct inode *inode,
> * This must happen under buffer lock for
> * ext4_xattr_block_set() to reliably detect freed block
> */
> - if (ea_block_cache)
> - mb_cache_entry_delete(ea_block_cache, hash,
> - bh->b_blocknr);
> + if (ea_block_cache) {
> + struct mb_cache_entry *oe;
> +
> + oe = mb_cache_entry_delete_or_get(ea_block_cache, hash,
> + bh->b_blocknr);
> + if (oe) {
> + unlock_buffer(bh);
> + mb_cache_entry_wait_unused(oe);
> + mb_cache_entry_put(ea_block_cache, oe);
> + goto retry_ref;
> + }
> + }
> get_bh(bh);
> unlock_buffer(bh);
>
> @@ -1867,9 +1884,20 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
> * ext4_xattr_block_set() to reliably detect modified
> * block
> */
> - if (ea_block_cache)
> - mb_cache_entry_delete(ea_block_cache, hash,
> - bs->bh->b_blocknr);
> + if (ea_block_cache) {
> + struct mb_cache_entry *oe;
> +
> + oe = mb_cache_entry_delete_or_get(ea_block_cache,
> + hash, bs->bh->b_blocknr);
> + if (oe) {
> + /*
> + * Xattr block is getting reused. Leave
> + * it alone.
> + */
> + mb_cache_entry_put(ea_block_cache, oe);
> + goto clone_block;
> + }
> + }
> ea_bdebug(bs->bh, "modifying in-place");
> error = ext4_xattr_set_entry(i, s, handle, inode,
> true /* is_block */);
> @@ -1885,6 +1913,7 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
> goto cleanup;
> goto inserted;
> }
> +clone_block:
> unlock_buffer(bs->bh);
> ea_bdebug(bs->bh, "cloning");
> s->base = kmalloc(bs->bh->b_size, GFP_NOFS);
> @@ -1991,18 +2020,13 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
> lock_buffer(new_bh);
> /*
> * We have to be careful about races with
> - * freeing, rehashing or adding references to
> - * xattr block. Once we hold buffer lock xattr
> - * block's state is stable so we can check
> - * whether the block got freed / rehashed or
> - * not. Since we unhash mbcache entry under
> - * buffer lock when freeing / rehashing xattr
> - * block, checking whether entry is still
> - * hashed is reliable. Same rules hold for
> - * e_reusable handling.
> + * adding references to xattr block. Once we
> + * hold buffer lock xattr block's state is
> + * stable so we can check the additional
> + * reference fits.
> */
> - if (hlist_bl_unhashed(&ce->e_hash_list) ||
> - !ce->e_reusable) {
> + ref = le32_to_cpu(BHDR(new_bh)->h_refcount) + 1;
> + if (ref > EXT4_XATTR_REFCOUNT_MAX) {
So far, we have mb_cache_entry_delete_or_get() and
mb_cache_entry_wait_unused(), so used cache entry cannot be concurrently
removed. Removing check 'hlist_bl_unhashed(&ce->e_hash_list)' is okay.
What's affect of changing the other two checks 'ref >=
EXT4_XATTR_REFCOUNT_MAX' and '!ce->e_reusable'? To make code more
obvious? eg. To point out the condition of 'ref <=
EXT4_XATTR_REFCOUNT_MAX' rather than 'ce->e_reusable', we have checked
'ce->e_reusable' in ext4_xattr_block_cache_find() before?
> /*
> * Undo everything and check mbcache
> * again.
> @@ -2017,9 +2041,8 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
> new_bh = NULL;
> goto inserted;
> }
> - ref = le32_to_cpu(BHDR(new_bh)->h_refcount) + 1;
> BHDR(new_bh)->h_refcount = cpu_to_le32(ref);
> - if (ref >= EXT4_XATTR_REFCOUNT_MAX)
> + if (ref == EXT4_XATTR_REFCOUNT_MAX)
> ce->e_reusable = 0;
> ea_bdebug(new_bh, "reusing; refcount now=%d",
> ref);
>
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