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Message-ID: <49d5b109-7cc3-6717-b3c6-6858310aa3ba@huawei.com>
Date: Mon, 15 Apr 2024 12:28:01 +0800
From: Baokun Li <libaokun1@...wei.com>
To: Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>, Theodore
Ts'o <tytso@....edu>
CC: <linux-ext4@...r.kernel.org>, <adilger.kernel@...ger.ca>,
<ritesh.list@...il.com>, <linux-kernel@...r.kernel.org>,
<jun.nie@...aro.org>, <ebiggers@...nel.org>, <yi.zhang@...wei.com>,
<yangerkun@...wei.com>, <yukuai3@...wei.com>,
<syzbot+a158d886ca08a3fecca4@...kaller.appspotmail.com>, Baokun Li
<libaokun1@...wei.com>
Subject: Re: [PATCH v2] ext4: fix race condition between buffer write and
page_mkwrite
On 2023/6/5 23:08, Jan Kara wrote:
> On Mon 05-06-23 15:55:35, Matthew Wilcox wrote:
>> On Mon, Jun 05, 2023 at 02:21:41PM +0200, Jan Kara wrote:
>>> On Mon 05-06-23 11:16:55, Jan Kara wrote:
>>>> Yeah, I agree, that is also the conclusion I have arrived at when thinking
>>>> about this problem now. We should be able to just remove the conversion
>>>> from ext4_page_mkwrite() and rely on write(2) or truncate(2) doing it when
>>>> growing i_size.
>>> OK, thinking more about this and searching through the history, I've
>>> realized why the conversion is originally in ext4_page_mkwrite(). The
>>> problem is described in commit 7b4cc9787fe35b ("ext4: evict inline data
>>> when writing to memory map") but essentially it boils down to the fact that
>>> ext4 writeback code does not expect dirty page for a file with inline data
>>> because ext4_write_inline_data_end() should have copied the data into the
>>> inode and cleared the folio's dirty flag.
>>>
>>> Indeed messing with xattrs from the writeback path to copy page contents
>>> into inline data xattr would be ... interesting. Hum, out of good ideas for
>>> now :-|.
>> Is it so bad? Now that we don't have writepage in ext4, only
>> writepages, it seems like we have a considerably more benign locking
>> environment to work in.
> Well, yes, without ->writepage() it might be *possible*. But still rather
> ugly. The problem is that in ->writepages() i_size is not stable. Thus also
> whether the inode data is inline or not is not stable. We'd need inode_lock
> for that but that is not easily doable in the writeback path - inode lock
> would then become fs_reclaim unsafe...
>
> Honza
Hi Honza!
Hi Ted!
Hi Matthew!
Long time later came back to this, because while discussing another similar
ABBA problem with Hou Tao, he mentioned VM_FAULT_RETRY, and then I
thought that this could be used to solve this problem as well.
The general idea is that if we see a file with inline data in
ext4_page_mkwrite(),
we release the mmap_lock and grab the inode_lock to convert the inline data,
and then return VM_FAULT_RETRY to retry to get the mmap_lock.
The code implementation is as follows, do you have any thoughts?
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 537803250ca9..e044c11c9cf6 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -6056,15 +6056,36 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
if (unlikely(IS_IMMUTABLE(inode)))
return VM_FAULT_SIGBUS;
+ /*
+ * The ext4 writeback code does not expect dirty page for a file with
+ * inline data, so inline data needs to be converted. But it needs to
+ * hold the inode_lock when converting, and trying to lock the inode
+ * while holding the mmap_lock may result in an ABBA deadlock. So here
+ * we release the mmap_lock for conversion and retry after conversion.
+ * Only one retry is allowed to avoid endless loops.
+ * Acquire xattr_sem to avoid race with inline data conversion.
+ */
+ down_read(&EXT4_I(inode)->xattr_sem);
+ if (ext4_has_inline_data(inode)) {
+ up_read(&EXT4_I(inode)->xattr_sem);
+
+ if (!fault_flag_allow_retry_first(vmf->flags))
+ return VM_FAULT_SIGBUS;
+
+ release_fault_lock(vmf);
+
+ inode_lock(inode);
+ ext4_convert_inline_data(inode);
+ inode_unlock(inode);
+ return VM_FAULT_RETRY;
+ }
+ up_read(&EXT4_I(inode)->xattr_sem);
+
sb_start_pagefault(inode->i_sb);
file_update_time(vma->vm_file);
filemap_invalidate_lock_shared(mapping);
- err = ext4_convert_inline_data(inode);
- if (err)
- goto out_ret;
-
/*
* On data journalling we skip straight to the transaction handle:
* there's no delalloc; page truncated will be checked later; the
--
With Best Regards,
Baokun Li
.
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