Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Mon, 15 Apr 2024 12:28:01 +0800 | Subject | Re: [PATCH v2] ext4: fix race condition between buffer write and page_mkwrite | From | Baokun Li <> |
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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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