On 2026/1/16 23:36, Christoph Hellwig wrote:
Sorry, just getting to this from my overful inbox by now.
On Fri, Jan 16, 2026 at 09:55:41AM +0000, Hongbo Li wrote:
2.1. file open & close
----------------------
When the file is opened, the ->private_data field of file A or file B is
set to point to an internal deduplicated file. When the actual read
occurs, the page cache of this deduplicated file will be accessed.
So the first opener wins and others point to it? That would lead to
some really annoying life time rules. Or you allocate a hidden backing
file and have everyone point to it (the backing_file related subject
kinda hints at that), which would be much more sensible, but then the
above descriptions would not be correct.
Your latter thought is correct, I think the words above
are ambiguous.
When the file is opened, if the corresponding erofs inode is newly
created, then perform the following actions:
1. add the erofs inode to the backing list of the deduplicated inode;
2. increase the reference count of the deduplicated inode.
This on the other hand suggests the fist opener is used approach again?
Not quite sure about this part, assuming you read the
patches, it's just similar to the backing_file approach.
Assuming the deduplication inode's page cache is PGCache_dedup, there
What is PGCache_dedup?
Maybe it's just an outdated expression from the older versions
from Hongzhen. I think just ignore this part.
Iomap and the layers below will involve disk I/O operations. As
described in 2.1, the deduplicated inode itself is not bound to a
specific device. The deduplicated inode will select an erofs inode from
the backing list (by default, the first one) to complete the
corresponding iomap operation.
What happens for mmap I/O where folio->mapping is kinda important?
`folio->mapping` will just get the anon inode, but
(meta)data I/Os will submit to one of the real
filesystem (that is why a real inode is needed to
pass into iomap), and use the data to fill the
anon inode page cache, and the anon inode is like
backing_file, and vma->vm_file will point to the
hidden backing file backed by the anon inode .
Thanks,
Gao Xiang
Also do you have a git tree for the whole feature?
