Re: [PATCH v2] erofs: support splice() in inode_share mode

[Chao Yu <chao@kernel.org>]

On 8/21/26 09:56, Zhan Xusheng wrote:
> From: Zhan Xusheng <zhanxusheng1024@gmail.com>
> 
> From: Zhan Xusheng <zhanxusheng@xiaomi.com>
> 
> erofs_ishare_fops routes everything that touches the page cache to the
> backing file in ->private_data: read_iter clones the iocb onto it, mmap
> does vma_set_file(), fadvise calls vfs_fadvise() on it.  splice_read was
> left as filemap_splice_read(), which works on the user file's own mapping:
> it does init_sync_kiocb(&iocb, in), and filemap_get_pages() then takes
> iocb->ki_filp->f_mapping.
> 
> So splice() and sendfile() fill the per-inode page cache rather than the
> shared one.  The content is the same either way, since erofs_fill_inode()
> sets a_ops on that mapping too, which is why this went unnoticed.  Two
> identical 8 MiB files under inode_share, reading one of them with splice(2)
> alone, in pages:
> 
>    before   own 2048   shared 0
>    after    own 0      shared 2048
> 
> "own" is cachestat(fd), which reports the file's own mapping; "shared" is
> mmap()+mincore(), which erofs_ishare_mmap() redirects to the backing file.
> 
> Read through the backing file, as read_iter already does.
> 
> Link: https://lore.kernel.org/all/b7dc7192-d586-45a2-bc4a-b41dc681c9bb@linux.alibaba.com/
> Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>

Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,