On Sat, Jul 5, 2025 at 12:58 PM Gao Xiang <hsiang...@linux.alibaba.com> wrote: > > Hi Amir, > > On 2025/7/5 16:25, Amir Goldstein wrote: > > ... > > > > > 2. When is it ok to use the backing_file helpers? > > > > The current patch allocates a struct file with > > alloc_file_pseudo() and not a struct backing_file, > > so using the backing_file helpers is going to be awkward and > > misleading and I think in this case it will we wize to refrain > > from using a local var name backing_file. > > > > The thing that you need to ask yourself is do you want > > backing_file_set_user_path() for the erofs shared inode. > > Yes, agreed, that should be improved as you said. > > > > > That means, what do you want users to see when they > > look at /proc/self/map_files symlinks. > > > > With the current patch set I believe they will see a symlink to > > something like "[erofs_pcs_f]" for any mapped file > > which is somewhat odd. > > Agreed. > > > > > I think it would have been nice if users saw something like > > "[erofs_pcs_md5digest:XXXXXXX]" > > But if we use `overlay.metacopy` for example, it's not > a string anymore. IOWs, I'd like to support binary > footprint too. > > And I tend to avoid md5 calcuation or something in > the kernel. The kernel just uses footprint xattrs > instead. > > > IMO, making the page cache dedupe visible in map_files is > > a very nice feature. > > > Overlayfs took the approach that users are expecting to see > > the actual path of the (non-anonymous) file that they mapped > > when looking at map_files symlinks. > > If you do not display the path to erofs mount in map_files, > > then lsof will not be able to blame a process with mapped files > > as the reason for keeping erofs mount busy. > > I think users should see `the actual path` rather than > "[erofs_pcs_xxxxx]" too, but I don't have any chance to > check this part yet. > > If it's possible for overlayfs, I guess it's possible for > erofs page cache sharing, maybe? >
Yes, I am sorry if that wasn't clear. If you want the map_files to show the user mapped file's path, you can use the backing_file helpers and more specifically backing_file_open() and all should work as in overlayfs. Obviously, you'd need to wrap the back_file helper with erofs specific logic, such as don't allow dio and such. Thanks, Amir.
