On Thu, Jul 19, 2018 at 10:09 AM, David Howells <dhowells@xxxxxxxxxx> wrote: > Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote: > >> Stacking file operations in overlay will store an extra open file for each >> overlay file opened. >> >> The overhead is just that of "struct file" which is about 256bytes, because >> overlay already pins an extra dentry and inode when the file is open, which >> add up to a much larger overhead. >> >> For fear of breaking working setups, don't start accounting the extra file. > > Sounds useful for cachefiles too, though Christoph Hellwig objected strongly > last time I tried this, so you might want to check with him directly. What the cachefiles use case would be? Overlayfs wants the "shadow" file open only as long as the real file is open. Is the cachefiles case the same? For overlays the real cost of pinned memory by open files is already sunk in the layered dentry references. Which means overlayfs is already pinning substantially more memory for an open file than other fs (by about a factor of 2-3, due to pinned lower and/or upper dentries and inodes). If this was a problem in real life, then we'd be already be in trouble. The stacked open file patch only adds the overhead of the actual struct file, which is small compared to the multiple pinned dentry and inode structs. Christoph already commented on a previous version of the patch and I pointed out the above. Thanks, Miklos
