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    SubjectRe: [PATCH 05/39] vfs: optionally don't account file in nr_files
    On Mon, Jun 4, 2018 at 10:47 AM, Christoph Hellwig <hch@infradead.org> wrote:
    > On Tue, May 29, 2018 at 04:43:05PM +0200, Miklos Szeredi 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.
    >
    > But that overhead is exactly what nr_files accounts for, so this looks
    > bogus to me.

    According to comment above files_maxfiles_init() one open file uses
    roughly 1k, which is the total from struct file + pinned dentry +
    pinned inode. The actual struct file is just a quarter of that.

    So while overlayfs does currently pin almost 2k per file and,
    according to that calculation should already be using two nr_file
    slots, it isn't. And switching to using two slots means current
    setups might well have regressions due to that.

    I'm not against switching to two slots, but it's something that would
    need to come with backward compatibility guarantees (e.g. explicitly
    enabled with boot option, or whatever) and I don't think it's worth
    the trouble.

    Maintaining the two versions of overlayfs (with and without stacked
    fops) also makes little sense.

    Thanks,
    Miklos

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