Re: [PATCH v12 00/31] Speculative page faults

[Peter Zijlstra]

On Mon, Apr 22, 2019 at 02:29:16PM -0700, Michel Lespinasse wrote:
> The proposed spf mechanism only handles anon vmas. Is there a
> fundamental reason why it couldn't handle mapped files too ?
> My understanding is that the mechanism of verifying the vma after
> taking back the ptl at the end of the fault would work there too ?
> The file has to stay referenced during the fault, but holding the vma's
> refcount could be made to cover that ? the vm_file refcount would have
> to be released in __free_vma() instead of remove_vma; I'm not quite sure
> if that has more implications than I realize ?

IIRC (and I really don't remember all that much) the trickiest bit was
vs unmount. Since files can stay open past the 'expected' duration,
umount could be delayed.

But yes, I think I had a version that did all that just 'fine'. Like
mentioned, I didn't keep the refcount because it sucked just as hard as
the mmap_sem contention, but the SRCU callback did the fput() just fine
(esp. now that we have delayed_fput).