Re: [PATCH v4 0/3] Avoid live-lock in btrfs fault-in+uaccess loop

From: Andreas Gruenbacher
Date: Mon Apr 25 2022 - 07:09:08 EST

Hi Catalin,

On Sat, Apr 23, 2022 at 8:40 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> On Sat, Apr 23, 2022 at 09:35:42AM -0700, Linus Torvalds wrote:
> > On Sat, Apr 23, 2022 at 3:07 AM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > >
> > > The series introduces fault_in_subpage_writeable() together with the
> > > arm64 probing counterpart and the btrfs fix.
> >
> > Looks fine to me - and I think it can probably go through the arm64
> > tree since you'd be the only one really testing it anyway.
> I'll queue it via arm64 then.

sounds good to me, thank you.

> > I assume you checked that btrfs is the only one that uses
> > fault_in_writeable() in this way? Everybody else updates to the right
> > byte boundary and retries (or returns immediately)?
> I couldn't find any other places (by inspection or testing). The
> buffered file I/O can already make progress in current fault_in_*() +
> copy_*_user() loops.

This started working correctly with commit bc1bb416bbb9
("generic_perform_write()/iomap_write_actor(): saner logics for short
copy") by Al from last May.

> O_DIRECT either goes via GUP (and memcpy() doesn't
> fault) or, if the user buffer is not PAGE aligned, it may fall back to
> buffered I/O. That's why I simplified the series, AFAICT it's only btrfs
> search_ioctl() with this problem.