Re: [PATCH] zsmalloc: move LRU update from zs_map_object() to zs_malloc()

[Nhat Pham <nphamcs@gmail.com>]

On Tue, May 9, 2023 at 12:24 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Tue, May 09, 2023 at 11:20:02AM -0700, Minchan Kim wrote:
> > Hi Folks,
> >
> > On Tue, May 09, 2023 at 01:44:01PM -0400, Johannes Weiner wrote:
> > > On Tue, May 09, 2023 at 12:00:30PM +0900, Sergey Senozhatsky wrote:
> > > > On (23/05/08 09:00), Nhat Pham wrote:
> > > > > > The deeper bug here is that zs_map_object() tries to add the page to
> > > > > > the LRU list while the shrinker has it isolated for reclaim. This is
> > > > > > way too sutble and error prone. Even if it worked now, it'll cause
> > > > > > corruption issues down the line.
> > > > > >
> > > > > > For example, Nhat is adding a secondary entry point to reclaim.
> > > > > > Reclaim expects that a page that's on the LRU is also on the fullness
> > > > > > list, so this would lead to a double remove_zspage() and BUG_ON().
> > > > > >
> > > > > > This patch doesn't just fix the crash, it eliminates the deeper LRU
> > > > > > isolation issue and makes the code more robust and simple.
> > > > >
> > > > > I agree. IMO, less unnecessary concurrent interaction is always a
> > > > > win for developers' and maintainers' cognitive load.
> > > >
> > > > Thanks for all the explanations.
> > > >
> > > > > As a side benefit - this also gets rid of the inelegant check
> > > > > (mm == ZS_MM_WO). The fact that we had to include a
> > > > > a multi-paragraph explanation for a 3-line piece of code
> > > > > should have been a red flag.
> > > >
> > > > Minchan had some strong opinion on that, so we need to hear from him
> > > > before we decide how do we fix it.
> > >
> > > I'd be happy if he could validate the fix. But this fixes a crash, so
> > > the clock is ticking.
> > >
> > > I will also say, his was a design preference. One we agreed to only
> > > very reluctantly: https://lore.kernel.org/lkml/Y3f6habiVuV9LMcu@google.com/
> > >
> > > Now we have a crash that is a direct result of it, and which cost us
> > > (and apparently is still costing us) time and energy to resolve.
> > >
> > > Unless somebody surfaces a real technical problem with the fix, I'd
> > > say let's do it our way this time.
> > >
> >
> > Sorry for being too late to review. The reason I insisted on it was
> > I overlookeded the bug and thought it was trivial change but better
> > semantic since zsmalloc provides separate API between allocation and
> > access unlike other allocators. Now, Nhat and Johannes provided it's
> > more error prone, I am totally fine with this fix and will live it
> > until the LRU writeback will move out of allocator.
> >
> > Sorry for wasting your time to hunt this bug down and thank you for fix!
> >
> > Acked-by: Minchan Kim <minchan@kernel.org>
>
> Thanks Minchan!
>
> Domenico is working on the LRU refactor right now, and should have
> patches for review soon. This will indeed get rid of all the zsmalloc
> warts and make our lives much easier going forward!

That's the dream! I look forward to Domenico's patches.
And thanks for the ack, Minchan!