Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in zswap_store().

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Sep 26, 2024 at 9:40 AM Sridhar, Kanchana P
<kanchana.p.sridhar@intel.com> wrote:
>
> > -----Original Message-----
> > From: Yosry Ahmed <yosryahmed@google.com>
> > Sent: Wednesday, September 25, 2024 9:52 PM
> > To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>; linux-kernel@vger.kernel.org;
> > linux-mm@kvack.org; nphamcs@gmail.com; chengming.zhou@linux.dev;
> > usamaarif642@gmail.com; shakeel.butt@linux.dev; ryan.roberts@arm.com;
> > Huang, Ying <ying.huang@intel.com>; 21cnbao@gmail.com; akpm@linux-
> > foundation.org; Zou, Nanhai <nanhai.zou@intel.com>; Feghali, Wajdi K
> > <wajdi.k.feghali@intel.com>; Gopal, Vinodh <vinodh.gopal@intel.com>
> > Subject: Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in
> > zswap_store().
> >
> > [..]
> > >
> > > One thing I realized while reworking the patches for the batched checks is:
> > > within zswap_store_page(), we set the entry->objcg and entry->pool before
> > > adding it to the xarray. Given this, wouldn't it be safer to get the objcg
> > > and pool reference per sub-page, locally in zswap_store_page(), rather than
> > > obtaining batched references at the end if the store is successful? If we
> > want
> > > zswap_store_page() to be self-contained and correct as far as the entry
> > > being created and added to the xarray, it seems like the right thing to do?
> > > I am a bit apprehensive about the entry being added to the xarray without
> > > a reference obtained on the objcg and pool, because any page-
> > faults/writeback
> > > that occur on sub-pages added to the xarray before the entire folio has been
> > > stored, would run into issues.
> >
> > We definitely should not obtain references to the pool and objcg after
> > initializing the entries with them. We can obtain all references in
> > zswap_store() before zswap_store_page(). IOW, the batching in this
> > case should be done before the per-page operations, not after.
>
> Thanks Yosry. IIUC, we should obtain all references to the objcg and to the
> zswap_pool at the start of zswap_store.
>
> In the case of error on any sub-page, we will unwind state for potentially
> only the stored pages or the entire folio if it happened to already be in zswap
> and is being re-written. We might need some additional book-keeping to
> keep track of which sub-pages were found in the xarray and zswap_entry_free()
> got called (nr_sb). Assuming I define a new "obj_cgroup_put_many()", I would need
> to call this with (folio_nr_pages() - nr_sb).
>
> As far as zswap_pool_get(), there is some added complexity if we want to
> keep the existing implementation that calls "percpu_ref_tryget()", and assuming
> this is extended to provide a new "zswap_pool_get_many()" that calls
> "percpu_ref_tryget_many()". Is there a reason we use percpu_ref_tryget() instead
> of percpu_ref_get()? Reason I ask is, with tryget(), if for some reason the pool->ref
> is 0, no further increments will be made. If so, upon unwinding state in
> zswap_store(), I would need to special-case to catch this before calling a new
> "zswap_pool_put_many()".
>
> Things could be a little simpler if zswap_pool_get() can use "percpu_ref_get()"
> which will always increment the refcount. Since the zswap pool->ref is initialized
> to "1", this seems Ok, but I don't know if there will be unintended consequences.
>
> Can you please advise on what is the simplest/cleanest approach:
>
> 1) Proceed with the above changes without changing percpu_ref_tryget in
>      zswap_pool_get. Needs special-casing in zswap_store to detect pool->ref
>     being "0" before calling zswap_pool_put[_many].

My assumption is that we can reorder the code such that if
zswap_pool_get_many() fails we don't call zswap_pool_put_many() to
begin with (e.g. jump to a label after zswap_pool_put_many()).

> 2) Modify zswap_pool_get/zswap_pool_get_many to use percpu_ref_get_many
>     and avoid special-casing to detect pool->ref being "0" before calling
>     zswap_pool_put[_many].

I don't think we can simply switch the tryget to a get, as I believe
we can race with the pool being destroyed.

> 3) Keep the approach in v7 where obj_cgroup_get/put is localized to
>     zswap_store_page for both success and error conditions, and any unwinding
>     state in zswap_store will take care of dropping references obtained from
>     prior successful writes (from this or prior invocations of zswap_store).

I am also fine with doing that and doing the reference batching as a follow up.


>
> Thanks,
> Kanchana
>
> >
> > >
> > > Just wanted to run this by you. The rest of the batched charging, atomic
> > > and stat updates should be Ok.
> > >
> > > Thanks,
> > > Kanchana
> > >
> > > >
> > > > Thanks,
> > > > Kanchana