RE: [PATCH v10 6/7] mm: zswap: Support large folios in zswap_store().

[Sridhar, Kanchana P]

> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Sent: Tuesday, October 1, 2024 10:04 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> linux-mm@xxxxxxxxx; nphamcs@xxxxxxxxx; chengming.zhou@xxxxxxxxx;
> usamaarif642@xxxxxxxxx; shakeel.butt@xxxxxxxxx; ryan.roberts@xxxxxxx;
> Huang, Ying <ying.huang@xxxxxxxxx>; 21cnbao@xxxxxxxxx; akpm@linux-
> foundation.org; willy@xxxxxxxxxxxxx; Zou, Nanhai <nanhai.zou@xxxxxxxxx>;
> Feghali, Wajdi K <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh
> <vinodh.gopal@xxxxxxxxx>
> Subject: Re: [PATCH v10 6/7] mm: zswap: Support large folios in
> zswap_store().
> 
> On Tue, Oct 1, 2024 at 10:01 AM Sridhar, Kanchana P
> <kanchana.p.sridhar@xxxxxxxxx> wrote:
> >
> > > -----Original Message-----
> > > From: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > Sent: Tuesday, October 1, 2024 4:11 AM
> > > To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> > > yosryahmed@xxxxxxxxxx; nphamcs@xxxxxxxxx;
> > > chengming.zhou@xxxxxxxxx; usamaarif642@xxxxxxxxx;
> > > shakeel.butt@xxxxxxxxx; ryan.roberts@xxxxxxx; Huang, Ying
> > > <ying.huang@xxxxxxxxx>; 21cnbao@xxxxxxxxx; akpm@linux-
> foundation.org;
> > > willy@xxxxxxxxxxxxx; Zou, Nanhai <nanhai.zou@xxxxxxxxx>; Feghali, Wajdi
> K
> > > <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh <vinodh.gopal@xxxxxxxxx>
> > > Subject: Re: [PATCH v10 6/7] mm: zswap: Support large folios in
> > > zswap_store().
> > >
> > > On Mon, Sep 30, 2024 at 10:32:21PM -0700, Kanchana P Sridhar wrote:
> > > > zswap_store() will store large folios by compressing them page by page.
> > > >
> > > > This patch provides a sequential implementation of storing a large folio
> > > > in zswap_store() by iterating through each page in the folio to compress
> > > > and store it in the zswap zpool.
> > > >
> > > > zswap_store() calls the newly added zswap_store_page() function for
> each
> > > > page in the folio. zswap_store_page() handles compressing and storing
> each
> > > > page.
> > > >
> > > > We check the global and per-cgroup limits once at the beginning of
> > > > zswap_store(), and only check that the limit is not reached yet. This is
> > > > racy and inaccurate, but it should be sufficient for now. We also obtain
> > > > initial references to the relevant objcg and pool to guarantee that
> > > > subsequent references can be acquired by zswap_store_page(). A new
> > > function
> > > > zswap_pool_get() is added to facilitate this.
> > > >
> > > > If these one-time checks pass, we compress the pages of the folio, while
> > > > maintaining a running count of compressed bytes for all the folio's pages.
> > > > If all pages are successfully compressed and stored, we do the cgroup
> > > > zswap charging with the total compressed bytes, and batch update the
> > > > zswap_stored_pages atomic/zswpout event stats with folio_nr_pages()
> > > once,
> > > > before returning from zswap_store().
> > > >
> > > > If an error is encountered during the store of any page in the folio,
> > > > all pages in that folio currently stored in zswap will be invalidated.
> > > > Thus, a folio is either entirely stored in zswap, or entirely not stored
> > > > in zswap.
> > > >
> > > > The most important value provided by this patch is it enables swapping
> out
> > > > large folios to zswap without splitting them. Furthermore, it batches
> some
> > > > operations while doing so (cgroup charging, stats updates).
> > > >
> > > > This patch also forms the basis for building compress batching of pages in
> > > > a large folio in zswap_store() by compressing up to say, 8 pages of the
> > > > folio in parallel in hardware using the Intel In-Memory Analytics
> > > > Accelerator (Intel IAA).
> > > >
> > > > This change reuses and adapts the functionality in Ryan Roberts' RFC
> > > > patch [1]:
> > > >
> > > >   "[RFC,v1] mm: zswap: Store large folios without splitting"
> > > >
> > > >   [1] https://lore.kernel.org/linux-mm/20231019110543.3284654-1-
> > > ryan.roberts@xxxxxxx/T/#u
> > > >
> > > > Co-developed-by: Ryan Roberts
> > >
> > > I would change that to
> > >
> > > Originally-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > >
> > > > Signed-off-by:
> > >
> > > and drop this for now.
> >
> > Thanks Johannes. Sure, this sounds good. Should I post a v11 for just this
> > specific patch which this change, or a v11 for the entire series?
> 
> Andrew could probably make the change for you while applying the
> patches to mm-unstable if you ask nicely :)
> 
> Also since we agreed further cleanup can be done as a followup:
> Acked-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>

Thanks Yosry!