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

[Sridhar, Kanchana P]

> -----Original Message-----
> From: Johannes Weiner <hannes@xxxxxxxxxxx>
> Sent: Tuesday, October 1, 2024 4:11 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>
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> 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?

> 
> > Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@xxxxxxxxx>
> 
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Many thanks! Really appreciate the code review and comments.

Thanks,
Kanchana