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

[Johannes Weiner <hannes@cmpxchg.org>]

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@arm.com/T/#u
> 
> Co-developed-by: Ryan Roberts

I would change that to

Originally-by: Ryan Roberts <ryan.roberts@arm.com>

> Signed-off-by:

and drop this for now.

> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>