RE: [PATCH v9 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 4:39 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> hannes@xxxxxxxxxxx; 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 v9 6/7] mm: zswap: Support large folios in zswap_store().
> 
> [..]
> > > > > > > > >  store_failed:
> > > > > > > > >         zpool_free(entry->pool->zpool, entry->handle);
> > > > > > > > > -put_pool:
> > > > > > > > > -       zswap_pool_put(entry->pool);
> > > > > > > > > -freepage:
> > > > > > > > > +put_pool_objcg:
> > > > > > > > > +       zswap_pool_put(pool);
> > > > > > > > > +       obj_cgroup_put(objcg);
> > > > > > > >
> > > > > > > > I think if we reorder the function we can drop these calls, make
> the
> > > > > > > > comments positioned a bit better, and centralize the entry
> > > > > > > > initializations. I am also not a fan of passing a semi-initialized
> > > > > > > > entry to zswap_compress() to get the pool pointer.
> > > > > > > >
> > > > > > > > Does the following diff improve things or did I miss something?
> > > > > > >
> > > > > > > We shouldn’t be adding the entry to the xarray before initializing its
> > > pool
> > > > > > > and objcg, right? Please let me know if I am misunderstanding
> what
> > > > you're
> > > > > > > proposing in the diff.
> > > > > >
> > > > > > It should be safe. We already initialize entry->lru after we insert
> > > > > > the entry in the tree. See the comment above the call to
> > > > > > zswap_lru_add(). Basically we are protected against concurrent
> > > > > > stores/loads through the folio lock, and are protected against
> > > > > > writeback because the entry is not on the LRU yet.
> > > > >
> > > > > Thanks for the clarification, Yosry. Since this is a change in the entry
> > > > > initialization wrt the mainline, is it Ok if this is done in a follow-up
> patch?
> > > >
> > > > Sure. We can discuss it separately. Do you want me to send a patch or
> > > > do you intend to?
> > >
> > > Thanks Yosry! I will send the patch separately.
> >
> > Hi Yosry,
> >
> > I am preparing the follow-up patch so I can submit it once this patch-series
> is
> > merged to mm-unstable (since these changes have dependencies on my
> > existing patch).
> >
> > Is my understanding correct that the folio lock also protects against swapoff
> > happening in between addition of the entry to the xarray and initializing its
> > members, which will need to be valid for
> > swapoff --> ... -> free_swap_slot() --> zswap_invalidate() ? Would
> appreciate
> > it if you can confirm.
> 
> Yes, the folio lock should protect against swapoff, as the folio must
> be in the swapcache.
> 
> For shmem, try_to_unuse() (called by swapoff()) will end up calling
> shmem_swapin_folio(), which will lookup the folio in the swapcache,
> find it, then lock it before proceeding to delete it from the swap
> cache and ultimately freeing the swap entry.
> 
> For anonymous memory, try_to_unuse() will call unuse_mm() -> .. ->
> unuse_pte_range(), which will also lookup the folio and lock it before
> deleting it from the swap cache and freeing the entry.
> 
> try_to_unuse() will also loop over any remaining swapcache entries,
> lock the folios and then try to free the swap entry.

Sounds good Yosry. Thanks for the explanations!

Thanks,
Kanchana