linux-kernel - Re: [PATCH v7 4/8] mm: zswap: Refactor code to delete stored offsets in case of errors.

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Message-ID: <CAJD7tkbdRPKxOoVJMg5XdQuoByE1yuOjEENuM=wDnh_cOQZ7mA@mail.gmail.com>
Date: Tue, 24 Sep 2024 12:20:15 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, hannes@...xchg.org, 
	nphamcs@...il.com, chengming.zhou@...ux.dev, usamaarif642@...il.com, 
	shakeel.butt@...ux.dev, ryan.roberts@....com, ying.huang@...el.com, 
	21cnbao@...il.com, akpm@...ux-foundation.org, nanhai.zou@...el.com, 
	wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v7 4/8] mm: zswap: Refactor code to delete stored offsets
 in case of errors.

On Mon, Sep 23, 2024 at 6:17 PM Kanchana P Sridhar
<kanchana.p.sridhar@...el.com> wrote:
>
> Added a new procedure zswap_delete_stored_offsets() that can be
> called to delete stored offsets in a folio in case zswap_store()
> fails or zswap is disabled.

I don't see the value in this helper. It will get called in one place
AFAICT, and it is a bit inconsistent that we have to explicitly loop
in zswap_store() to store pages, but the loop to delete pages upon
failure is hidden in the helper.

I am not against adding a trivial zswap_tree_delete() helper (or
similar) that calls xa_erase() and  zswap_entry_free() to match
zswap_tree_store() if you prefer that.

>
> Refactored the code in zswap_store() that handles these cases,
> to call zswap_delete_stored_offsets().
>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> ---
>  mm/zswap.c | 33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index fd35a81b6e36..9bea948d653e 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1435,8 +1435,37 @@ static bool zswap_store_entry(struct xarray *tree,
>         return true;
>  }
>
> +/*
> + * If the zswap store fails or zswap is disabled, we must invalidate the
> + * possibly stale entries which were previously stored at the offsets
> + * corresponding to each page of the folio. Otherwise, writeback could
> + * overwrite the new data in the swapfile.
> + *
> + * This is called after the store of an offset in a large folio has failed.
> + * All zswap entries in the folio must be deleted. This helps make sure
> + * that a swapped-out mTHP is either entirely stored in zswap, or entirely
> + * not stored in zswap.
> + *
> + * This is also called if zswap_store() is invoked, but zswap is not enabled.
> + * All offsets for the folio are deleted from zswap in this case.
> + */
> +static void zswap_delete_stored_offsets(struct xarray *tree,
> +                                       pgoff_t offset,
> +                                       long nr_pages)
> +{
> +       struct zswap_entry *entry;
> +       long i;
> +
> +       for (i = 0; i < nr_pages; ++i) {
> +               entry = xa_erase(tree, offset + i);
> +               if (entry)
> +                       zswap_entry_free(entry);
> +       }
> +}
> +
>  bool zswap_store(struct folio *folio)
>  {
> +       long nr_pages = folio_nr_pages(folio);
>         swp_entry_t swp = folio->swap;
>         pgoff_t offset = swp_offset(swp);
>         struct xarray *tree = swap_zswap_tree(swp);
> @@ -1541,9 +1570,7 @@ bool zswap_store(struct folio *folio)
>          * possibly stale entry which was previously stored at this offset.
>          * Otherwise, writeback could overwrite the new data in the swapfile.
>          */
> -       entry = xa_erase(tree, offset);
> -       if (entry)
> -               zswap_entry_free(entry);
> +       zswap_delete_stored_offsets(tree, offset, nr_pages);
>         return false;
>  }
>
> --
> 2.27.0
>