[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <1ee09786-ed3c-485e-99e4-48c4d2b92ced@linux.alibaba.com>
Date: Mon, 8 Sep 2025 11:17:06 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Kairui Song <kasong@...cent.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
Chris Li <chrisl@...nel.org>, Barry Song <baohua@...nel.org>,
Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>,
Kemeng Shi <shikemeng@...weicloud.com>,
Ying Huang <ying.huang@...ux.alibaba.com>,
Johannes Weiner <hannes@...xchg.org>, David Hildenbrand <david@...hat.com>,
Yosry Ahmed <yosryahmed@...gle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 08/15] mm/shmem, swap: remove redundant error handling
for replacing folio
On 2025/9/6 03:13, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>
>
> Shmem may replace a folio in the swap cache if the cached one doesn't
> fit the swapin's GFP zone. When doing so, shmem has already double
> checked that the swap cache folio is locked, still has the swap cache
> flag set, and contains the wanted swap entry. So it is impossible to
> fail due to an Xarray mismatch. There is even a comment for that.
>
> Delete the defensive error handling path, and add a WARN_ON instead:
> if that happened, something has broken the basic principle of how the
> swap cache works, we should catch and fix that.
>
> Signed-off-by: Kairui Song <kasong@...cent.com>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> ---
> mm/shmem.c | 42 ++++++++++++------------------------------
> 1 file changed, 12 insertions(+), 30 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 4e27e8e5da3b..cc6a0007c7a6 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1698,13 +1698,13 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
> }
>
> /*
> - * The delete_from_swap_cache() below could be left for
> + * The swap_cache_del_folio() below could be left for
> * shrink_folio_list()'s folio_free_swap() to dispose of;
> * but I'm a little nervous about letting this folio out of
> * shmem_writeout() in a hybrid half-tmpfs-half-swap state
> * e.g. folio_mapping(folio) might give an unexpected answer.
> */
> - delete_from_swap_cache(folio);
> + swap_cache_del_folio(folio);
> goto redirty;
> }
You should reorganize your patch set, as the swap_cache_del_folio()
function is introduced in patch 9.
> if (nr_pages > 1)
> @@ -2082,7 +2082,7 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
> new->swap = entry;
>
> memcg1_swapin(entry, nr_pages);
> - shadow = get_shadow_from_swap_cache(entry);
> + shadow = swap_cache_get_shadow(entry);
Ditto.
> if (shadow)
> workingset_refault(new, shadow);
> folio_add_lru(new);
> @@ -2158,35 +2158,17 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
> /* Swap cache still stores N entries instead of a high-order entry */
> xa_lock_irq(&swap_mapping->i_pages);
> for (i = 0; i < nr_pages; i++) {
> - void *item = xas_load(&xas);
> -
> - if (item != old) {
> - error = -ENOENT;
> - break;
> - }
> -
> - xas_store(&xas, new);
> + WARN_ON_ONCE(xas_store(&xas, new));
> xas_next(&xas);
> }
> - if (!error) {
> - mem_cgroup_replace_folio(old, new);
> - shmem_update_stats(new, nr_pages);
> - shmem_update_stats(old, -nr_pages);
> - }
> xa_unlock_irq(&swap_mapping->i_pages);
>
> - if (unlikely(error)) {
> - /*
> - * Is this possible? I think not, now that our callers
> - * check both the swapcache flag and folio->private
> - * after getting the folio lock; but be defensive.
> - * Reverse old to newpage for clear and free.
> - */
> - old = new;
> - } else {
> - folio_add_lru(new);
> - *foliop = new;
> - }
> + mem_cgroup_replace_folio(old, new);
> + shmem_update_stats(new, nr_pages);
> + shmem_update_stats(old, -nr_pages);
> +
> + folio_add_lru(new);
> + *foliop = new;
>
> folio_clear_swapcache(old);
> old->private = NULL;
> @@ -2220,7 +2202,7 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
> nr_pages = folio_nr_pages(folio);
> folio_wait_writeback(folio);
> if (!skip_swapcache)
> - delete_from_swap_cache(folio);
> + swap_cache_del_folio(folio);
> /*
> * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
> * won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)
> @@ -2459,7 +2441,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> folio->swap.val = 0;
> swapcache_clear(si, swap, nr_pages);
> } else {
> - delete_from_swap_cache(folio);
> + swap_cache_del_folio(folio);
> }
> folio_mark_dirty(folio);
> swap_free_nr(swap, nr_pages);
Powered by blists - more mailing lists