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Message-ID: <f90a6072-b75a-40df-a58c-9a98e9ca10ad@linux.alibaba.com>
Date: Mon, 23 Jun 2025 11:26:32 +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>, Hugh Dickins
<hughd@...gle.com>, Matthew Wilcox <willy@...radead.org>,
Kemeng Shi <shikemeng@...weicloud.com>, Chris Li <chrisl@...nel.org>,
Nhat Pham <nphamcs@...il.com>, Baoquan He <bhe@...hat.com>,
Barry Song <baohua@...nel.org>, linux-kernel@...r.kernel.org,
stable@...r.kernel.org
Subject: Re: [PATCH v2 1/4] mm/shmem, swap: improve cached mTHP handling and
fix potential hung
Hi Kairui,
On 2025/6/20 01:55, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>
>
> The current swap-in code assumes that, when a swap entry in shmem
> mapping is order 0, its cached folios (if present) must be order 0
> too, which turns out not always correct.
>
> The problem is shmem_split_large_entry is called before verifying the
> folio will eventually be swapped in, one possible race is:
>
> CPU1 CPU2
> shmem_swapin_folio
> /* swap in of order > 0 swap entry S1 */
> folio = swap_cache_get_folio
> /* folio = NULL */
> order = xa_get_order
> /* order > 0 */
> folio = shmem_swap_alloc_folio
> /* mTHP alloc failure, folio = NULL */
> <... Interrupted ...>
> shmem_swapin_folio
> /* S1 is swapped in */
> shmem_writeout
> /* S1 is swapped out, folio cached */
> shmem_split_large_entry(..., S1)
> /* S1 is split, but the folio covering it has order > 0 now */
>
> Now any following swapin of S1 will hang: `xa_get_order` returns 0,
> and folio lookup will return a folio with order > 0. The
> `xa_get_order(&mapping->i_pages, index) != folio_order(folio)` will
> always return false causing swap-in to return -EEXIST.
>
> And this looks fragile. So fix this up by allowing seeing a larger folio
> in swap cache, and check the whole shmem mapping range covered by the
> swapin have the right swap value upon inserting the folio. And drop
> the redundant tree walks before the insertion.
>
> This will actually improve the performance, as it avoided two redundant
> Xarray tree walks in the hot path, and the only side effect is that in
> the failure path, shmem may redundantly reallocate a few folios
> causing temporary slight memory pressure.
>
> And worth noting, it may seems the order and value check before
> inserting might help reducing the lock contention, which is not true.
> The swap cache layer ensures raced swapin will either see a swap cache
> folio or failed to do a swapin (we have SWAP_HAS_CACHE bit even if
> swap cache is bypassed), so holding the folio lock and checking the
> folio flag is already good enough for avoiding the lock contention.
> The chance that a folio passes the swap entry value check but the
> shmem mapping slot has changed should be very low.
Thanks for fixing the issue. Sadly, I haven't reproduced this issue from
my previous test cases :(
And I have a question below.
> Cc: stable@...r.kernel.org
> Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
> Signed-off-by: Kairui Song <kasong@...cent.com>
> Reviewed-by: Kemeng Shi <shikemeng@...weicloud.com>
> ---
> mm/shmem.c | 30 +++++++++++++++++++++---------
> 1 file changed, 21 insertions(+), 9 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index eda35be2a8d9..4e7ef343a29b 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -884,7 +884,9 @@ static int shmem_add_to_page_cache(struct folio *folio,
> pgoff_t index, void *expected, gfp_t gfp)
> {
> XA_STATE_ORDER(xas, &mapping->i_pages, index, folio_order(folio));
> - long nr = folio_nr_pages(folio);
> + unsigned long nr = folio_nr_pages(folio);
> + swp_entry_t iter, swap;
> + void *entry;
>
> VM_BUG_ON_FOLIO(index != round_down(index, nr), folio);
> VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> @@ -896,14 +898,24 @@ static int shmem_add_to_page_cache(struct folio *folio,
>
> gfp &= GFP_RECLAIM_MASK;
> folio_throttle_swaprate(folio, gfp);
> + swap = iter = radix_to_swp_entry(expected);
>
> do {
> xas_lock_irq(&xas);
> - if (expected != xas_find_conflict(&xas)) {
> - xas_set_err(&xas, -EEXIST);
> - goto unlock;
> + xas_for_each_conflict(&xas, entry) {
> + /*
> + * The range must either be empty, or filled with
> + * expected swap entries. Shmem swap entries are never
> + * partially freed without split of both entry and
> + * folio, so there shouldn't be any holes.
> + */
> + if (!expected || entry != swp_to_radix_entry(iter)) {
> + xas_set_err(&xas, -EEXIST);
> + goto unlock;
> + }
> + iter.val += 1 << xas_get_order(&xas);
> }
> - if (expected && xas_find_conflict(&xas)) {
> + if (expected && iter.val - nr != swap.val) {
> xas_set_err(&xas, -EEXIST);
> goto unlock;
> }
> @@ -2323,7 +2335,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> error = -ENOMEM;
> goto failed;
> }
> - } else if (order != folio_order(folio)) {
> + } else if (order > folio_order(folio)) {
> /*
> * Swap readahead may swap in order 0 folios into swapcache
> * asynchronously, while the shmem mapping can still stores
> @@ -2348,15 +2360,15 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>
> swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
> }
> + } else if (order < folio_order(folio)) {
> + swap.val = round_down(swp_type(swap), folio_order(folio));
Why rounding down the swap type? do you mean rounding down the swap offset?
> }
>
> alloced:
> /* We have to do this with folio locked to prevent races */
> folio_lock(folio);
> if ((!skip_swapcache && !folio_test_swapcache(folio)) ||
> - folio->swap.val != swap.val ||
> - !shmem_confirm_swap(mapping, index, swap) ||
> - xa_get_order(&mapping->i_pages, index) != folio_order(folio)) {
> + folio->swap.val != swap.val) {
> error = -EEXIST;
> goto unlock;
> }
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